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Transcript of Ag3 Rtm Chap 2
2‐1
CHAPTER TWO
Aviation Weather
2.1 INTRODUCTION
In this chapter, the Terminal Aerodrome Forecast (TAF), the Pilot Report (PIREP), the DD 175‐1, and
different tools used to generate aviation weather briefs, as well as the computer based programs used
by Navy and Marine Corps personnel in support of aviation are presented. Next, a detailed review of
the Upper air code and its operational applications, followed by a detailed review of the Skew‐T, Log
P Diagram wrap up the contents of this chapter.
2.2 TERMINAL AERODROME FORECAST (TAF) AND FM51‐XII ENCODING
Learning Objectives
Describe the forecasted weather elements contained within an encoded TAF
Explain Change Lines and when each is properly utilized
Describe the specific conditions that require an AMD, COR or RTD
Interpret specific forecast weather elements for translation into Flight Weather Briefer and a DD175‐
1, Flight Weather Briefing
Naval Meteorology and Oceanography Command Instruction 3143.1, Terminal Aerodrome Forecast
(TAF) and the FM51‐XII TAF Code, is the governing instruction for all U.S. Navy and Marine Corps weather
activities and should be read in its entirety. The United States, as a member of the World
Meteorological Organization (WMO), is obligated to follow general coding procedures and to advise the
WMO of differences in national coding practices. The WMO Manual on Codes, No. 306 Volume II Part A,
is the basic document outlining Terminal Aerodrome Forecast (TAF) Codes.
The Terminal Aerodrome Forecast (TAF) code provides information about the expected weather con‐
ditions that will occur at an airfield or station control zone through a 24 or 30‐hour forecast period, and
covers a relatively an area around the airfield, averaging about 75 square miles. The TAF is a forecast for
the most probable conditions expected for the airfield or station control zone, and includes very specific
elements that can control flight operations. The assistant forecaster and forecaster must pay particular
attention to; low visibility, thunderstorms, freezing precipitation, low ceilings and low level wind sheer as
these elements determine whether an airfield is available or a mission is feasible. Only certified
forecasters are authorized to write TAFs, however, as the assistant forecaster, you will often be tasked
2‐2
to prepare the latest TAF for local and long line transmission. The TAF code is presented here to enable the
accurate interpretation of the various coding elements as well as the ability to spot encoding errors if they
occur.
2.2.1 FM51‐XII TAF ENCODING
Navy and Marine Corps TAFs are issued three times a day every 8 hours, or four times a day every 6
hours. For 6‐hour TAFs, they shall be filed at the intermediate synoptic times of 0300, 0900, 1500, and
2100 UTC, and have a valid period of 24 hours. Zulu time, equivalent to UTC, Universal Time Coordinate,
is used on all meteorological products to include the TAF.
The complete TAF format is shown in table 2‐1 and explained in the following sections.
Table 2‐1. TAF Code Format and Sample
CCCC TAF (AMD or COR or RTD) Y1Y1G1G1/Y2Y2G2G2 dddffGfmfmKT VVVV w'w' NsNsNshshshs or SKC or VVhshshs
(WShwshwshws/dddffKT or WSCONDS) (6IchihihitL) (5BhbhbhbtL) QNHPIPIPIPIINS (Remarks) TTTTT GGGeGe or TTGGgg)
(TTFTF/D1D1GFGFZ) (AMD or COR GGgg)
KNGU TAF 2109/2209 23012KT 4800 ‐SN BKN005 OVC012 620107
QNH3002INS
TEMPO 0914 0800 +SNRA ‐BLSN VV002
BECMG 0506 33018KT 510804 QNH3015INS T01/15Z
2.2.1.1 Message Header: CCCC TAF (AMD or COR or RTD) Y1Y1G1G1G2G2
CCCC is the four‐letter ICAO identifier for the aerodrome and TAF is the message identifier.
AMD, COR, and RTD are utilized in very specific situations as described below.
AMD is included when certain weather conditions exist or are developing that were
not forecast, or when certain weather conditions were forecast, but are not
developing, or are no longer expected to develop. AMD is discussed in depth further
in this chapter.
COR is included when a previously transmitted TAF contained syntax errors and
required a correction. COR is not to be used to correct forecast errors.
RTD is included when the TAF is not transmitted, or disseminated by the scheduled
times.
AMD/COR is included when the TAF is a correction to an amended TAF
2‐3
The next group in the message header is Y1Y1G1G/Y2Y2G2G2, representing the date time group and valid
time of the TAF where; Y1Y1 represents the day of the month, G1G1 represents the beginning time of the
TAF; 03, 09, 15, or 21 as appropriate, and Y2Y2 represents the ending date while G2G2 represents the end
time of the TAF. Written together, Y1Y1G1G/Y2Y2G2G2 will read 2103/2203, 2109/2209, 2115/2215 or
2121/2221 as appropriate, where a date time group of 2109/2209 would indicate the TAF was valid on the
21st day of the month beginning at 0900Z, and valid until 0900Z on the 22nd as in the example in Table 2‐1.
In accordance with the most recent Naval Meteorology and Oceanography Command policy, the date time
group (DTG) format directed by the recently released 30‐Hour TAF format is currently in use in order for
electronic distribution programs to identify the TAF correctly. The TAF, however, remains valid for 24
hours in the majority of cases.
2.2.1.2 Wind group: dddffGfmfmKT
Encode the expected mean wind direction, speed and gusts, if any, for the initial forecast period and all subsequent
FM change lines, and BCMG change lines that include an expected change. KT is included in all wind groups.
ddd represents the forecasted true wind direction to the nearest 10 degrees and is expressed using 3
digits. If the direction is expected to vary by more than 60 degrees during the forecast period, encode
the prevailing direction for ddd and append the limits of variability to the remarks at the end of the final
line of the TAF (WND 270V350). Encode the contraction VRB only when the wind speed is forecast to be
6 knots or less.
ff is the forecast mean wind speed in whole knots using two digits. If the wind speed is expected to
exceed 99 knots during the forecast period, use three digits. Calm conditions are encoded as 00000KT
and variable wind conditions are encoded as VRB05KT. Remember, VRB is only used for forecasted
wind speeds of 6 knots or less.
Gfmfm represents gusts during the forecast period. A gust is a rapid fluctuation in wind speed with a variation of 10
knots between peaks and lulls so this group is omitted whenever forecasted winds will not meet these criteria. If
gusts are expected to exceed 99 knots during a forecast period, use 3 digits.
2‐4
2.2.1.3 Visibility Group: VVVV
Encode the forecast prevailing visibility for the initial forecast period and any subsequent FM or BCMG
group that includes an expected change to the visibility. Visibility is reported in meters and rounded
down to the nearest reportable value. Reportable values are listed in Table 2‐2. Whenever the forecast
decreases visibility to 9000 meters or less, a weather group must be included to indicate the cause of
the decrease in visibility. If the visibility is expected to alternate between significant values, as
presented in Table 2‐2, describe the situation in a TEMPO group as variable visibility remarks are not
authorized.
Table 2‐2. Reportable Visibility Values
STATUTE MILES METERS STATUTE MILES METERS 0 0 1 1/4 2000
1/16 0100 1 3/8 2200 1/8 0200 1 1/2 2400
3/16 0300 1 5/8 2600 1/4 0400 1 3/4 2800
5/16 0500 1 7/8 3000 3/8 0600 2 3200 1/2 0800 2 1/4 3600 5/8 1000 2 1/2 4000 3/4 1200 3 4800 7/8 1400 4 6000 1 1600 5 8000
1 1/8 1800 6 9000 7 or higher 9999
2.2.1.4 Weather Group: w’w’
Encode the forecasted weather and obstructions to vision, if any, for the initial forecast period and any
FM or BCMG groups that follow that include an expected change. Use standard abbreviations in the
appropriate order of precedence as presented in Table 2‐3. If significant weather is not expected in the
initial time period, or subsequent FM of BCMG groups, omit the group. Although a weather group can
be included anytime weather is forecast, a forecasted visibility of 9000 meters or less must be
accompanied by weather or an obstruction to visibility. For weather conditions that will occur for less
than one hour each occurrence, and will not cumulatively occur for more than half the forecast period, a
TEMPO, or temporary group is used. TEMPO is discussed later in this section.
Utilize Table 2‐3, reading from left to right, to identify the correct abbreviations for the forecasted
weather phenomenon. The order of precedence for entry into the TAF code is; thunderstorms,
2‐5
precipitation with predominant intensity, and obstructions to vision. Review NAVMETOCCOMINST
3143.1, section 3.2.4.1 for expanded information. A forecast of freezing precipitation is an exception to
the rule concerning Table 2‐3. When one or more types of precipitation are forecast, and one is
freezing, that type shall be encoded first, regardless of intensity.
Thunderstorms may be encoded without associated precipitation, however, whenever a thunderstorm
is included in the weather group, even if in the vicinity, the cloud group shall include a forecast cloud
type of cumulonimbus such as, BKN010CB.
Table 2‐3 Weather and Obstructions to Vision Identifiers
QUALIFIER WEATHER PHENOMENON
INTENSITY OR
PROXIMITY DESCRIPTOR PRECIPITATION OBSCURATION OTHER
1 2 3 4 5
‐ LIGHT MI Shallow DZ Drizzle BR Mist PO Well Developed Dust/Sand Whirls
MODERATE (no symbol)
PR Partial RA Rain FG Fog SQ Squalls
+ HEAVY BC Patches SN Snow FU Smoke FC Funnel Cloud, Tornado, Waterspout
VC in the Vicinity
DR Low Drifting SG Snow Grains VA Volcanic Ash
SS Sandstorm
BL Blowing IC Ice Crystals DU Widespread Dust
DS Duststorm
SH Showers PL Ice Pellets SA Sand
TS Thunderstorms
GR Hail HZ Haze
FZ Freezing
GS Small Hail and/or Snow Pellets
PY Spray
When forecasting fog, use BR if the prevailing visibility is expected to be 5/8 statute mile (1000 meters)
or more and use FG when prevailing visibility is expected to be less than 5/8 of a statute mile. MIFG, or
shallow mist, shall be encoded when the fog depth is forecast to be less than 6 feet and not expected to
obscure any part of the sky. MIFG often occurs in the morning hours when radiational cooling has
developed a surface based inversion and radiation fog. This type of fog does not obscure the sky or
restrict visibility, since the depth is less than 6 feet, but it can pose a hazard to aircraft on approach and
landing as the runway can be completely obscured from sight.
2‐6
With the exception of volcanic ash (VA), low drifting snow (DRSN), shallow fog (MIFG), partial fog
(PRFG), patchy fog (BCFG), drifting dust (DRDU), and drifting sand (DRSA), obscurations are forecast only
when the prevailing visibility is 9000 meters or less. VA, DRSN, MIFG, PRFG, BCFG, DRDU, and DRSA can
be forecast even when prevailing visibility is expected to remain unrestricted.
The vicinity qualifier, VC, is used to forecast weather phenomena between 5 to 10 statute miles of the
airfield, but not expected to occur within 5 statute miles of the airfield. VC shall only be used in
combination with TS, SH, and FG and shall be placed before the weather phenomena entry; i.e. VCTS.
When the predominant forecast condition has an encoded w’w’ group, and is followed by a change
group (BECMG or TEMPO) in which no significant weather is expected; the w’w’ in that change group
shall be encoded as NSW, No Significant Weather, to signify that significant weather phenomenon is no
longer expected.
2.2.1.5 Sky Cover Group: NsNsNshshshs or SKC or VVhshshs
This group is reported as often as necessary to indicate all forecasted cloud layers to the first overcast 8/8 cloud
layer. The sky cover group shall be included in the initial time period line, all subsequent FM lines, and
other change lines as necessary. Arrange sky cover layers in ascending order of cloud bases, from
lowest to highest, using the summation principle, just as you would when taking a surface weather
observation. All cloud layers included in the TAF are considered opaque.
The sky cover amount, represented as NsNsNs, shall be encoded as: SKC – Sky Clear, or no clouds, FEW – 1 to
2 oktas (eights), SCT – 3 to 4 oktas, BKN – 5 to 7 oktas, OVC – 8 oktas. Height information is not included
with SKC. Remember, the summation principle, as in observing, applies to all forecasted cloud decks in
the TAF code. The lowest level aloft at which the cumulative cloud cover equals 5/8, BKN, is considered
the ceiling, and once the sky condition OVC is reached, no further cloud decks are forecast.
If the sky condition is expected to alternate between two conditions, (BKN020 alternating with SCT020),
describe this condition by using a TEMPO group which is discussed later in this section.
The cloud layer height, represented as hshshs in the TAF code, is encoded as the height of the base of each sky
cover layer in hundreds of feet AGL. Layers from the surface to 50 feet are considered surface based and are
encoded as 000. The height of each layer from the surface to 5,000 is expressed to the nearest 100 feet, from
5,000 to 10,000 feet to the nearest 500 feet, and above 10,000 feet to the nearest 1,000 feet.
2‐7
The only cloud type included in the TAF is cumulonimbus (CB). Cumulonimbus is identified by adding
the contraction CB as a suffix to the height in a cloud group, for example, BKN020CB. CB may be
forecast without the forecast of a thunderstorm on station, however, if a thunderstorm is forecast, even
if only in the vicinity (VCTS), CB shall be forecast.
A partial obscuration, described as a surface based obscuration that obscures only part of the celestial
dome, is encoded with other sky cover layers, but the height is entered as 000 to indicate a surface
based obscuration.
A surface based obscuration is always the first layer reported with additional remarks in the remarks
section of the TAF. For example, if a cloud group of BKN000 is forecasted, the remark FG BKN000 would
be entered into the remarks if the obscuring phenomenon was in fact fog. Surface based partial
obscurations are not considered a ceiling, but are included in the summation principle when
determining the ceiling.
When a total obscuration is forecasted, use the TAF code group VVhshshs where VV is the indicator for the
group and hshshs indicates the expected vertical visibility up into the obscuring phenomena. To forecast a totally
obscured sky with a 500 foot vertical visibility, enter the group as VV005 in the TAF code. No remarks are
necessary when using this group, unlike the surface based partial obscuration above.
2.2.1.6 Non‐Convective Low‐Level Wind Shear (LLWS) Group: WShwshwshws/dddffKT or WSCONDS
Non‐convective Low‐Level Wind Shear poses great risk to the safe operation of aircraft, especially during
takeoff and landing. The LLWS group is included in the TAF whenever LLWS conditions not associated
with convective activity are expected to exist from the surface up to and including 2000 feet. Aviators
are trained to expect wind shear associated with convective activity, so a forecast of TS or CB will alert
them to the associated dangers, but non‐convective LLWS may not have associate visual clues as to its
existence, so by including this group in the TAF code, aviators will have forewarning of dangerous
conditions.
LLWS is defined as a vector difference of greater than 20 knots from the surface to the wind shear
height. WS is the indicator for the group, while hwshwshws represents the upper limit of the LLWS, and dddffKT
indicates the forecasted wind direction and speed at the wind shear height. For example, if the surface wind is
forecast to be from 270 degrees/ 10 knots, and from 030 degrees/ 15 knots at 1,600 feet, by subtracting the
surface wind from the upper wind at 1,600 feet, we find a vector difference of 050 degrees/ 23 knots. Figure 2‐1,
Vector Subtraction, explains the process of vector subtraction.
2‐8
If conditions favorable for wind shear are expected to occur but the forecaster is not confident in the wind
direction, speed, or the height of the wind shear height, WSCONDS can be used to document the dangerous
condition. The LLWS group is omitted when no low level wind shear is forecast, and LLWS shall never be included
in a TEMPO group.
The LLWS group shall be included in the initial period or a FM group whenever any of the following occur:
One or more pilots report LLWS at or in the vicinity of the airfield, where the pilots experience a
loss or gain in airspeed of 20 knots or more, AND the forecaster determines that the reports reflect
a valid occurrence of shear rather than mechanical turbulence.
When vertical, non‐convective wind shears of 10 knots or more per 100 feet occur in a layer more
than 200 feet thick within 2,000 of the surface are expected or reliably reported.
If meteorological conditions are such that LLWS can be inferred from pilot reports.
If meteorological conditions exist, or are forecast to exist, that are similar to past occurrences of
LLWS.
Figure 2‐1. Vector Subtraction. (Source: PDC)
2‐9
2.2.1.7 Icing Group: 6IchihihitL
The icing group is used to forecast icing at and in the vicinity
of the airfield, not associated with thunderstorms. Do not
use this group for icing in thunderstorms, as icing is implied
in and near thunderstorms.
This group can be repeated as often as necessary to indicate
multiple individual layers or a layer with a thickness greater
than 9,000 feet. The number 6 is the indicator for this group,
while Ic indicates the type of icing from Table 2‐4. hihihi
represents the height of the base of the icing layer, while tL
represents the thickness of the layer expressed in thousands
of feet using the appropriate thickness code from Table 2‐5.
If the layer thickness is greater than 9,000 feet, repeat the 6
group placing the base of the continued icing layer 9,000 feet
above the base of the previous layer. For example, a layer of
light icing, in cloud is forecast to develop. This layer is forecast to begin at 10,000 and extend upward to 21,000 feet.
Since this layer is greater than 9,000, one icing group will not suffice to describe the entire layer. This example is
encoded in the TAF as; 621009 621902, where the first layer is from 10,000 to 19,000 feet, and the second layer is
from 19,000 to 21,000 feet.
2.2.1.8 Turbulence Group: 5BhbhbhbtL
The turbulence group is used to forecast turbulence, at and in the
vicinity of the airfield, not associated with thunderstorms. Do not use
this group for turbulence in thunderstorms, as turbulence is implied in
and near thunderstorms.
This group can be repeated as often as necessary to indicate multiple
individual layers or a single layer with a thickness greater than 9,000
feet. The number 5 is the indicator for this group, while B indicates the
type of turbulence from Table 2‐6. hbhbhb,, as in icing, indicates the
height of the base of the turbulence layer, and tL represents the
thickness of the layer using the same table as for icing, Table 2‐5.
Table 2‐4 Icing Type (Ic)
Ic= Type of forecast ice accretion on external parts of aircraft
Code Figure
0 No Icing
1 Light Icing
2 Light Icing in Cloud
3 Light Icing in Precipitation
4 Moderate Icing
5 Moderate Icing in Cloud
6 Moderate Icing in Precipitation
7 Severe Icing
8 Severe Icing in Cloud
9 Severe Icing in Precipitation
Table 2‐5 Thickness of Icing and Turbulence Layers (tL)
Code Figure Thickness
1 1,000 feet
2 2,000 feet
3 3,000 feet
4 4,000 feet
5 5,000 feet
6 6,000 feet
7 7,000 feet
8 8,000 feet
9 9,000 feet
2‐10
2.2.1.9 Predicted Lowest Altimeter Setting:
QNHPIPIPIPIINS
The lowest altimeter setting is the lowest
expected altimeter setting during the initial
forecast period and for each BECMG and FM
group. QNH is not included in any TEMPO group.
QNH is the indicator for this group, while PIPIPIPI
represents the tens, units, tenths, and hundredths
of an inch. Omit the period between the tens and
tenths of an inch. INS simply indicates the unit of
measure.
2.2.1.10 Remarks
Remarks should be infrequent and relate to only
operationally significant forecast elements.
Remarks should never be used to replace a BECM or FM group to change weather conditions during a forecast
period. It is acceptable to include a remark concerning partial obscurations, as previously discussed, the time of an
AMD or COR TAF, as well as optional temperature data if so directed.
2.2.1.11 Change Lines Within the TAF Code
Change lines are used to indicate operationally significant changes from the initial forecast condition at some
intermediate hour within the 24 or 30‐hour forecast period. Change lines enable the forecaster to fully convey the
expected weather over an entire forecast period.
With each change line, start a new line of text within the TAF code and use only those change lines necessary to
explain changing weather conditions significant to operations in order for the meaning of the whole TAF code to
remain clear. Although several change lines may be required to accurately describe the full 24‐hour forecast period,
too many change lines and the overlapping of several TEMP and BECMG lines can cause confusion and inaccuracies
in the interpretation of the TAF product. As such, no more than two consecutive BECMG and TEMPO (combined)
modifications shall be used to change the initial or subsequent FM lines.
NATOPS requires that information contained in BECMG and TEMPO lines be used when considering the need for
alternate airfields. In other words, the inclusion of a TEMPO line with a visibility of 1/8 statute mile will essentially
close that airfield for the forecast time indicated, no matter how good the predominant weather is expected to be.
Table 2‐6. Turbulence Type and Intensity (B)
B = Turbulence
Code Figure
0 None
1 Light Turbulence
2 Moderate Turbulence in clear air, occasional
3 Moderate Turbulence in clear air, frequent
4 Moderate Turbulence in cloud, occasional
5 Moderate Turbulence in cloud, frequent
6 Severe Turbulence in clear air, occasional
7 Severe Turbulence in clear air, frequent
8 Severe Turbulence in cloud, occasional
9 Severe Turbulence in cloud, frequent
X Extreme Turbulence
2‐11
The initial line of the TAF is considered the predominant condition until changed by one of the change lines beginning
with either FM or BECMG. TEMPO is considered a temporary fluctuation to the predominant weather conditions
during the times indicated, with the initial line, or previous FM or BECMG line remaining predominant until the next
FM or BECMG line.
The FM change line is represented in the TAF code as FMDDGGgg where FM is the indicator of the group, DD
represents the day, and GGgg represent the hour and minute respectively. FM is the primary change group of the
TAF code and is used to describe significant change from the previously prevailing conditions at a specific time.
Changes indicated by the FM group occur in less than an hour with the time indicated representing the beginning
time of this new self contained part of the forecast. For example, a FM group beginning with FM101700 indicates
that predominant conditions will change from what they were in the previous predominant group to the conditions
contained in the FM101700 line beginning on the 10th day of the month at 1700Z, and will completely change within
one hour. The FM101700 group becomes the predominant forecast group until the end of the TAF period, or until
superseded by another FM or BECMG line.
When the FM change group is used, all forecast conditions preceding this group are superseded by the conditions
forecast in this group, and all TAF elements, except for those authorized to be omitted, are required in a FM group.
The BECMG change line is represented in the TAF code as BECMG DDGG/DeDeGeGe where BECMG is the indicator,
DDGG represent the day and hour the weather conditions will begin to change, while DeDeGeGe represents the day
and hour the changes are complete. BECMG shall be used to indicate a change to the predominant forecast
conditions expected to occur at either a regular or irregular rate at an unspecified time within the period indicated.
A BECMG group such as, BECMG 1019/1021, indicates the predominant weather conditions will begin to change on
the 10th day of the month at 1900Z at a regular or irregular rate to the conditions within the BECMG line, and will
complete the change to those weather conditions by the 10th day of the month at 2100Z. The duration of change
denoted by the BECMG line shall not exceed 2 hours.
Note the difference between the FM and BECMG change groups. While the FM time indicates the hour and minute
changes will begin to occur with all changes complete within one hour, the BECMG time indicates the day and hour at
which the change will begin, and the day and hour the change will be complete.
The BECMG group must contain all weather elements forecasted to change from the previously predominant line.
Any groups omitted from a BECMG line indicate that that particular weather element from the initial line or previous
FM group is still valid. In the following example, the BECMG line 2422/2500 does not contain a visibility element. In
this case refer to the initial line for the visibility of 9999, unrestricted.
2‐12
KNGU 2415/2515 02008KT 9999 FEW030 SCT120 BKN250 QNH2995INS
BECMG 2422/2500 VRB03KT FEW030 SCT250 QNH3000INS
When a BECMG line is used to forecast a change in one or more groups, the entire group must be repeated. For
example, if the BECMG line was used to forecast a lowering of the ceiling, but all other cloud layers are forecast to
remain the same, the entire cloud group must be repeated, not just the ceiling layer.
An incorrect use of the BECMG change line:
FM241800 35007KT 9999 FEW020 SCT120 SCT200 QNH3003INS BECMG 2421/2423 VRB03KT BKN020
The above BECMG line was necessary to denote the formation of a ceiling at 2,000 but was not intended to indicate
the middle and high cloud decks would dissipate. In order to correct the above BECMG line, it should be encoded as:
BECMG 2421/2423 VRB03KT BKN020 BKN120 OVC200
In this corrected version of the BECMG line, the cloud groups are all repeated to indicate the lowering of the ceiling as
well as the persistence of the middle and high cloud layers.
The final change line in the TAF code is the TEMPO change line. This change line is never a predominant group, but
rather a change from the predominant conditions that precede this line. The temporary conditions expressed within
a TEMPO group are those expected to occur during the time defined by the two‐digit begin time, expressed in whole
hours, and the two‐digit ending time also expressed in whole hours.
Weather conditions expressed in a TEMPO group, are those conditions that are expected to last less than one hour
each instance, with the aggregate of the TEMPO weather conditions occurring for less than half of the period
indicated by the begin and end times. In other words, a change line of TEMPO 1016/1020, indicates the conditions
listed in the TEMPO group will occur between 1600Z and 2000Z on the 10th day of the month, for less than one hour
each occurrence, and no more than 2 hours, or half of the valid time of the TEMPO line.
If the conditions contained within the TEMPO line are expected to occur for a period of greater than one hour or
greater than half of the period indicated, a FM or BECMG line must be used.
The time period of any TEMPO group should not exceed 6 hours and any weather element omitted from a TEMPO
line indicates the previous predominant condition remains valid.
TEMPO change lines cannot be used to forecast periods of non‐convective low‐level wind shear, icing, or turbulence.
2‐13
2.2.1.12 Change Lines and the Use of NSW
The no significant weather entry, NSW, shall be used in BECMG and TEMPO lines whenever the previous
predominant forecast condition has significant weather encoded but NO significant weather is expected during the
BECMG or TEMPO period, including weather forecast to occur in the vicinity.
2.2.1.13 Modifying the TAF
There are several reasons the forecaster may need to modify an existing TAF. If the observed weather is significantly
different than forecasted, the TAF may need to be amended (AMD). A syntax error in the TAF discovered after
transmission would call for a correction (COR) to be issued.
Once issued, it is the responsibility of the assistant forecaster, along with the forecaster to maintain a watchful eye on
weather conditions as they evolve to ensure the existing TAF adequately describes the situation. There are specific
weather criteria that will trigger a requirement for the TAF to be amended, but the forecaster may amend an existing
TAF any time it is considered advisable in the interest of safety, efficiency of operations, flight planning, operational
control, or flight assistance to aircraft. Safety should always be the first and foremost factor when deciding whether
or not to amend a TAF.
Because the TAF directly affects the safety of flight, the Navy and Marine Corps have issued universal criteria for
issuing an amendment to an existing TAF.
The TAF shall be amended if visibilities not predicted in the most recent forecast are observed, or are later forecasted
to increase to equal or exceed, or decrease to less than:
3 miles, or 4800 meters
1 mile, or 1600 meters
½ mile, or 800 meters
In other words, if the predominant visibility is forecast to decrease to 4 miles, 6000 meters, but instead decreases to 2
miles, 3200 meters, the TAF requires amendment because the visibility decreased to less than the 4800 meter
threshold. In another TAF example, say the forecast is for visibility to decrease to ½ mile, 800 meters, from 1200Z to
1600Z, but the observed visibility at 1245Z is 4000 meters and has improved from 3200 meters an hour earlier. This
trend of improving visibility is now expected to continue. Since a visibility of 800 meters was forecast, but the visibility
has exceeded the visibility threshold of 1600 meters, the TAF requires an amendment. Since several airfields are
closed when visibilities decrease to ranges as low as 800 meters, it is very important to inform aviators that conditions
have not and will not decrease as much as forecast, so an amendment must be issued.
2‐14
The TAF shall also be amended if ceilings not predicted in the most recent forecast are observed, or are later
forecasted to increase to equal or exceed, or decrease to less than:
3000 feet
1 000 feet
200 feet, or field minimums.
Say, for example, the existing TAF indicates the airfield will experience a low broken (BKN) cloud deck at 5,000 feet all
day, but at 1800Z the ceiling lowers to 2500 feet in a low deck of stratocumulus following the passage of a cold front,
and upstream stations indicate the deck will remain there for several hours. A flight plan with that airfield as a
destination does not require an alternate airfield with a 5,000 foot ceiling, but does require an alternate with a ceiling
below 3,000 feet, so, an amendment to the TAF is required since the ceilings lowered below the threshold of 3,000
feet.
In another TAF example, say the forecasted ceiling is 1,500 feet, but the ceiling increases to 3,000 feet mid‐way
through the forecast period. Satellite and upstream airfields indicate the ceiling layer is beginning to dissipate and in
addition to increasing in height, the cloud deck is now expected to become scattered within the next few hours. A
TAF amendment must be issued because the height of the lowest broken deck, the ceiling, increased to the
threshold value of 3,000 feet and will become scattered (SCT) soon resulting in no ceiling layer.
Other criteria that require an amendment to the TAF be issued include, thunderstorm and/or tornadic activity,
precipitation, significant differences between forecasted surface winds and observed surface winds, non‐convective
LLWS, and the minimum altimeter setting. With thunderstorms and tornadic activity: If thunderstorms or tornadoes
were forecasted but are no longer expected, or if thunderstorms or tornadoes were not forecasted but are now
expected, the TAF must be amended to note the change. The precipitation criterion is much the same as with
thunderstorms. If precipitation was forecasted but later it is determined that precipitation will not develop, or if
precipitation was not forecasted but later develops, the TAF must be amended to reflect the change. Non‐convective
LLWS amendment criteria is the same as for thunderstorms, tornadoes, and precipitation in that if LLWS was
forecasted but later it is determined it will not develop, or if it was not forecasted but later develops, the TAF must be
amended. For the minimum altimeter, QNH, any time the minimum altimeter decreases to less than forecast, the
TAF must be amended.
Surface wind amendment criteria are different than other criteria. The specific amendment criteria for surface winds
are as follows:
If the sustained wind speed changes more than 10 knots from the forecast
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If the wind direction changes more than 30 degrees or more from the forecast when the mean wind
speed or gusts are in excess of 15 knots
If the wind direction or speed changes from the forecast cause a change in the active runway
2.2.1.14 Sample TAF
Take a few minutes to review the below TAF example in order to practice decoding the weather conditions at
particular times throughout the TAF forecast period.
KNGU TAF 1421/1521 08008KT 9999 SCT010 SCT120 QNH3008INS
BECMG 1503/1505 08005KT 6000 BR BKN010 BKN120
FM150630 00000KT 0800 FG VV008 QNH3004INS
TEMPO 1507/1512 0500 DZ FG VV005
BECOMG 1512/1514 24010KT 9999 NSW SCT010 BKN080 QNH2998INS
FM151800 25014G28KT 9999 VCSHRA BKN020 OVC080 WS00512025 QNH2992INS
In the above example TAF for Navy Norfolk, the forecast is valid from 2100Z on the 14th until 2100Z on the 15th of the
month. Beginning at 142100Z, winds will be from 080 degrees at 8 knots with unrestricted visibility. Sky condition is
1,000 scattered, 12,000 scattered and the minimum altimeter expected is 30.08 inches.
At 150300 Z, weather conditions will begin to change at a regular or irregular rate until 150500Z when the
predominant weather conditions become; winds 080 degrees at 5 knots, visibility 4 miles in mist. Sky condition will
deteriorate to 1,000 broken, 12,000 broken and the minimum altimeter expected remains 30.08 inches. Since the
altimeter was not included in the BECMG line, we revert back to the previous predominant group.
Weather conditions will again change, beginning at 150630Z, and will have completely changed by 150730Z to a calm
wind with ½ mile visibility in fog. A surface based total obscuration will exist with a vertical visibility of 800 feet. The
lowest altimeter expected from 150730Z is 30.04 inches. During the period from 150700Z until 151200Z, while the
predominant conditions in the FM150630Z remain, the visibility is expected to temporarily decrease to 500 meters in
drizzle and fog and the vertical visibility in the total obscuration will temporarily decrease to 500 feet, with each
occurrence lasting less than one hour and the total time of the lower visibility and ceiling not exceeding 2 and one half
hours, or not more than half of the 5 hours between 0700Z and 1200Z.
At 151200Z, the weather conditions will begin to change, at a regular or irregular rate, and the change will be
completed by 151400Z to become winds of 240 degrees at 10 knots with unrestricted visibility. The previous
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condition of drizzle and fog is expected to be completely diminished, as indicated by NSW, with a scattered cloud
layer at 1,000 and a ceiling, or broken layer at 8,000 feet. The lowest altimeter is now expected to be 29.98 inches.
The last line of the TAF indicates that at 151800Z, weather conditions will again begin to change, and be completely
changed by 151900Z to winds from 250 degrees at 14 knots, with gusts to 28 knots. Visibility will become
unrestricted with rain showers in the vicinity of the airfield and a ceiling will form at 2,000 with an overcast layer at
8,000. Non‐convective low level wind shear is expected with winds at the height of the 500 shear level of 120
degrees at 25 knots. The lowest altimeter expected between 151900Z and 152100Z is 29.92 inches.
2.2.2 VARIATIONS IN THE TAF CODE
2.2.2.1 National Weather Service (NWS) Responsibilities
The National Weather Service (NWS) is responsible for issuing TAFs for all civilian airfields throughout the United
States. As Navy and Marine Corps assistant forecasters, there is frequently a need to refer to NWS TAFs, so it
becomes vitally important to understand the TAF code variations used by the NWS.
The NWS does not forecast for any of the following weather elements:
Variable winds
Partial obscurations
Altimeter Group
Icing Group
Turbulence Group
Temperature Group
The NWS also encodes a few of the weather elements differently than does the military. While military TAFs use
meters to forecast visibility, the NWS forecasts visibility in statute miles and for a forecasted visibility greater than 6
statute miles is encoded as P6SM.
For cloud layers, the NWS only includes up to three cloud layers, in ascending order, no matter how many layers are
actually expected to exist. The military forecasts all cloud layers, no matter the number, up until the first overcast
(8/8 oktas) layer. In addition to a maximum three cloud layers, if a ceiling exists below 15,000 feet, the NWS does not
normally include any layers above 15,000.
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The NWS also uses a group called PROB. PROB is used to forecast any thunderstorms or other precipitation that has
a low probability of occurrence, 30 to 40 percent, and will include all other elements associated with these conditions,
such as, clouds, visibility, etc., as necessary. When PROB is used, it is encoded as PROB, followed by the probability of
occurrence, then the beginning and ending times. PROB301215 indicates there is a 30% chance of some weather
event occurring between 1200 and 1500Z.
2.2.2.2 International Differences From the Military TAF Code
Internationally there are also minor differences from the military TAF code. The World Meteorological
Organization (WMO) states that if a forecast is devoid of visibilities less than 10 kilometers, cloud layers below 5,000
feet, cumulonimbus clouds, and significant weather; CAVOK, ceiling and visibility okay, may be encoded into the
forecast.
Other differences include:
NSC may be encoded if no clouds layers are forecast below 5,000 feet
The international BECMG change duration can cover a period of up to four hours (remember the
military change period is up to two hours)
Cloud heights are reported to the nearest 100 feet at all levels
The PROB change group must be immediately followed by a TEMPO, such as PROB30 TEMPO1215.
Remember the NWS did not use PROB and TEMPO together
2.2.3 TAF ADMINISTRATION AND OTHER CONSIDERATIONS
2.2.3.1 Record Keeping
Activities responsible for the production of terminal forecasts shall have a system in place to locally
maintain TAF’s for 31 days. Any forecast that involves an aircraft incident, along with any data
supporting the creation of the TAF, shall be maintained until no longer required by higher authority.
2.2.3.2 Standard Operating Procedures
Aviation activities with the responsibility of producing TAFs shall monitor these products and their preparation using
their best professional judgment to optimize the forecast timeliness and representation of weather events.
Forecasters shall perform logic and continuity checks prior to the release of any individual forecast and will ensure a
comparison of the TAF to other area forecasts and warnings is conducted to ensure continuity across a region.
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Aviation activities that support remote stations shall develop a set of Standard Operating Procedures (SOP) to
incorporate the following concepts that are the responsibility of the assistant forecaster and forecaster:
Maintain a continuous weather watch over the region to ensure the TAF remains accurate throughout
the forecast period, since a remote aviation activity is not on station to directly observe weather
conditions.
Be familiar with the local topography, geography, and all associated local effects for each remote site.
Continually monitor the station’s surface weather observations issued by the Automated Surface
Observing System (ASOS).
Evaluate Doppler radar data for all supported stations.
Make every effort to determine the presence of thunderstorm activity or freezing precipitation.
2.2.4 THE 30 HOUR TAF
The International Civil Aviation Organization (ICAO) Annex 3, "Meteorological Service for International
Air Navigation", was updated at the request of long‐haul carriers to include a provision for a 30 hour
TAF. In order to accommodate this change, the TAF format was updated to include the date with every
time reference.
With the new format, the National Weather Service began to issue 30 hour TAF's for 32 airports across
the country. Most military and NWS activities continue issuing TAFs for a 24 hour period, but all have
switched to the new 30 hour TAF format in order to maintain consistency. The complete list of NWS TAF
sites changing to the 30 hour duration is located at http://www.prh.noaa.gov/hnl/pages/taf30.php.
2.2.4.1 Format Changes
Time reference in the TAF will now include the date. For example:
Table 2‐7 TAF Format Change. Where: dd: two digit day; hh: two digit hour
Old Format New Format
BECMG hhhh BECMG ddhh/ddhh
TEMPO hhhh TEMPO ddhh/ddhh
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2.2.4.2 Examples
The following are examples of the new TAF format. Differences from the old format are highlighted.
Example of a 30 hour TAF with TEMPO groups:
KEWR 241740Z 2418/2524 29007KT P6SM SCT040 SCT200 TEMPO 2419/2422 BKN045
FM250100 34005KT P6SM SCT045 FM251400 02006KT P6SM FEW040 FM251800 20008KT P6SM FEW040
2.3 PILOT REPORTS (PIREP)
Learning Objectives
Describe the format, elements, and abbreviations used in a PIREP.
Identify when a PIREP should be submitted by pilots.
Identify when PIREPS should be forwarded to data collection centers.
Identify the primary reference publication concerning pilot weather reports (PIREP).
Identify the means by which PIREP data is collected, encoded and disseminated.
Pilot‐reported weather conditions are used throughout the world to supplement weather conditions
observed by remote sensing and from the ground. There are several types of reports routinely used that
must be understood by Navy and Marine Corps meteorology and oceanography professionals, so they can
incorporate this valuable data into forecast products.
Many countries throughout the world use national code forms to transmit pilot‐reported weather
conditions, but most of these code forms are not readily disseminated outside the originating country.
Within the United States, its territories, and in some countries where U.S. military forces are stationed, a
national code form, the PIREP code, is used to encode and transmit significant weather observed
by pilots. The Federal Meteorological Handbook No. 12 (FMH‐12), and Naval
Meteorology and Oceanography Command Instruction 3142.1 outline procedures that
govern the proper encoding and dissemination of pilot reports in a standard format to facilitate
processing, transmission, storage, and retrieval of in‐flight weather reports.
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2.3.1 PILOT‐REPORTED CRITERIA
In the United States, pilots are encouraged to provide in‐flight weather data whenever they encounter
significant weather of any type during take‐off, ascent to flight level, at flight level, during descent, or on
landing that are of meteorological significance to other aircraft or to surface activities. Significant
weather is defined as any weather that may affect the flight performance of an aircraft, or is capable of
causing injury or damage to personnel or property on the ground. Phenomena such as low‐level wind
shear (LLWS), thunderstorms and associated thunderstorm phenomena, icing, and turbulence are all
considered significant.
Pilots are also encouraged to make negative reports for conditions that are forecast but not observed in
flight. For instance, if clear‐air turbulence (CAT) or thunderstorms are briefed as occurring in the area and
no evidence of the phenomena is observed by the pilot, the pilot should report these conditions as "not
occurring."
In particular situations, a pilot may be asked to provide information that is not observable from the ground.
This may include information on the height of cloud bases and/or tops, the presence or absence of clear
levels within what appears to be a solid cloud deck, or the presence or absence of enroute weather over
data sparse areas. Pilots are also encouraged to report actual measurements of flight level winds and
temperatures.
In‐flight reports are extremely useful in the knowledge‐centric forecasting environment because they can
reveal significant weather features that may not be evident through the routine use of surface weather
observations, satellite imagery, or radar data. Clear air turbulence, icing layers, and LLWS are but a few
hazardous phenomena that may not be evident by other means.
To provide a means to properly asses the report, pilots are asked to provide specific information with all
reports. The minimum data elements required with any PIREP are; the location of the aircraft with
respect to a navigational aid, the flight level of the aircraft, the type of aircraft, and at least one
meteorological element observed, with the time of occurrence. The best weather information is
worthless if it cannot be referenced geographically and by altitude.
2.3.2 RECORDING AND ENCODING INFORMATION
All meteorology and oceanography professionals must be thoroughly familiar with both the FMH‐12 and
NMOC instruction 3142.1. In addition, forecasters should monitor all received PIREPs, paying particular
attention to PIREPs that contain hazardous flight conditions. Upon receipt of a PIREP that contains
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hazardous meteorological elements, the forecaster must verify to ensure the applicable regional
forecast product or products previously issued cover the flight hazards.
Data from in‐flight PIREPs are normally received via direct pilot to ground communications using the
Pilot to METRO Service (PMSV). However, there are very few Naval Air Stations manned with AG’s and
only minimal staffing at most Marine Corps Air Stations so PMSV is not utilized as often as in the past.
Air Traffic controllers are always in communication with aircraft in their vicinity and can request specific
information from pilots if asked to do so.
There are a wide variety of conditions that a pilot may report. All reported information is entered as it is
received on the PIREP report form, Figure 2‐2.
The upper portion of the form is used to record the reported information as it is reported. The lower
portion of the form is used to encode the PIREP for transmission. Abbreviated plain language is used in
the encoded portion of the message to enter each reported element. The abbreviations permitted for use
are found in FAA Order 7340.1. The most frequently used abbreviations are contained in Table 2‐8.
Figure 2‐2.—NMOC 3140/10, the PIREP report form.
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Locations are referenced only with respect to electronic navigation aid stations using VOR (very‐
high‐frequency omni‐directional range), TACAN (tactical air navigation), or VORTAC (a combined facility).
These locations are identified using the three‐letter national identifier, as listed in the Location
Identifiers Publication. The DOD Flight Information Publication (Enroute) and IFR Supplement lists all VHF,
TACAN, and VORTAC facilities, along with the four‐letter International Civil Aviation Organization
(ICAO) identifier of the facility. The last three letters of the ICAO identifier are the national identifier. For
Table 2‐8—Frequently Used Abbreviations.
ABV above MOD
BKN broken (sky coverage)
moderate ( ic ing, turbulence, or precipitation)
BLO below MOV moving
CAT clear air turbulence MX mixed
CHOP chop (turbulence) N north
CLR clear (icing) NE northeast
CTC contact NEG negative (not present)
DURGC during climb NMRS numerous (area coverage)
DURGD during descent NW northwest
E east OCNL occasional (occurrence)
EXTRM extreme OVC overcast (sky coverage)
FEW few (area/sky coverage) RIME rime icing
FRQ frequent RY runway
FV flight level visibility S south
GND ground SCT scattered (sky coverage)
HVY heavy (precipitation) SE southeast
ISOL isolated (area coverage) SEV severe (icing or turbulence)
LGT light (turbulence, icing, or precipita‐ SFC surface
tion) SKC sky clear
LLWS low‐level wind shear SW southwest
LN line (area coverage) TRACE trace (icing)
LTGCA cloud to air lightning TS thunderstorm
LTGCC cloud to cloud lightning UNKN unknown
LTGCG cloud to ground lightning W west
LTGIC in cloud lightning — to or through (layer)
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example: NAS Norfolk (Chambers Field) has a national identifier NGU while the ICAO identifier is KNGU.
The K is the Country code for the continental United States.
The Text Element Indicators (TEIs), from the bottom of the PIREP form, Figure 2‐3, are denoted
by a forward slash followed by a two‐letter abbreviation. TEI’s are used in the code to indicate which
element is being reported and are used as appropriate for each pilot report, but are omitted if that
element is not included in the report. The type of information that can be entered for each TEI is
indicated on the PIREP code form in the line below the space provided that TEI. For example, for the TEI
/SK, the authorized entries are Sky Conditions in the format Amount/Base/Tops. An arrow after the TEI
means a space must follow the TEI before the abbreviated information. Table 2‐9 provides an
explanation for each TEI and a few sample entries.
The PIREP code is flexible concerning entries for each element. As long as standard abbreviations
are used, nearly all significant information may be reported. Reports of elements that are difficult to
encode after a TEI, such as low‐level wind shear, are entered after the last TEI ‐ "/RM" for remarks. The
reported occurrence of a tornado, funnel cloud, or waterspout may be abbreviated in the weather TEI
"/WX", however, when any of these three elements occur, they must be spelled out in the "/RM"
remarks TEI, along with any supplemental information such as the approximate location, direction, and
speed of movement.
Figure 2‐3: Bottom Portion of PIREP Form
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Table 2‐9. PIREP Coded Text Element Indicators and Examples of Entries.TEI MEANING EXAMPLE DECODED MESSAGE
/OV OVer location /OV KNGU /OV KNGU 120035
directly over KNGU 35 miles Southeast of KNGU
/TM TiMe (UTC) /TM 1135 phenomena occurred at 1135Z /FL Flight Level /FL120 aircraft flying at 12,000 feet (MSL) /TP aircraft TyPe /TP F16 /TP C5 reported by an F‐16 reported by a C‐5
/SK SKy cover /SK SCT030‐060 /SK OVC065‐UNKN
scattered cloud layer bases 3,000 ft, tops 6,000 ft (MSL) in overcast layer, bases 6,500 ft (MSL), tops unknown
WX Weather /WX FV02SM TSRA GR /WX FV99SM
FL visibility 02 statute miles, in thunderstorm with rain and hail FL visibility unrestricted
/TA Temperature (outside Air)
/TA 01 /TA M10 outside air temperature 1°C outside air temperature ‐10°C
WV Wind dir/spd /WV 09060KT wind from 090°(true) at 60 knots /TB TurBulence /TB NEG BLO 080 /TB MOD
120‐180 /TB MOD‐SEV CAT forecast turbulence not present below 8,000 ft turbulence moderate 12,000 to 18,000 ft clear air turbulence moderate to severe (at flight level)
/IC ICing /IC MOD RIME 035‐075 moderate rime icing 3,500 to 7,500 ft
/RM ReMark /RM WATERSPOUT MOV ENE waterspout sighted, moving east‐northeast
Weather elements reported after the "/WX" TEI should conform to the METAR Surface Meteorological
Observation code standard abbreviations as displayed in Table 2‐10. No more than three weather groups
should be reported in a single PIREP. Consult the Federal Meteorological Handbook No. 12 for the specific
use options for /WX PIREP entries.
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2.3.3 TRANSMITTING PIREPS
All pilot reports received shall be disseminated both locally and longline. The only exceptions to the 100%
dissemination rule are PIREPs that:
Contain essentially the same information for the same area as another PIREP just received,
only the most recent is sent out.
Report sky conditions which have been incorporated into a METAR or SPECI observation.
Normally, all PIREPs are prefixed with the message header UA. When sent out in a collective, several
PIREPs sent out in a group, the UA header is included only as a group header, not on the individual
reports.
Any PIREP reporting hazardous phenomena is considered an urgent PIREP and must be prefixed with the
header UUA. Hazardous phenomena are defined as reported tornadoes, funnel clouds, waterspouts, hail,
severe icing, severe or extreme turbulence (including CAT), low‐level wind shear, volcanic eruptions or
any condition that in the judgment of the person entering the PIREP would present a hazard to flight.
Table 2‐10. Weather and Obstructions to Vision Identifiers.
QUALIFIER WEATHER PHENOMINON
INTENSITY OR PROXIMITY
1
DESCRIPTOR 2
PRECIPITATION 3
OBSCURATION 4
OTHER 5
‐ Light MI Shallow DZ Drizzle BR Mist PO Well‐Developed Dust/Sand Whirls
Moderate2 PR Partial RA Rain FG Fog
+ Heavy BC Patches SN Snow FU Smoke SQ Squalls
VC in the vicinity
DR Low Drifting SG Snow Grains VA Volcanic Ash FC Funnel Clouds, Tornado, Waterspout
BL Blowing IC Ice Crystals DU Widespread Dust
SH Showers PE Ice Pellets SA Sand SS Sandstorm
TS Thunderstorms GR Hail HZ Haze DS Duststorm
FZ Freezing GS Small Hail and/or Snow Pellets
PY Spray
1. The /WX groups shall be constructed by considering columns 1 to 5 in the above table in sequence, i.e.: intensity, descriptor then weather phenomenon. 2. No symbol is required to denote moderate intensity.
2‐26
PIREPs are disseminated and queried via web based applications. A typical PIREP would be entered
for transmission as follows:
UA/OV KNGU090100/TM 2213/FL250/TP C5/SK BKN160‐180 /WV 23057KT/TB LGT‐MOD CAT 250‐
270/IC LGT RIME 160‐180
An urgent PIREP may be entered with only limited data as follows:
UUA/OV PHNL270150/TM 0933/FL290/TP C9B/TB SEV CAT 310
2.3.4 RECORDS
A PIREP log, such as a two or three‐ring binder, should be maintained to keep all completed PIREP code
forms. When transmitted, a printed copy of the transmitted message is normally attached to the PIREP
code form. These records should be reviewed frequently to ensure proper coding. Completed
PIREP forms may be retained on board for as long as they may be of use, usually one year, and then
destroyed.
2.4 DD Form 175‐1
Learning Objectives.
Explain the pertinent data available from a DD 175 Military Flight Plan.
Describe the various methods that may be used to describe the intended route of flight on the DD 175
Military Flight Plan.
Describe the criteria that require completion of a DD 175‐1 Flight Weather Briefing.
Describe the five Parts of the DD 175‐1 Flight Weather Briefing.
Explain the sources of weather information available to complete the specific blocks in
Parts II and III of the DD 175‐1 Flight Weather Briefing.
Describe the types and intensities of turbulence and icing.
Explain the significance of the phrase, “Storm development has not progressed as
forecast”.
Describe the conditions that require an alternate air field and the weather criteria that
must be met at that alternate air field.
Explain the process of determining the Briefing Void time.
In order to accurately complete a DD Form 175‐1, it is first necessary to understand the DD Form 175,
Military Flight Plan, as seen in Figure 2‐4.
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The DD form 175 is the official form the pilot uses to file a flight plan and contains pertinent information
such as, the aircraft type and call sign, the type of flight plan (IFR or VFR), the point of departure and
estimated time of departure (ETD), the flight level, destination or destinations for a multi‐leg flight plan,
estimated time enroute, any alternate airfields desired and additional time enroute to the alternate
airfield (if an alternate airfield is required) and finally, but certainly not least, the route the pilot intends
to take in order to reach the destination.
A thorough understanding of the intended route of flight is crucial to providing the most accurate
weather data possible. There are several ways a pilot can enter the route data into the flight plan. The
simplest method is to enter “Direct” because, as the term would suggest, “Direct” indicates the pilot will
take the most direct path possible to the destination. Other means of entering the intended route of
flight are point to point referencing of ICAO’s (4‐letter station identifiers) or WMO station identifiers,
navigation aids, jet routes, or a combination of ICAO’s, navaids, and jet routes.
Figure: 2‐4. DD Form 175‐1, Military Flight Plan
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In Figure 2‐4, the first route of flight uses ICAO’s and is rather easy to discern. If this route were
described using WMO station identifiers it would appear as NPA – LIT – TIK.
The use of navigation aids is a little trickier to decode. The second route of flight displayed in Figure 2‐4
follows the exact line of flight as the first; it is just described using a combination of WMO station
identifiers and navigation aids. SJI, for example, is the Semmes VORTAC near Mobile, AL.
There are different types of aeronautical navigation aids that can be included in the route of flight on a
military flight plan, the most common of which are; VHF Omni‐Directional Range (VOR), VOR with DME,
TACAN, and VORTAC.
A VOR broadcasts a VHF radio signal that includes the stations identifier in morse code and allows the
aircraft equipment to derive a magnetic bearing from the VOR. If DME, Distance Measuring Equipment,
is added to a VOR, called a VOR with DME system, aircraft can receive both bearing and range
information. A TACAN is an aeronautical navigation system used by military aircraft which provides
range and bearing information, and a VORTAC is a combination of a VOR and TACAN system. These
navigation aids are pictured in Figures 2‐5, 2‐6, 2‐7 and 2‐8. To geographically locate the navigation aids
listed on a flight plan, you may refer to an Enroute Low Altitude Chart which displays the geographical
location of all navigation aids.
Figure 2‐5. VOR (Source: NOAA)
Figure 2‐6. VOR with DME (Source: NOAA)
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Jet routes may also be used to define the intended
flight path. Jet routes are depicted on Enroute High Altitude Charts, 18,000 feet to 45,000 feet, and are
identified by the prefix “J” as in J12. An example of a route of flight that includes jet routes may appear
as; CHS J121 SWL J174 HTO. This route departs Charleston, SC along jet route 121 to the Snow Hill, MD
VORTAC, then follows jet route 174 into the destination of East Hampton, NY.
If you do not completely understand the route identified on the Military Flight Plan; ask the pilot or
navigator of the flight. A wrong guess of the intended route of flight could expose the aircraft and crew
to unexpected flight hazards and increased risk.
Once you received a military flight plan from the pilot, navigator and other member of the flight crew,
you have enough information to begin completing the DD Form 175‐1, Flight Weather Briefing. The DD
175‐1 provides a common format for all military (and DoD) aircrews to receive a weather briefing
from an Aviation Weather Regional Forecasting Hub regardless of their location. OPNAVINST 3710.7T,
Naval Air Training and Operating Procedures Standardization (NATOPS), prescribes general flight and
operating instructions and procedures applicable to the operation of all naval aircraft and related
activities. NATOPS section 4.6.3.1 directs that, “Pilots are responsible for being thoroughly familiar with
weather conditions for the area in which flight is contemplated. Where NMOC or USMC weather
services are available, a flight weather briefing shall be obtained from a qualified meteorological
forecaster.” Notice that NATOPS specifically uses the term “shall” which indicates the practice is
mandatory.
Figure 2‐7. TACAN (Source: NOAA)
Figure 2‐8. VORTAC (Source: NOAA)
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NATOPS continues with, “Navy and Marine Corps Forecasters are required to provide flight weather
briefings using either the DD‐175‐1 Flight Weather Briefing, or a VFR Certification Stamp. A DD‐175‐1
Flight Weather Briefing form shall be completed whenever an IFR flight plan or a combination IFR/VFR
flight plan is filed.” Pilots, who file a VFR flight plan present a DD‐175 Flight Plan to the forecaster and
specifically request a VFR Stamp, may receive a VFR Certification stamp in lieu of a completed DD 175‐1
so long as VFR criteria can be maintained throughout the entire route of flight. Figure 2‐9 represents
the NATOPS authorized VFR Stamp.
It is the ultimate responsibility of the forecaster to ensure a complete and accurate weather briefing is
accomplished, but it is ultimately the forecaster’s final decision whether a VFR Stamp will suffice in lieu
of a completed DD 175‐1, flight weather brief. If there is any question along any part of the intended
route of flight, it is advisable to complete the DD 175‐1.
Flight Weather Briefer (FWB) is a web‐enabled software application capable of providing either of the
above briefings, DD 175‐1 or VFR Certification Stamp. Aviators are encouraged to file their flight plan
using FWB, and forecasters shall use FWB to conduct all DD 175‐1 weather briefings.
Figure 2‐10 shows that the DD 175‐1 Flight Weather Briefing Form is divided into five sections: Part I ‐
Takeoff Data, Part II ‐ Enroute & Mission Data, Part III ‐ Aerodrome Forecasts, Part IV ‐
Comments/Remarks, and Part V ‐ Briefing Record.
“BRIEFING VOID _____Z, FLIGHT AS PLANNED CAN BE CONDUCTED UNDER
VISUAL FLIGHT RULES. VERBAL BRIEFING GIVEN AND HAZARDS EXPLAINED.
FOLLOWING SIGMETS ARE KNOWN TO BE CURRENTLY IN EFFECT ALONG
PLANNED ROUTE OF FLIGHT.”
Figure 2‐9. VFR Stamp.
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Figure 2‐10. DD 175‐1 Flight Weather Briefing Form
2‐32
Parts I, II, and III provide very specific information regarding hazards or other significant weather events
the pilot and crew are expected to encounter along the intended route of flight and are designed to
provide specific information corresponding to the three phases of any flight which are take‐off,
enroute, and landing as shown in Figure 2‐10.
Specific guidance for completing the DD 175‐1 is found in NAVMETOCCOMINST 3140.14, Flight Weather
Briefing Manual.
2.4.1 PART I: TAKEOFF DATA
Part I, Figure 2‐11, identifies the flight for which the flight weather briefing is being prepared and
provides forecasted surface conditions for the estimated time of departure, ETD. Enter any existing or
forecasted weather watches, warning, or advisories valid at the ETD in block 13. This significant weather
entry consists of the name of the warning and time that the warning is valid (e.g., TSTM COND II until
13Z). Notice the required format for all of Part I data as displayed in Figure 2‐11.
2.4.2 PART II: ENROUTE & MISSION DATA
The Enroute & Mission Data section of the DD 175‐1 provides space for specific information about the
expected weather conditions within 25 nautical miles either side of the intended route of flight, and
from the surface to 5,000 feet above the flight level, in addition to the destination weather at the
estimated time of arrival (ETA).
(Note: All thunderstorm activity along the route of flight will be briefed, no matter what the flight level.)
To aid in the interpretation of similar weather conditions occurring in various geographical locations
along a route of flight, for example two lines of rain showers or thunderstorms in different states,
briefers may elect to use different indicators to correlate weather entries to specific geographical areas
while entering data into blocks 22 through 25 of the briefing form. Notice in Figure 2‐12, block 25,
25 AUG 09 T-45C/VV3E276 KNPA/1230 52F/11C 50F/10C -4C +50 +100
05009 05010/150; 32020/300; 21249/380 NONE DRY
N/A
25 AUG 09 T-45C/VV3E276 KNPA/1230 52F/11C 50F/10C -4C +50 +100
05009 05010/150; 32020/300; 21249/380 NONE DRY
N/A
Figure 2‐11. Part I ‐ Takeoff Data
2‐33
Precipitation, the briefer has elected to use a star shape to specify moderate drizzle in western Arkansas,
and large dots to indicate heavy rain showers from eastern Oklahoma into Tinker, AFB.
Also, an up arrow (↑) may be used to indicate conditions during the climb, and a down arrow (↓) may
be used to indicate conditions during the descent.
From block 23, Turbulence, of Figure 2‐12, turbulence from 18,000 to 12,000, and from 2,000 to the
surface can be expected while at flight level and on descent throughout eastern Oklahoma into Tinker
AFB.
Block 14, flight level/winds/temp, allows for the entry of the flight level, flight level winds, and the
temperature at flight level along the intended route of flight. Enter the flight level in hundreds of feet
Mean Sea Level (MSL) in three digits. In the above example, the flight level is 38,000 feet, entered as
380. If there are significant wind speed and direction changes along the route of flight, break the
forecast into legs. Enter true wind direction at flight level in tens of degrees and wind speed to the
nearest 5 knots. From block 14 above, flight level winds are forecast from 240 degrees at 50 knots with
a temperature of ‐45C from Pensacola to the SEMMES VORTAC, from 110 degrees, 35 knots with a
temperature of ‐44C from SEMMES to Texarkana, and from 040 degrees, 50 knots with a temperature of
‐42 from Texarkana into Tinker, AFB.
A single flight level wind, direction and temperature may be used if there is little change along the entire
route of flight.
Figure 2‐12. Part II – Enroute & Mission Data
2‐34
Blocks 15, Space Weather, and 16, Solar/Lunar, are optional for Navy and Marine Corps aviators.
Unless specifically requested, the forecaster may enter N/A in these two blocks.
Block 17 notifies the pilot of clouds at the intended flight level. Yes is checked if the flight will be in
cloud at least 45 percent of the time, No implies less than 1 percent of the time, and In and Out implies
between 1 and 45 percent of the time. If in cloud flight is expected in more than one location along the
route, break the locations into legs as previously explained. Some pilots may choose to change their
intended flight level if a majority of their flight will be in cloud. Information on the height of cloud bases
and tops may be determined with PIREPs, upper level charts, satellite data, and/or Skew‐T Log P
Diagrams.
Block 18 is used to brief the pilot on any obscurations to visibility at flight level. If “Yes” is checked,
enter the type of obscuration that is expected to restrict in‐flight visibility at flight level. Indicate the
intensity and location along the route, along with the type of obscuring phenomena, i.e., fog, haze,
smoke, etc.)
Block 19 is the Minimum Ceiling entry and indicates the lowest ceiling along the route of flight in
hundreds of feet Above Ground Level (AGL), and the geographical location. Block 19 of Figure 2‐12
indicates the lowest ceiling anywhere along the route is in Oklahoma at 400 feet AGL. If the minimum
ceiling is over hilly or mountainous terrain, or in thunderstorms, make an appropriate entry such as, 010
BOSTON MTNS, for 1,000 over the Boston Mountains. If an AIRMET is issued for the phenomena, enter
the name and number of the AIRMET and be sure to brief that data as well.
Block 20 identifies the geographical location of Maximum Cloud Tops not associated with
thunderstorms. Thunderstorm maximum tops are identified in block 22. Identify cloud layers
appropriate to the flight level, i.e. with bases below the flight level that extend into and through the
flight level. For example, if the flight level is 16,000 feet, the forecaster will identify middle etage
cloudiness that may extend vertically into or near the flight path, and would disregard cirrus that forms
above the 16,000 flight level. In block 20 of Figure 2‐12, since the flight level is 38,000, the high etage,
cirrus cloud deck is identified with maximum cloud tops to 22,000 feet MSL. A Radar summary,
enhanced infrared satellite image, Skew‐T data, and/or PIREPs may be used to determine the maximum
cloud tops.
2‐35
In block 21, identify the minimum height of the freezing level and the geographical location along the
entire route of flight. If the lowest freezing level is at the surface, enter SFC and geographical location.
Block 22 is used to brief any and all thunderstorms along the route of flight, even if the flight level is above
the expected maximum cloud tops of the strongest thunderstorms. Thunderstorms are one
meteorological phenomenon most obvious when building an overview of the weather. To recognize
existing areas of thunderstorm activity, the Radar Summary, local and national WSR‐88D NEXRAD
composites, lightning data, and satellite imagery provide invaluable information.
To identify areas favorable for the development of thunderstorms, severe thunderstorms and tornadoes,
utilize Naval Aviation Forecast Center (NAFC), Fleet Numerical Meteorology and Oceanography Center
(FNMOC), Air Force Weather Agency (AFWA), and NOAA Storm Prediction Center (SPC) forecast
products. SPC publishes several very useful storm prediction products and forecasts up to 8 days in
advance that are available at: http://www.spc.noaa.gov.
The National Weather Service Storm Prediction Center issues unscheduled Weather Watch (WW) bulletins
as graphical advisories for the Continental United States whenever a high probability exists for severe
weather. The Air Force also issues scheduled Military Weather Advisories (MWA) in graphical form for the
same geographical areas. Both provide estimates of the potential for convective activity for a specific time
period, will be provided to pilots or certified crewmembers upon request, and are included with all
briefings.
Except for operational necessity, emergencies, and flights involving all‐weather research projects or
weather reconnaissance, pilots shall not file into or through areas for with the Storm Prediction Center has
issued a WW unless one of the following exceptions apply:
Storm development has not progressed as forecast for the planned route. In such situations
the pilot may file a VFR flight plan through an existing WW if existing and forecast weather
for the planned route permits such flights, and may file an IFR flight plan if the aircraft
carries proper radar equipment or if the route of flight is in controlled air space and visual
meteorological conditions can be maintained.
The performance characteristics of the aircraft permit an enroute flight alititude above
existing or developing severe storms.
2‐36
Figure 2‐13 and 2‐14 provide graphical and alphanumeric examples of an SPC issued WW.
Convective SIGMETs are issued by NOAA’s
Aviation Weather Center and must also be
briefed when they exist along any portion of
the route of flight. Convective SIGMETs are
issued for existing severe thunderstorms
producing surface winds greater than or equal to 50 knots, hail at the surface greater than or equal to ¾
inches in diameter, or tornadoes. They are also issued for embedded thunderstorms (thunderstorms
that develop within multi‐layered stratiform cloud decks), lines of thunderstorms, and for
thunderstorms with radar returns of 51 dBZ or higher that affect 40% or more of an area at least 3000
square miles.
A Convective SIGMET implies severe or greater turbulence, severe icing, and low level wind shear. In
addition to the above criteria for the issuance of a convective SIGMET, they may also be issued for any
convective situation which the forecaster feels is hazardous to all categories of aircraft.
URGENT – IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 133 NWS STORM PREDICTION CENTER NORMAN OK 250 PM EDT FRI APR 10 2009‐09‐01 THE NWS STORM PREDICTION CENTER HAS ISSUED A TORNADO WATCH FOR PORTIONS OF CENTRAL INTO EASTERN KENTUCKY EASTERN TENNESSEE FOR WESTERN VIRGINIA EFFECTIVE THIS FRIDAY AFTERNOON AND EVENING FROM 250 PM UNTIL 1000 PM EDT.
TORNADOES. . .HAIL TO 2 INCHES IN DIAMETER. .
.THUNDERSTORM WIND GUSTS TO 70 MPH. .
.AND DANGEROUS LIGHTNING ARE POSSIBLE IN
THESE AREA.
THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 60 STATUTE MILES EAST AND WEST OF A LINE FROM 40 MILES NORTH OF JACKSON KENTUCKY TO 80 MILES SOUTH SOUTHEAST OF LONDON KENTUCY. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU3).
Figure 2‐13. SPC Weather Watch
Figure 2‐14. SPC Alphanumeric
2‐37
Figures 2‐15 and 2‐16 provide examples of graphical and alphanumeric Convective SIGMETs as provided
by NOAA’s Aviation Weather Center’s, Aviation Digital Data Service.
To enter information pertaining to a WW or convective SIGMET, enter WW followed by the number, or
CON SIGMET followed by the number as appropriate. For the Tornado Watch presented in Figures 2‐13
and 2‐14, “WW 133” would be entered into block 22. For the Convective SIGMET in Figures 2‐15 and 2‐
16, CON SIGMET 68E would be entered into block 22.
Check the appropriate blocks from the Thunderstorm block 22 in order to accurately represent any
thunderstorm activity along the route of flight. Referring back to Figure 2‐12, the pilot can expect to
encounter a line of scattered to numerous thunderstorms with maximum tops of 41,000 feet from
Texarkana into Tinker AFB. CON SIGMET 19C is briefed with specific detail to the line of thunderstorms.
Block 23 is used to brief turbulence not associated with thunderstorms, since the existence of
thunderstorm activity implies the existence of severe turbulence.
Forecasting the existence and type of turbulence is a challenge. Factors that create turbulence in one
situation may not cause turbulence in an identical situation on a later date. To complicate matters, aircraft
that differ in size, speed and character react differently within the same air space.
There are four classifications of turbulence; light, moderate, severe and extreme.
In areas of light turbulence, an aircraft will experience slight, erratic changes in attitude or altitude. A
slight variation of airspeed from 5‐14 knots is encountered in light turbulence, and loose objects in the
aircraft usually remain at rest. Light turbulence can be found in mountain areas, even with light winds, in
WSUS31 KKCI 011755 SIGE CONVECTIVE SIGMET 68E VALID UNTIL 1955Z NC CSTL WTRS FROM 170E ECG‐200ESE ECG‐130SSE ILM‐60SSE ILM‐170E ECG DMSHG AREA TS MOV FROM 26010KT. TOPS TO FL410
Figure 2‐16. Convective SIGMET Alphanumeric
Figure 2‐15. Convective SIGMET Graphic
2‐38
and near cumulus clouds, near the tropopause, and at low altitudes when the surface winds exceed 25
knots.
In areas of moderate turbulence, the aircraft experiences moderate changes in attitude or altitude, but
remains in positive control at all times. There are usually small variations in airspeed of 15‐24 knots and a
change in vertical velocity of 20‐35 feet per second. Occupants feel a definite strain against seat restraints,
and unsecured objects in the aircraft become dislodged. Moderate turbulence can be found in mountain
waves as far as 300 miles downwind of a mountain ridge, when the wind is perpendicular and exceeds 50
knots, in towering cumuliform clouds and thunderstorms, within 100 NM of the jet stream on the cold air
side, and in low altitudes over rough terrain when the wind exceeds 25 knots.
In areas of severe turbulence, the aircraft experiences abrupt changes in attitude or altitude and may be
out of control for short periods. There are usually large variations in airspeed (> 25 knots) and
significant changes in vertical velocity of 36‐49 feet per second. Occupants are thrown violently against
seat restraints and unsecured objects in the aircraft are tossed about. Severe turbulence can be found
up to 150 downwind of a mountain ridge and within 5,000 feet of the tropopause when a mountain
wave exists and when the wind is perpendicular to the mountain range in excess of 50 knots, in and near
mature thunderstorms, and near the jet stream altitude and 50 to 100 miles on the cold air side of the
jet core.
In areas of extreme turbulence, the aircraft is violently tossed about, is practically impossible to control,
and may suffer structural damage. Rapid fluctuations in airspeed, similar to severe turbulence, and
vertical velocity fluctuations as great as > 50 knots may occur. Extreme turbulence is rarely encountered
but may be found in mountain waves in or near the rotor cloud and in sever thunderstorms, especially in
organized squall lines.
Different fixed wing aircraft react differently to turbulence depending on the weight, wing surface area,
wing sweep angle, airspeed, and aircraft flight attitude. Generally speaking, light weight, large wing
surface area, decreased wing sweep angle (wings more perpendicular to the fuselage), increased
airspeed and non‐level flight (ascending or descending) all increase an aircraft’s sensitivity to turbulence.
So, a light, large winged, fast aircraft ascending to flight level will experience more turbulence than a
heavy, small winged, slow aircraft flying at flight level through the same area.
The intensity of turbulence experienced by rotary‐wing aircraft is directly dependent to the speed of the
aircraft (faster more intense, slower less intense), inversely proportional to the weight of the aircraft
2‐39
(lighter more intense, heavier less intense), inversely proportional to the lift velocity (faster
ascend/descend rate equals more intense, slower ascend/descend rate equals less intense), and directly
proportional to the arc of the rotor blades (the longer the blade, the more intense the turbulence.)
To brief turbulence using the DD 175‐1, enter the AIRMET or SIGMET, if issued and as appropriate to the
route of flight, and the phonetic word assigned to the AIRMET or SIGMET. Figure 2‐17 displays a NOAA
AWC product which graphically depicts all
valid AIRMETs and SIGMETs throughout
the continental United States, while
Figure 2‐18 provides an example of the
text version of an AIRMET with the
phonetic word TANGO assigned. If Figure
2‐18 represented an AIRMET to be briefed
on a DD 175‐1, AIRMET TANGO would be
Figure 2‐17. Plotted AIRMET/SIGMETS (Source: NOAA)
WAUS43 KKCI 011445 CHIT WA 011445 AIRMET TANGO UPDT 2 FOR TURB VALID UNTIL 012100 AIRMET TURB. . .ND SD NE KS OK TX FROM 50SSE MOT TO 60S FAR TO 20S FSD TO 40NW END TO 60NNW SPS TO 20ENE CDS TO 20SSE LBB TO TXO TO 50W LBL TO GLD TO 20 WNW RAP TO 20W BIS TO 50SSE MOT MOD TURB BLW 080. CONDS CONTG BYD 21Z ENDG 00‐02Z
Figure 2‐18. AIRMET TANGO
2‐40
entered into block 23. Extensive information pertaining to active AIRMET/SIGMETs is available on the
NWS ADDS at http://adds.aviationweather.gov/airmets/.
Block 24 is used to brief icing not associated with thunderstorms, since the existence of thunderstorms
implies severe icing.
Icing interferes with the performance of aircraft by increasing drag and decreasing lift, while icing within
the engine reduces the effective power of the aircraft.
There are three types of icing that may be briefed using the DD 175‐1 including, rime, clear, and mixed
icing.
Rime icing, Figure 2‐19, forms when small, super cooled water droplets freeze upon contact with the
aircraft. The small drops strike the aircraft, instantaneously freeze, and form a rough, brittle, milky, and
opaque ice layer on the skin of the aircraft.
Clear icing, Figure 2‐20, forms when larger, super cooled water droplets strike the aircraft surface,
spread over the surface of the airframe before freezing, and then completely freeze. Clear ice, as the
name implies, forms as a clear or translucent layer of ice which adheres firmly to exposed surfaces and is
much more difficult than rime ice to remove with deicing equipment.
Mixed icing forms from a combination of rime and clear conditions. As the icing accumulates, snow or
ice particles become embedded into the clear ice forming a very rough layer of ice.
Four factors determine the rate of icing accumulation.
Figure 2‐19. Rime Icing
Figure 2‐20. Clear Icing
2‐41
The amount of available super‐cooled (temperature of the water droplets is below freezing)
liquid water present in the atmosphere.
The size of the super cooled water droplets. Larger super‐cooled water droplets increase
the rate of accumulation. Smaller droplets tend to deflect away from the wings with the air
stream and will not readily collect on the wing surface.
The rate of ice formation increases with higher airspeeds, however, at very high speeds,
speeds attained by jet aircraft, friction creates enough heat on the skin of the aircraft to
melt structural ice. Icing is seldom a problem at airspeeds in excess of 575 knots.
The size and shape of the aircraft is the fourth factor. Icing accumulates more rapidly on
large, non‐streamlined aircraft with rough surface features than it will on thin, smooth,
highly streamlined aircraft. However, icing that forms on even a thin, smooth, streamlined
aircraft will increase the accumulation rate by increasing the surface area upon which the
droplets can freeze.
There are four intensities of icing that can be briefed using the DD 175‐1.
Trace icing is the least worrisome and has little effect on aircraft performance, unless it is
encountered for more than one hour.
Light icing may create a problem if the aircraft is exposed to the icing conditions for over an
hour. Occasional use of deicing equipment is adequate to remove and prevent
accumulation.
Moderate icing accumulates at such a rate that even short encounters become potentially
hazardous and the use of deicing equipment or diverting the route of flight is necessary.
Severe icing presents a rate of accumulation such that deicing equipment fails to control or
reduce the icing hazard. Immediate diversion of the route of flight is necessary in conditions
of severe icing.
On the DD 175‐1, enter the name, number or date/time group of the icing forecast product used (e.g.,
AIRMET, SIGMET, etc.) and the type, intensity, levels (base and top of the icing layer), and geographical
locations. NOAA’s ADDS provides extensive information pertaining to icing conditions at
http://adds.aviationweather.gov/icing/, including a map with all valid AIRMETs and SIGMETs and a link
to retrieve recent PIREPs.
2‐42
Block 25 is used to brief precipitation not associated with thunderstorms. Enter the type, intensity,
character, and geographical location of precipitation occurring at the point of departure, flight level
along the route of flight, and the destination or destinations if a multi‐leg flight. From Figure 2‐21,
moderate drizzle is expected from western Arkansas into Tulsa, OK (briefed due to the proximity of
precipitation to the alternate airfield), and heavy rain shower from eastern Oklahoma into Tinker AFB.
2.4.3 PART III: AERODROME FORECASTS
The Aerodrome Forecasts section of the DD 175‐1 provides space for information about forecasted
weather conditions at the destination and alternate airfields, plus any planned intermediate stops. Enter
the worst conditions forecasted from the predominant TAF line for the destination, and any temporary
conditions (TEMPO) expected from one hour prior to one hour after the estimated time of arrival (ETA). In
Figure 2‐21, the ETA into Tinker, AFB is 1404Z so the forecasted conditions entered into the DD 175‐1,
from the KTIK TAF, are valid from 1304Z to 1504Z. If, for example, the flight is estimated to arrive at 1404
and the destination is expected to have OVC004 sky conditions until 1400, then improve to OVC020 by
1430, the cloud layers entered into the DD 175‐1 remain OVC004 due to the one hour prior to one hour
after ETA requirement.
NATOPS, OPNAVINST 3710.7, imposes very strict criteria when determining the requirement for an
alternate air field, as well as the weather conditions that must exist at the alternate air field. The criteria
to determine if an alternate is necessary are based on the lowest forecasted predominant or TEMPO
ceiling and/or visibility conditions valid one hour prior to one hour after the ETA.
Figure 2‐21. Part III – Aerodrome Forecasts
2‐43
Any time the destination forecast, valid one hour prior to one hour after ETA, contains a ceiling less than
3,000 feet, and/or a visibility less than 3 miles, an alternate air field is required. Weather conditions at
the alternate airfield in this situation must be, at a minimum, published minimums for the air field plus
300 feet and 1 mile visibility for non‐precision aircraft, or published minimums plus 200 feet and ½ mile
visibility for precision aircraft. (Published minimums are the ceiling/visibility criteria that will close an
airport). For example, the published minimums for KSPS are a 500 foot ceiling and 1 mile visibility.
Since the forecast for the destination, Tinker AFB, is a 400 ceiling and ½ mile visibility (below 3,000 and
3), an alternate air field is required. KSPS is requested as the alternate air field so we‐ather conditions
equal or exceed a 700 foot ceiling and 1 ½ miles visibility (500 + 200 = 700 foot ceiling and 1 + ½ mile
visibility for a precision aircraft). The forecast for KSPS, depicted on Figure 2‐21, is 800 and 4 so the air
field is a valid alternate.
If the destination weather conditions are forecasted to be below published field minimums during the
one hour prior to one hour after ETA, NATOPS requires the alternate airfield to have, at a minimum, a
3,000 foot ceiling and a forecasted visibility of at least 3 miles.
The runway temperature and pressure altitude are normally not required entries.
2.4.4 PART IV: COMMENTS/REMARKS
This section provides space for miscellaneous information concerning any portion of the flight (Figure
2‐22). Remarks include any significant details or data not covered elsewhere and deemed pertinent,
such as low‐ level wind shear or runway conditions. Provide amplifying remarks regarding any
WWs, SIGMETs, AIRMETs, or similarly issued warnings or advisories are required in Block 35. The
latest hourly surface observation for the destination may also be included here. If space is a problem,
an additional DD 175‐1 may be used as a continuation sheet.
Figure 2‐22 Part IV – Comments/Remarks
2‐44
2.4.5 PART V: BRIEFING RECORD SECTION
The actual time the briefing is completed is entered in Block 36 of Part V (Figure 2‐23). Block 37 is used
to document the flimsy briefing number which is used to identify the particular briefing for archive
purposes. The flimsy briefing number is comprised by the two‐digit month, followed by the sequential
briefing number for that month. From Figure 2‐23, the briefing was conducted at 1045Z and was the
231st briefing in the month of September. This briefing number is transferred to the DD 175 flight plan
before submitting it to Base Operations, indicating the aircrew has received a weather brief from a
qualified forecaster.
Be certain to complete blocks 38 and 39 with your initials and the name of the individual receiving the
brief.
Block 40 serves to document the void time of the flight weather briefing. Weather conditions can
change rapidly which renders every brief perishable. The NATOPS rule for assigning the void time is 30
minutes past the ETD, not to exceed two and one half hours from the weather briefed time. For
example, with an ETD of 1200Z and a weather briefed time of 1030Z, the void time of the briefing is
1230Z. With the same 1200Z ETD, if the weather briefed time was 0945Z, the void time would be 1215Z
since 30 minutes past the ETD of 1200Z exceeds the two and one half hours from the weather briefed
time limitation.
In some cases the pilot or crewmember must receive a brief that will void even prior to the ETD. In such
a case, or if the flight is delayed, or the briefing becomes void for any other reason, the pilot or
crewmember who received the briefing must receive an extension or an updated weather brief, as
determined by the forecaster, prior to takeoff. An extension or update may be accomplished by
telephone, PMSV, or any other means of communication necessary to speak with a certified forecaster.
Block 41 is completed for and extension and block 42 for weather re‐brief, as applicable. In either case,
the new void time is subject to the same limitations as the original void time: one‐half hour after the
Figure 2‐23. Part V – Briefing Record
2‐45
new ETD not to exceed 2 and ½ hours past the weather re‐brief time. The pilot or crewmember
receiving the extension or re‐brief must complete block 41 or 42 as appropriate.
2.4.6 RECORD KEEPING
All paper DD Form 175‐1’s and locally prepared substitute forms (whether conducted locally or by
remote means), shall be retained for a minimum of one month.
2.5 COMPUTATION OF AIRCRAFT PERFORMANCE INDICATORS FROM OBSERVED DATA
Learning Objectives
Identify three aircraft performance indicators computed from observed data.
Define the terms pressure altitude, density altitude, and specific humidity.
Describe the procedures used to compute pressure altitude and density altitude.
Identify the procedure used to determine specific humidity.
Air density and water vapor content of the air have a critical effect on aircraft engine performance and
takeoff characteristics. This section describes some of these effects and explains how to compute the
necessary values. The three most common elements that must be computes are pressure altitude,
density altitude, and specific humidity. All may be determined by using a Density Altitude Computer.
However, pressure and density altitudes can be obtained from ASOS. Pressure and density altitudes are
specified in feet; specific humidity in grams per gram or pounds per pound.
2.5.1 PRESSURE ALTITUDE
Pressure altitude is defined as the indicated altitude of a pressure altimeter at an altimeter setting of
29.92 inches of mercury or the U.S. Standard Atmosphere. The pressure altitude of a given pressure is
usually a fictitious altitude, since it is rarely equal to the true altitude and is equal to true altitude only
when the pressure at sea level (or the flight‐level pressure) corresponds to the pressure of the U.S.
Standard Atmosphere. A pressure altitude higher than the actual altitude indicates the air is less dense
than normal, and the aircraft may not be able to carry a full cargo load. A pressure altitude lower than
the actual altitude means the air is more dense than normal and the aircraft may be able to takeoff
successfully with a larger cargo load.
2‐46
Aircraft altimeters are constructed for the pressure‐height relationship that exists in the standard
atmosphere. Therefore, when the altimeter is set to standard sea‐level pressure (29.92 inches of
mercury), it indicates pressure altitude, not the true altitude. Aircraft with settings based on an
altimeter setting of 29.92 inches, rather than true altitudes, are flown above 18,000 feet in the United
States, and on transoceanic flights more than 100 miles offshore. The quickest method for
approximating the pressure altitude is by using the Pressure Reduction Computer as seen in Figure 2‐24.
Detailed calculation instructions are listed on the computer. If you find yourself in a situation where the
pressure reduction computer is unavailable, there are two alternate methods that enable personnel to
calculate approximations of the pressure altitude. Pressure altitude varies directly with the change in
pressure multiplied by a complex variable. The variable amount takes into account temperature and
station elevation. Both methods simplify the equation but still provide close pressure altitude
approximations.
The first method uses a set of pre‐calculated pressure altitudes based on pressure differences from
standard pressure. From Table 2‐11, identify the pressure altitude value which corresponds to your
current or forecasted altimeter setting, or the current or forecasted altimeter setting for another station
of interest. Take the value you identify from Table 2‐11 and add that value to your station elevation or
the elevation of the other station of interest to determine the pressure altitude. For example, if the
altimeter setting at the station of interest is 29.87 inches and the station elevation is 150 feet, locate
29.8 along the left side of the table and locate the intersection of the 29.8 row and the "0.07" column to
find 47 feet. Add 47 feet to the station elevation of 150 feet, to determine a pressure altitude of 197
feet.
Figure 2‐24 Pressure Reduction Computer
2‐47
You may also use the table to find pressure altitude by using station pressure. Station elevation should
NOT be added to the value when using station pressure.
The second alternate method is useful when you do not have access to the pressure reduction
computer, or Table 2‐11. To calculate pressure altitude, using this method, apply the formula PA =
HA+PAV, where PA = pressure altitude, HA = station elevation, and PA V = pressure altitude variation
approximation (or 29.92 minus the current altimeter setting multiplied by 1,000).
For example, use 150 feet for the station elevation and 29.87 for the station altimeter setting:
PA = HA + PAV
PA = 150 + 1,000(29.92 ‐ 29.87)
PA = 150 + 1,000 x .05
PA = 150 + 50
PA = 200 feet
Notice that we used the exact numbers in both of the alternate methods with a result that differs by
only 3 feet.
2‐48
Table 2‐11 Pressure Altitude Values
Hundredths 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
PRESSURE
inches & tenths PRESSURE ALTITUDE (FEET)
28.0 1824 1814 1805 1795 1785 1776 1766 1756 1746 1737
28.1 1727 1717 1707 1698 1688 1678 1668 1659 1649 1639
28.2 1630 1620 1610 1601 1591 1581 1572 1562 1552 1542
28.3 1533 1523 1513 1504
1494 1484 1475 1465 1456 1446
28.4 1436 1427 1417 1407 1398 1388 1378 1369 1359 1350
28.5 1340 1330 1321 1311 1302 1292 1282 1273 1263 1254
28.6 1244 1234 1225 1215
1206 1196 1186 1177 1167 1138
28.7 1148 1139 1129 1120 1110 1100 1091 1081 1072 1062
28.8 1053 1043 1034 1024 1015 1005 995 986 976 967
28.9 957 948 938 929 919 910 900 891 881 872
29.0 863 853 844 834 825 815 806 796 787 111
29.1 768 758 749 739 730 721 711 702 692 683
29.2 673 664 655 645 636 626 617 607 598 589
29.3 579 570 560 551 542 532 523 514 504 495
29.4 485 476 467 457 448 439 429 420 410 401
29.5 392 382 373 364 354 345 336 326 318 308
29.6 298 289 280 270 261 252 242 233 224 215
29.7 205 196 187 177 168 159 149 140 131 122
29.8 112 103 94 85 75 66 57 47 38 29
29.9 20 10 +1 ‐8 ‐17 ‐26 ‐36 ‐45 ‐54 ‐63
30.0 ‐73 ‐82 ‐91 ‐100 ‐100 ‐119 ‐128 ‐137 ‐146 ‐156
30.1 ‐165 ‐174 ‐183 ‐192 ‐202 ‐211 ‐220 ‐229 ‐238 ‐248
30.2 ‐257 ‐266 ‐275 ‐284 ‐293 ‐303 ‐312 ‐321 ‐330 ‐339
30.3 ‐348 ‐358 ‐367 ‐376 ‐385 ‐394 ‐403 ‐412 ‐421 ‐431
30.4 ‐440 ‐449 ‐458 ‐467 ‐476 ‐485 ‐494 ‐504 ‐513 ‐522
30.5 ‐531 ‐540 ‐549 ‐558 ‐567 ‐576 ‐585 ‐594 ‐604 ‐613
30.6 ‐622 ‐631 ‐640 ‐649 ‐658 ‐667 ‐676 ‐685 ‐694 ‐703
30.7 ‐712 ‐721 ‐730 ‐740 ‐749 ‐758 ‐767 ‐776 ‐785 ‐794
30.8 ‐803 ‐812 ‐821 ‐830 ‐839 ‐848 ‐857 ‐866 ‐875 ‐884
30.9 ‐893 ‐902 ‐911 ‐920 ‐929 ‐938 ‐947 ‐956 ‐965 ‐974
31.0 ‐983 ‐992 ‐1001 ‐1010 ‐1019 ‐1028 ‐1037 ‐1046 ‐1055 ‐1064
INPUT STATION PRESSURE: READ PRESSURE ALTITUDE DIRECTLY FROM TABLE INPUT ALTIMETER SETTING: READ VALUE FROM TABLE AND ADD STATION ELEVATION TO FIND PRESSURE ALTITUDE.
2‐49
Pilots of rotary wing aircraft frequently request the maximum pressure altitude for takeoff and all
destinations to help them calculate load and power restrictions. This is calculated using the lowest
expected altimeter setting (QNH) for the destination. The forecaster may have to interpret the other
station's forecast to determine if the forecasted QNH will be valid during the time the aircraft will be in
the vicinity. Many rotary wing aircraft have a table in their aircraft that uses the maximum pressure
altitude and maximum temperature to determine the maximum permissible load that can be carried.
Maximum pressure altitude can be used by the pilot in lieu of density altitude.
2.5.2 DENSITY ALTITUDE
The density altitude is defined as the pressure altitude corrected for temperature deviations from the
standard atmosphere and is the altitude at which a given air density is found in the standard
atmosphere.
For a given altitude, density altitude changes with changes in pressure, air temperature, and humidity.
An increase in pressure increases air density, so it decreases the density altitude. An increase in
temperature decreases air density, so it increases the density altitude. An increase in humidity
decreases air density, so it increases the density altitude. Changes in pressure and temperature have
the greatest effect on density altitude, while changes in humidity have the least effect.
If, for example, the pressure at Cheyenne, Wyoming, (elevation 6,140 feet) is equal to the pressure of
the standard atmosphere at that elevation, and the temperature is 101°F, the density would be the
same as that found at 10,000 feet. Therefore, the air is less dense than normal, and an aircraft on
takeoff will take longer to get airborne. Air density also affects airspeed. True airspeed and indicated
airspeed are equal only when density altitude is zero. True airspeed exceeds indicated airspeed when
density altitude increases. No instrument is available to measure density altitude directly; it must be
computed from the pressure (for takeoff, station pressure) and the virtual temperature at the particular
altitude under consideration.
The quickest method of calculating density altitude (DA) is to use an online Density Altitude Calculator or
a handheld Density Altitude Computer. The density altitude must be computed from the pressure (for
takeoff, station pressure) and the virtual temperature at the particular altitude under consideration. If
an online DA calculator and a density altitude computer is not available, calculate the density altitude by
applying the following formula: DA = PA + (120 Vt), where DA = density altitude, PA = pressure altitude
at the level you are calculating the density altitude, 120 = a temperature constant (120 feet per 1°C),
2‐50
and Vt = the actual temperature minus the standard temperature for the level you are calculating the
density altitude.
For example, the surface temperature is 30°C and the pressure altitude is 2,010 feet. Using Figure 2‐25
locate the 2,000‐foot line and follow it toward the center of the table until it intersects the Standard
Temperature line. At 2,000 feet the value of 11°C is identified. To calculate the density altitude at the
surface based on these values,
DA = PA + (120 Vt)
DA = 2,010 feet + [120(30°C ‐ 11°C)]
Figure 2‐25 Density Altitude diagram.
2‐51
DA = 2,010 + 120(19)
DA = 2,010 + 2,280
DA = 4,290 feet
For an acceptable result with slightly less precision, you may also use the density altitude diagram
(Figure 2‐25) to obtain density altitude. This method ignores the effect of humidity on density altitude
and is only accurate to within 200 feet.
Enter the bottom of the diagram with the air temperature, in this case 30°C, and proceed vertically to
the intersection of the pressure altitude line, in this case 2,010 feet. The pressure altitude lines are
curved from upper right to lower left. Once the intersected point of 30°C and 2,010 feet is identified,
move horizontally to the left side of the diagram to find the density altitude, in this case about 4,100
feet.
2.5.3 SPECIFIC HUMIDITY
Specific humidity is the mass of water vapor present in a unit mass of air. Where temperatures are high
and rainfall is excessive, the specific humidity of the air reaches high proportions.
Fog and humidity affect the performance of aircraft. During takeoff, two things are done to compensate
for their effect on takeoff performance. First, since humid air is less dense than dry air, the allowable
takeoff gross weight is reduced for operations in areas that are consistently humid. Second, because
power output is decreased by humidity, pilots must compensate for the power loss. Pilots may request
humidity values as either relative humidity or specific humidity and the forecaster is responsible to
ensure the pilot has the most accurate information available.
Specific humidity can be determined from the density altitude computer following instructions printed
on the computer. The air temperature, dew‐point temperature, and pressure from the observation are
necessary in order to perform the calculation.
2.6 FLIGHT WEATHER BRIEFER (FWB)
Learning Objectives
Describe the purpose of Flight Weather Briefer (FWB).
Describe the different modules and tabs of Flight Weather Briefer (FWB).
Identify the publications that govern Flight Weather Briefing.
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Explain the workflow of FWB from initial filing of a DD 175 to a completed DD 175‐1.
2.6.1 FLIGHT WEATHER BRIEFER (FWB) APPLICATIONS
Flight Weather Briefer (FWB) is a web‐enabled software applications that Navy forecasters shall use to
conduct all DD Form 175‐1 flight weather briefings, and should be used by aviators to the maximum
extent possible to request weather briefings remotely, and for filing the DD Form175 Military Flight Plan.
The FWB software application is comprised of Pilot, Forecaster, AIROPS, and FWB Manager Software
modules that are installed on web servers and accessible via web browser from any location where an
Internet/NIPRNET access is available. The computer terminal used to access FWB must have a common
access card reader in order to be authorized into FWB.
The initial step to operate within FWB is to request a new account. Enter the URL:
https://fwb.metoc.navy.mil/fwb11/, into the internet browser and the login screen appears. (Figure 2‐
26) On the login screen is the “Request New Account” button. Complete the information and usually
within 24 hours an account is generated and access is enabled.
Figure 2‐26 FWB Log‐in screen.
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Once the account is active, and the forecaster logs into the system, the main menu, Figure 2‐27
becomes accessible. The main menu offers access into all operational and archive functions of FWB.
There are nine tabs on the FWB main menu.
Home tab provides access to the “Home” page from any page within the application.
Inbox tab is where the FDO will find DD Form 175, Military Flight Plans that were submitted by the pilot.
FWB operates in a workflow concept of data usage and storage that includes a function of ownership
that maintains a system of accountability throughout the filing process. Documents submitted to the
FDO Inbox indicate that the pilot has passed ownership of the flight plan to the Forecast Duty Officer
(FDO), who becomes accountable for the completion of the appropriate weather briefing for the flight
plan.
Outbox tab contains completed flight weather briefings. Completed DD 175‐1’s will reside in the
Outbox for approximately 6‐hours beyond the flight plan ETD, then are automatically transferred to
archives. Briefs finalized by the pilot will also reside in the FDO Outbox for about 6‐hours past the
estimated time of departure (ETD), then those briefs are transferred to the archive files, unless the pilot
uses the “recall” functionality. The Outbox includes Flight Weather Product (FWP), Work, and Workflow
functions.
Consoles tab provides an alert/messaging capability for the user. For the FDO, the Consoles tab
provides a visual listing of all briefs conducted for the assigned site (entity). The workflow function, in
turn, allows for a detailed narrative of a specific briefing. In the Consoles tab, alternate views of the
“Flight Weather Plan” and “Workflow” are also accessible. The “Delete” function will remove a row
from this table, but does NOT actually delete the flight plan or brief itself. The Consoles tab also allows
for the FDO to open the system console in a new window for consistent monitoring. The content lists
within the consoles tab auto‐refreshes every 50 seconds. Information will be added or removed from
the list automatically, depending on its priority and importance, as established in the software
configuration.
Figure 2‐27 FWB menu tab.
2‐54
New Brief Tab allows the FDO to create a new request for a DD 175/DD175‐1, DD 1801/DD175‐1, or a
Canned Route Brief. The DD 1801 is an International Flight Plan. These three (3) request forms are the
exact forms the pilot uses within the Pilot Application.
WX Functions tab allows the FDO to manage canned routes, establish and access WX graphic links, and
access METAR and TAF information.
Archive Briefs tab maintains both the completed DD Form 175‐1’s and canned route briefs for a period
of 30 days. Both archive functions allow the FDO to retrieve a specific date and forecasting entity
(activity) for display.
Other Functions tab contains access to miscellaneous maintenance functions or interest areas specific
to FDOs.
Logoff tab exits the user from the FWB program and returns the user to a login accessible page.
The pilot is the initiator in the process of filing a DD Form 175 and creates, modifies and finalizes the
flight plan using the New Brief tab and the DD 175/DD 175‐1, DD 1801/DD175‐1, or a Canned Route
Brief as appropriate, and as seen in Figures 2‐28 and 2‐29. Once the flight plan is finalized, the
weather briefing number is generated and the pilot will receive an updated DD Form 175, Military
Flight Plan for submission into the national airspace system. A courtesy copy of the flight plan is also
sent to base operations once a pilot submits the DD Form 175 to the forecasting location (Note –
Figure 2‐28. FWB DD 175 Top Portion.
2‐55
this flight plan will NOT have the weather briefing number on it, and is NOT the final flight plan).
FWB does not relieve the pilot of his or her responsibilities to contact a local or remote weather
forecast office to ensure receipt of the flight plan for the flight of interest and does not provide
100% two‐way communication between the pilot, base operations or the forecasting activity.
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The finalize function is the pilot’s ultimate responsibility. In such a case where the FDO marks the
Briefing Completion page as a “completion pending pilot” or “returned to pilot (problem)”, this option
becomes available. In these cases, once the FDO finalizes the weather briefing, the brief is directly sent
to the pilot’s inbox as “completed” and the pilot must re‐finalize the briefing to obtain a flimsy briefing
number and final submission to base operations. Finalizing the flight plan will automatically send the
Figure 2‐29. FWB DD 175 Bottom Portion
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completed flight plan to air ops and the FDO Outbox where it will remain for 6‐hours beyond the
estimated time of departure before automatically being sent to archives.
Finalizing the flight plan is the same process of “filing” a flight plan. Finalizing the flight plan will
automatically launch Adobe Acrobat Reader and display the DD Form 175‐1
FWB is managed by the Director of Aviation for the Naval Oceanography Operations Command and is
developed and supported by the Enterprise Engineering Department (EED), Naval Oceanographic Office.
The EED provides FWB Enterprise Engineering and Life Cycle Support to include: requirements
management, design, development, testing, configuration management, security accreditation,
distribution, installation, and training.
2.7 OPTIMUM PATH AIRCRAFT ROUTING SYSTEM (OPARS)
Learning Objectives
Describe the purpose of Optimum Path Aircraft Routing System (OPARS).
Explain the sub‐systems of Optimum Path Aircraft Routing System (OPARS).
Identify the publications that govern Optimum Path Aircraft Routing System (OPARS).
OPARS is a preflight planning aid that integrates forecasted atmospheric conditions with the pilot’s
proposed flight profile to provide an optimized
flight plan that minimizes fuel consumption
(Figure 2‐30). The primary purpose of OPARS
is to provide a flight planning service to the
Naval Aviation community. OPARS provides a
recommended customized flight plan by using
sophisticated computer programs to analyze
the latest environmental forecast data and the
most fuel‐efficient flight profile for a specific
aircraft. In addition to providing the optimum
route and flight level, based on environmental parameters, OPARS can also calculate the amount of fuel
to load in order to arrive with a specified reserve, the maximum time on station, the maximum
cargo/stores for a particular flight, mandatory over water reporting positions, and fuel usage for a
specific route and/or altitude. OPARS serves as a supplement to the DD 175 (Military Flight Plan) and
DD 175‐1 (Flight Weather Briefing). The OPARS software application is also used by the Naval Portable
Flight Planning System (N‐PFPS) to provide flight level winds for mission planning purposes.
Figure 2‐30. OPARS Program screen.
2‐58
Aircraft performance data is derived from the appropriate NATOPS or commercial performance manual
and is divided into climb, cruise, and descent profiles. Based on aircraft performance data, the
sophisticated computer models at Fleet Numerical Meteorology and Oceanography Center in Monterey,
CA collect atmospheric data from several sources to analyze and forecast wind and temperature data for
flight levels from 1,000 feet through 100,000 feet, and forecast periods out to 72 hours. OPARS uses the
Digital Aeronautical Flight Information File (DAFIF) database to obtain information about high altitude
airways, low and high altitude Navaids and Waypoints, and airports with runway longer than 5,000 feet.
To gain access to OPARS, the user must have access to a common access card enabled computer
terminal. OPARS is available through the Navy Oceanography Portal (NOP) at
https://portal.fnmoc.navy.mil/metoc/, or directly at https://portal.fnmoc.navy.mil/opars‐ufs/. Once an
account is created, the user can access the web version of OPARS. The web based version enables
access to the software from any command access card enabled computer that does not have OPARS
installed on the system.
2.7.1 OPARS SUB‐SYSTEMS
OPARS is comprised of four sub‐systems; Communications, Flight Planner, Aeronautical Database, and
the Environmental Database.
2.7.1.1 The Communications sub‐system provides an interface for the OPARS user to generate and
submit OPARS requests and for the OPARS Duty Petty Officer, at Fleet Numerical, to monitor, control,
and assist in the flight plan development.
2.7.1.2 The Flight Planner sub‐system computes the optimum route and performance parameters for
the specified aircraft configuration.
2.7.1.3 The Aeronautical Database consists of aircraft performance characteristics, route structures,
and boundary information required by the OPARS Flight Planner module. Information for the
Aeronautical Database is taken from the DAFIF and is updated every 28 days.
2.7.1.4 The Environmental Database consists of wind and temperature fields for flight levels 1,000 ft.
through 100,000 ft. Fields are produced 4 times daily and are derived from the Naval Operational Global
Atmospheric Prediction System (NOGAPS) forecast model. Wind and temperature fields based on
climatology are also available.
2‐59
2.7.2 OPARS FLIGHT PLAN PROCESSING
The OPARS user is the individual interacting through a personal computer linked with the Fleet
Numerical computer system. The OPARS user builds a flight plan request on a computer with the aid of
a graphical user interface and submits the flight plan request to the Fleet Numerical host computer for
processing. Included within this request is such information as aircraft type, the number of flight legs,
points of departure, times of departure/arrival, points of arrival, and other pertinent information.
After the flight plan request is submitted to and accepted by the host system, OPARS begins calculating
an optimum route for the aircraft to fly. During this building process, OPARS uses the aircraft
parameters and the wind data to simulate the flight on possible routes between the point of departure
and the point of arrival. After analysis, the route that provides optimum fuel consumption is selected
for the flight plan. As the final step in the process, the information is formatted as a flight plan and
downloaded to the OPARS user’s personal computer. Print out and delivery to flight personnel
completes the process.
The OPARS program provides "help" menus that explain individual elements. A jet‐route data base is
included with the software and allows users to visually determine the most efficient air routes on their
remote computer terminal. Once selected, an air route can be saved for future use to become a
commonly used air route known as a "canned" route. Flight requests can also be saved and made
available at a future time.
Once users obtain a flight plan from FNMOC, they can display it in many different formats, as a variety of
tools are available to customize and enhance the display. Wind fields, navigational aids (navaids),
and other features may be overlaid on any flight route. The flight plan is then downloaded to a printer and
delivered to the pilot.
The Optimum Path Aircraft Routing System User's Manual, provides detailed information for
processing OPARS flight plans. This manual is published by FNMOC is available in the Help menu of
the web based version of OPARS, and can also be downloaded from the FNMOC website through the
Navy Oceanography Portal.
2.8 UPPER‐AIR OBSERVATIONS
Recognize the uses of upper‐air observation data.
Identify the different types of upper‐air observations.
Determine which types of upper‐air observations are conducted by Navy and Marine Corps
METOC personnel.
2‐60
Identify the publications that govern upper‐air observations and observation codes.
Recognize the applications for upper‐air observation reporting codes.
Identify the observation location and time in an upper‐air report.
Identify the standard upper‐air observation times.
Identify the differences and similarities in the four forms of the TEMP code.
Describe the information contained in each part of the TEMP coded report.
Explain the format and the meaning of each coded part of the TEMP report.
Describe the modifications added to the International code form in WMO Region IV.
Describe the format and contents of an Early Transmission Message.
Identify the three forms of the PILOT code and explain the use of each form.
Identify the type of information contained in each PILOT code message part and the meaning of
each coded element.
Describe the special use of the PILOT code for rawinsonde observations conducted within WMO
Region IV.
Identify the records that must be maintained by upper‐air observers, and explain the proper disposition
of these records.
Upper‐air soundings, or upper‐air observations, are collected using a special instrument called a
radiosonde or rawinsonde that measures meteorological elements through the troposphere and lower
stratosphere . A radiosonde is attached to a helium filled balloon, released, and tracked by ground
equipment while the radiosonde transmits pressure, temperature and relative humidity data back
to the receiver on the ground. Most radiosondes also provide wind information by tracking the
horizontal movement as it ascends through the atmosphere using a Global Positioning System device
embedded within the radiosonde.
The information transmitted to the receiver is processed, encoded, and then transmitted over
automated weather networks. The National Weather Service, U.S. Air Force, and the U.S. Navy’s
production centers ingest this data to produce several, twice daily meteorological computer
models based on data received from the 0000Z and 1200Z upper‐air soundings. The computer
programs can use data up to 12‐hours old so all observations, regardless of the observation time are
used if received within 12‐hours after the observation. Additionally, all transmitted observations, even
those not used in forecasting programs, are automatically entered in the upper‐air climatic data base
2‐61
at the National Climatic Data Center in Asheville, North Carolina. This data is used extensively in
atmospheric research.
Locally, upper air observations provide an immediate vertical profile of the atmosphere and are
invaluable as a forecasting tool, particularly for severe weather and general aviation forecasts.
2.8.1 NAVY/MARINE CORPS UPPER‐AIR PROGRAMS
Upper air observations are routinely conducted by fleet units having embarked Strike Group
Oceanography Teams (SGOT), Mobile Environmental Teams (MET), or deployed USMC units.
Procedures for operating the hardware system(s) have been provided to designated units. No special
forms are required to record the observations since the equipment provides an automatic output in
accepted coded formats.
Upper air observations are to be conducted at the synoptic times of 0000Z and 1200Z. However, any
sounding, regardless of time taken should be transmitted after termination of sounding. Other special
observations and reporting schedules may be required and will be promulgated in pertinent
OPLAN/OPORD/OPTASKs.
Some sites located on islands or in remote areas are designated as Synoptic Upper‐air Observation Sites.
These activities routinely conduct upper‐air observations to support World Meteorological Organization
(WMO) data collection requirements, as well as operational commitments. Strike Group
Oceanography Teams (SGOT) use portable equipment aboard ship and at remote shore sites to
conduct upper‐air observations in support of Tactical Decision Aids, global computer generated weather
forecasting models, and atmospheric research efforts. Marine Corps Meteorological Mobile Facility
Replacement [METMF(R)], which is a well equipped van capable of collecting, developing, and
communicating meteorological data, members also use portable equipment to conduct upper‐air
observations in support of deployed forces.
Normally, all upper‐air observations from ships, designated Synoptic stations, and remote land locations
are encoded and transmitted. Special observations conducted for training at shore stations may be
encoded but are not usually transmitted.
NOTE: Currently, the upper‐air observing program of the U.S. is comprised of a network of rawinsonde
stations combined with a number of additional upper air observation systems including pibals, a
network of ground‐based remote sensing wind profilers, enroute commercial aircraft pilot reports, and
satellite‐based temperature profile and cloud‐motion wind capability. Together these systems provide
2‐62
upper‐air measurements that are basic to meeting the needs of operational weather forecasting,
climatological data bases, and meteorological research programs.
2.8.2 COMMON TYPES OF UPPER‐AIR OBSERVATIONS
A radiosonde is a balloon‐borne instrument used to simultaneously measure and transmit
meteorological data while ascending through the atmosphere. The instrument consists of sensors for
the measurement of pressure, temperature, and relative humidity. The sensors' information is
transmitted in a predetermined sequence to the ground receiving station where that information is
processed at some fixed time interval.
When wind information is processed by tracking the balloon's horizontal motion while ascending, the
instrument package is termed a rawinsonde. Thus, rawinsonde observations of the atmosphere describe
the vertical profile of temperature, humidity, and wind direction and speed as a function of pressure and
height from the surface to the altitude where the sounding is terminated. The rawinsonde system
consists of a balloon‐borne radiosonde, receiving and tracking equipment, and computer systems for
data processing.
Pibal (pilot balloon) observations are soundings that delineate the vertical profile of wind direction and
speed as a function of height. These deliniations are made by tracking of a balloon using optical means
or by radar equipment. The terms RAOB (RAdiosonde OBservation) and RAWIN (RAWINsonde
observation) are frequently used to refer to any type of upper‐air observation.
Reports of conditions measured during any of the various upper‐air observations are normally encoded in
World Meteorological Organization (WMO) International codes for dissemination.
International upper‐air observation reporting codes were established by the WMO to allow all countries
of the world to exchange data. Because there are many different types of upper‐air observations
conducted each day, several similar codes are in use to efficiently report the data collected. Table 2‐10
shows the different types of upper‐air observations conducted, the types of data observed and reported,
and the WMO International code form used to format the report.
Reports received in these code formats are routinely used by weather personnel for routine aviation
support, weather‐forecasting support, and as input for GFMPL. Additionally, these observations provide
primary input to the Navy,s environmental prediction system at the Fleet Numerical Meteorology and
Oceanography Center, and to the National Weather Service environmental prediction system at
the National Meteorological Center. Navy and Marine Corps METOC personnel must be able to
2‐63
decode all upper‐air observation codes. And, as stated earlier, they must be able to encode, or verify,
the MRS computer encoding of the various forms of the TEMP code.
The appropriate format for encoding upper air observations is in accordance with the World
Meteorological Organization (WMO), Manual on Codes. Several types of upper air observations and the
proper coding format are demonstrated below in Table 2‐12.
Table 2‐12. Upper‐Air Observation Types and Reporting Codes
WMO CODE ID OBSERVATION SITE DATA OB TYPE
FM 32‐IX‐PILOT PP— Fixed Land Site Upper PIBALsFM 34‐IX‐PILOT MOBIL EE— Mobil Land Site Wind FM 33‐IX‐PILOT SHIP Q Q ‐ Ship Reports
FM 35‐X‐TEMP TT— Fixed Land Site Upper level RAWIN‐,
FM 38‐X‐TEMP MOBIL II— Mobil Land Site Pressure RADIO‐, FM 36‐X‐TEMP SHIP UU— Ship Temperature DROP‐,FM 37‐X‐TEMP DROP XX— Aircraft Humidity SONDEs, and
Winds RABALs
FM 39‐VI‐ROCOB RRXX Fixed Land Site Upper level ROCKETSONDEs
Ship Air density Temperature FM 40‐VI‐ROCOB SHIP SSXX Winds
Upper level Pressure Aircraft
Temperature FM 41‐IV‐CODAR LLXX Aircraft
Winds
FM 42‐XI AMDAR none Aircraft Upper level Pressure
Temperature Dew point Winds Aircraft to satellite data relay
NOTE: "—" indicates multi‐part messages (AA, BB, CC, or DD).
2.8.2.1 Radiosonde Observations
Pressure, temperature, and humidity
are measured by a balloon‐borne
instrument as seen in Figure 2‐31. Data is
encoded and transmitted in the TEMP,
TEMP MOBIL, or TEMP SHIP code. In
addition to the WMO, Manual on Codes,
refer to the Federal Meteorological
Handbook No. 3 for proper encoding
Figure 2‐31 Radiosonde with Balloon (Source: USGS)
2‐64
format.
2.8.2.2 Rawinsonde Observations
Pressure, temperature, and humidity measured by a balloon‐borne instrument similar in
appearance to Figure 2‐32. Wind speed and direction information is obtained by the sonde
through use of GPS satellite tracking and is transmitted along with the rest of the data to the
ground receiving station. Collected data is encoded and disseminated in the TEMP, TEMP MOBIL
or TEMP SHIP code, with se lected information distributed in the PILOT, PILOT MOBIL, or PILOT
SHIP code. In addition to the WMO, Manual on Codes, refer to the Federal Meteorological Handbook No.
3 for proper encoding format.
2.8.2.3 RABAL Observations (Radiosonde Balloon)
These observations measure wind speed and direction by using a theodolite or fire‐control radar to
track a reflector attached to a radiosonde train. When conducted in conjunction with a
RAOB, data is encoded and distributed in the TEMP, TEMP MOBIL, or TEMP SHIP code. When only
wind information is obtained, data is distributed in the PILOT, PILOT MOBIL, or PILOT SHIP code.
2.8.2.4 PIBAL Observations (Pilot Balloon)
A balloon is tracked with an optical theodolite (or radar) to determine only low‐level wind speed
and direction. No radiosonde is attached to the balloon. Heights are based on assumed ascension
rates. When transmitted, data is encoded in PILOT, PILOT MOBIL, or PILOT SHIP formats. In addition to
the WMO, Manual on Codes, refer to the Federal Meteorological Handbook No. 3 for proper encoding
format.
With the introduction of compact, computerized rawinsonde systems containing navigational aid
(NAVAID) receivers, the Radiosonde and Rabal observations have become obsolete. Pibal
observations are still conducted by Marine Corps observers in the field to provide low‐level wind
observations in support of aviation operations and para‐drop operations. Pibal observations are
particularly important in situations where radio emissions would lead to detection by enemy forces.
2‐65
2.8.3 OTHER TYPES OF UPPER AIR OBSERVATIONS
Throughout the world, many countries conduct and transmit data from radiosonde, rawinsonde, rabal,
and pibal observations. Several countries, including the United States, routinely carry out additional types
of upper‐air observations as follows.
2.8.3.1 Rocketsonde Observations
A rocket containing pressure, temperature, and wind sensors is launched from a ship, land station, or
aircraft. After the rocket reaches apogee, an instrument package is deployed on a parachute measuring
the atmosphere as it descends. Observed data is transmitted in the ROCOB or ROCOB SHIP code.
2.8.3.2 Dropsonde Observations
Aircraft deploy a parachute‐carried sensor package; the sensors measure pressure, temperature,
humidity, and winds. This information is encoded and transmitted in the TEMP DROP code.
2.8.3.3 Aircraft Flight Level Observations
Aircraft flying routine flight levels may contain an automatic sensor unit that measures, encodes,
and automatically transmits an Aircraft Meteorological Data Relay (AMDAR) message which
contains pressure, temperature, dew point, and wind information. Similar data may be gathered
manually by the aircrew from onboard equipment and forwarded by voice radio or communication
link in the CODAR format.
2.8.4 UPPER‐AIR OBSERVATION PUBLICATIONS
All U.S. upper‐air observations, including military, are governed by procedures outlined in the Federal
Meteorological Handbook No. 3 (FMH‐3), rawinsonde and pibal Observations. The FMH‐3
prescribes federal standards for conducting rawinsonde and pibal observations, and for
processing, encoding, transmitting, and archiving observation data. Also provided are procedures for
quality control.
All information in the FMH‐3 is consistent with World Meteorological Organization (WMO) standards.
WMO publication number 306, Manual on Codes, Volumes I, International Codes, and Volume II,
Regional and National Coding Practices contain a complete breakdown of all upper‐air observation code
formats.
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2.8.5 UPPER‐AIR REPORTING CODES
Upper‐air codes are designed to allow transmission of a large amount of data using only a small number of
characters. More importantly, the standardization of numerically coded formats can be readily
transmitted by computer and then decoded by a weather person in any country, regardless of their spoken
language. These codes may be easily ingested into computer programs that run algorithms to
analyze the upper‐air data, plot graphical displays, and then calculate probable changes in the
reported conditions. The resulting meteorological, computer generated forecast models serve as
invaluable forecasting aids.
2.8.6 IDENTIFYING MESSAGE CODE FORM
Nearly all coded upper‐air‐report messages contain a four‐letter code identifier as the first group in the
first line of data. All upper‐air codes except the AMDAR code have a common format for the data
identification line. As encoded for transmission, identification data appears in the first line of the
message. The symbolic format for the identification data groups is as follows:
MiMiMjMj YYGGId IIiii (land stations) or
MiMiMjMj D.. ..D 99LaLaLa QcLoLoLoLo
MMMULaULo (hohohohoim) (ship/aircraft/mobile land stations)
The first group, MiMiMjMj, is found in nearly every international coded report, and is the code
identifier. The MiMi identifies the code type as presented in the second column of Table 2 ‐13. The MjMj
identifies which part of the multi‐part upper‐air reports is contained in the section of the report: AA for
Part A, BB for Part B, and so forth. If all of the observed data is routinely distributed as a single message,
such as the CODAR report, the MjMj is encoded XX. The first group of the coded report also contains the
observation time and the location of the sounding.
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Table 2‐13. Part "A" TEMP Coded Upper‐Air Report (Surface to 100‐hPa Level Mandatory Reporting Levels)
SEC SYMBOLIC FORMAT CONTENTS
1 MiMiMjMj YYGGId IIiii Example: (1) TTAA 64121 72306 MiMiMjMj D. . . .D YYGGId 99LaLaLa QcLoLoLoLo MMMULaULo (hohohohoim) Examples: (2) IIAA 64121 99352 70787 11658 01002 (3) UUAA NSHP 64121 99311 70721 11612 (4) XXAA 14121 99311 70721 11612
Identification Data (Land station) Identification Data (Mobile‐land station) (Ship) (Aircraft)
2 99PoPoPo ToToTaoDoDo dodofofofo (surface) PnPnhnhnhn TnTnTanDnDn dndnfnfnfn (all other levels) Example: 99030 05050 09015 00211 06060 09005 92353 02227 08021 85490 00646 07016 70010 06900 08527 50560 22764 09047 40718 33372 09045 30916 459/ / 09071 25090 543/ / 09096 20225 581/ / 09099 15475 595/ / 09615 10745 575/ / 09100
Mandatory Pressure Level data
3 88PtPtPt TtTtTatDtDt dtdtftftft or 88999 Example: 88225 589// 09098 Tropopause data
4 77PmPmPm dmdmfmfmfm (4vbvbvava) or 77999 Example: 77132 09628 40508
Max wind and wind‐shear value
2.8.7 IDENTIFYING OBSERVATION TIME AND LOCATION
The WMO established the synoptic hours of 0000Z, 0600Z, 1200Z, and 1800Z as standard times for
conducting upper‐air observations. Most balloon releases actually take place 30 to 45 minutes
before these times so that the scheduled observation time actually occurs near the middle of the
observation.
Because of time, personnel, and budget considerations, most stations do not conduct
observations at each of the synoptic hours. If only two upper‐air soundings are taken per day, they are
taken at 0000Z and 1200Z. If only one upper‐air sounding is conducted, it is taken at 0000Z or 1200Z,
whichever time is closest to local sunrise.
The observation time is coded in the third group of the first line of data in the form YYGGId. This is the
date/time group, with YY indicating the day of the month, and GG indicating the synoptic hour of the
observation. The Id is an indicator that is different in each code and will be discussed later.
On most reports the code identifier and date/time group and printed a message header at the top of the
bulletin. A single bulletin may contain several Part A TEMP SHIP reports from different ships, all under
a header such as UUAA 211200Z NOV.
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The location where the observation was conducted is identified differently in the various code forms. Ship
observations and mobile land‐observation sites are located by latitude, longitude, and Marsden
Square (Figure 2‐32) coordinates, which pinpoint the location. Established land stations are located
only by referencing the international block and station number (IIiii) on weather plotting charts or in
the Master Weather Station Catalog.
The identification groups "D. . . .D 99LaLaLa QcLoLoLoLo MMMULaULo" are used only by transient
observation platforms to identify the observation platform and the location of the
observation. All ships, mobile observation sites ashore, and aircraft use these groups. As in the Ship
Synoptic code FM 13‐X SHIP, discussed in Chapter 1, the "D....D" is the call sign of a ship or the call sign or
communications identifier assigned to a mobile unit. The "99LaLaLa QcLoLoLoLo" groups are the latitude
and longitude of the observation, exactly as used in the ship synoptic reports. The MMMULaULo group is a
second location reporting group, which contains the Marsden Square number of the location, MMM and
Marsden sub‐grid locations ULa (a repeat of the units digit of the latitude) and ULo (a repeat of the units
digit of the longitude). Some computers use only the Marsden group to enter the position of the upper‐
air report in the analysis program. The latitude and longitude groups are used to determine the exact
location. Both groups must be correct.
Figure 2‐32. Marsden Square.
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In place of the latitude, longitude, and Marsden Square groups, permanent shore stations report only
one group: IIiii. This is the WMO block (II) and station number (iii) exactly as used for the Land Synoptic
code. Mobile land stations include an additional group (hohohohoim) that reports the station elevation in
either meters or feet.
The breakdown of all the upper‐air reporting codes and coding formats is contained in the WMO
Publication 306, Manual on Codes, Volume 1, International Codes. The majority of these coded
messages are "read" automatically by computers and entered into analysis programs for use. In
selected situations where manual decoding is required, consult WMO Publication 306. Both the TEMP
code and the PILOT code forms are routinely used to encode observed data, and are addressed below.
2.8.8 TEMP CODE
The TEMP coded upper‐air information is automatically ingested into computer algorithms, and is also
used extensively in many manual applications. For detailed analysis, TEMP coded data is decoded and
plotted on a Skew‐T, Log P Diagram, or on horizontal or time section diagrams. The TEMP code is the
primary upper‐air reporting code so it is extremely important that all meteorology and oceanography
professionals be thoroughly familiar with this code.
The different forms of the TEMP codes are used to report data gathered in the rawinsonde, radiosonde, or
rabal observations, depending on the site used to launch the balloon. However, aircraft‐launched
dropsondes use a slightly different code format. The four different forms of the TEMP code are
presented in Table 2‐13.
2.8.9 COMPOSITION OF THE REPORTS (MESSAGE)
All four forms of the TEMP code are broken down into four parts to speed distribution. Additionally, each
code part is divided into data sections. The data sections contain information in five‐digit groups,
although letters are used in one or two groups in the identification data section. Each figure in each group
is significant to its position in the group and to its position in the message; therefore, the established
order of the groups in the messages must be maintained. When observed data is not available for an
element, a slant (/) is used instead. This is done to preserve continuity of the groups and sections as
required.
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2.8.9.1 Message Parts
Each TEMP code part may be transmitted as an independent message. This is done to speed distribution
of the reports because the collection of sounding data takes a considerable amount of time. A
radiosonde may continue to report usable data 2 to 3 hours after release.
The parts of the message are identified as A, B, C, and D. Data at and below the 100‐hPa level are
reported in Parts A and B, and data above 100 hPa are reported in Parts C and D. Parts A and C contain
data pertinent to the standard atmospheric pressure surfaces, which are also called the mandatory
reporting levels. Parts B and D contain data pertinent to the significant levels, these are the levels
determined significant due to temperature and/or humidity changes, and/or changes in wind speed or
direction. Table 2‐14 indicates the data available in each section:
Table 2‐14. Message Parts
PART A PART C
Mandatory Levels SFC Mandatory Levels 10 hP
to 100 hPa and higher
PART B PART D
Significant Levels SFC Significant Levels 10 hP
to 100 hPa and higher
All military stations designated to encode and transmit upper‐air observations will encode and
transmit the Early Transmission Message, and Parts A, B, C, and D. Each part of a code may be sent as a
separate message as soon as the data is evaluated and encoded.
The Early Transmission Message is a brief message of certain observed upper air data which are transmitted
as soon as possible after the radiosonde reaches the 500 hPa level. Normally these Early Transmission
Messages are manually encoded and released while the receiving equipment is still receiving data.
Part A of the TEMP form is available from the computer first, even while the observation of the higher
levels is still being measured, Part B is available a short time later, while Parts C and D are not available
until after the upper‐air sounding is terminated. Many stations send each part as a separate message.
Because of this, upper‐air reports may be received in parts at different times after the synoptic hour.
With rapid electronic equipment, the number of messages, rather than message length, is often the key
factor in speed of transmission. The MRS processor, when connected to a desktop computer or laptop,
rather than a printer, allows for properly formatted messages to be delivered transmission‐
ready. Whether all parts are included in a single message depends upon a number of factors that change
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from day to day. When broken into separate sections, the Early Transmission Message has first
transmission priority, Parts A and C have second priority, and Parts B and D have third priority.
2.8.10 IDENTIFICATION DATA SECTION
Each part of TEMP code contains data for up to 10 coded sections. These sections are not readily
apparent in the coded message, and except for Section 1, Identification Data, the type of data that
each section contains varies from part to part.
The identification data for each part of the code is nearly identical, and it is contained in the first line of
each message part. Data type TTAA indicates a TEMP code report from a fixed land station (message Part
A), while UUDD indicates a TEMP SHIP coded report from a ship (message Part D), and so forth.
The only difference in the identification data for the TEMP and the other upper‐air codes is the indicator in
the YYGGId group, and the method used to encode the UTC date for YY. The TEMP code uses indicator Id in
message Parts A and C, but contains indicator a4 in Part B. In Part D, the indicator is replaced by a "/." The Id
is the indicator for the highest mandatory pressure level for which winds are reported (WMO code table
1734). If, for example, winds are reported to the 50‐hPa level, the indicator as used in Part A would be
"1," because Part A would include winds to the 100‐hPa level; and the indicator in Part C would be "5,"
because the winds in Part C would be reported to the 50‐hPa level. The a4 in Part B is a code figure for the
type of measuring equipment used (WMO code table 0265), which should be reported as a 0 for the MRS
system.
The coding of the date, YY, identifies the wind‐speed reporting units. If wind speeds are reported in knots,
as are all U.S. Military observations, 50 is added to the UTC day of the month. For example a sounding taken
on the 22d day of the month is encoded as 72. When winds are reported in the standard meters per
second, YY is encoded as the actual day of the month.
With the exception of identification data, the data contained in each message part is the same for the
different "TEMP" code forms. A TEMP report, a TEMP MOBIL report, a TEMP SHIP report, and a TEMP
DROP report will all encode data using identical data formats.
Note: In the following sections, the subscript in the WMO form is dropped to simplify explanation.
2.8.11 PART A ‐ LOWER MANDATORY LEVELS
Part A of the coded message contains identification data, pressure, temperature, dew‐point
depression, winds, tropopause data, and maximum wind data. Table 2‐13 shows the symbolic
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representation for Part A, along with examples of coded data. Refer to Table 2‐13 for the following four
examples.
Example (1) shows typical coded identification data for a fixed land station 1200Z observation on the 14th
day of the month from block 72, station 306, with winds in knots: TTAA 64121 72306.
Example (2) shows similar data for a mobile land station: 1200Z observation on the 14th day of the month
from site 35.2° N, 078.7° W, Marsden Square 116, with winds in knots, and a station height of 100 feet:
IIAA 64121 99352 70787 11658 01002.
Example (3) is from a ship, the "NSHP" with a 1200Z observation on the 14th day of the month at 31.1°
N, 072.1° W, Marsden Square 116, with winds in knots: UUAA NSHP 64121 99311 70721 11612.
Example (4) is an aircraft dropsonde directly over NSHP at the same time, except wind speeds
are in meters per second: XXAA 14121 9311 70721 11612.
2.8.11.1 Mandatory Level Data
The Mandatory Level data in section 2 contains the bulk of the coded data in this part of the report. The
format for the surface data is slightly different from the format for the remaining mandatory atmospheric
levels reported in Part A. The mandatory levels reported in this part are the surface, 1,000 hPa, 925 hPa,
850 hPa, 700 hPa, 500 hPa, 400 hPa, 300 hPa, 250 hPa, 200 hPa, 150 hPa, and 100 hPa.
For the surface, the three five‐digit data groups report surface pressure, temperature and dew‐point
depression, and winds. For each of the mandatory levels, the three five‐digit groups report the
pressure level and the altitude of the level in meters if im is a 1, 2, 3, or 4, or in feet if im is a 5, 6, 7, or 8,
the temperature and dew‐point depression at each significant level, and wind direction and speed. The
three groups are repeated for each of the mandatory levels.
2.8.11.2 Surface Pressure
In the first of the three groups for surface information, the 99 is the indicator for "surface information,"
and the PPP is encoded in hundreds, tens, and units of the surface pressure in hectopascals. In the
example, 030 represents 1,030 hPa.
2.8.11.3 Pressure Level Altitude
The first of the three groups for the remaining "mandatory levels" contains PP, the hundreds and tens
digits of the reported pressure level, 850 hPa is encoded as 85, and hhh, the altitude in meters or
decameters of the reported pressure level. For levels up to and including 700 hPa, the altitude is reported in
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three digits to the nearest meter with the thousands value, if any, deleted, 85490, from Table 2‐13, is the
850 hPa mandatory level at 1,490 meters.
For all levels above 700 hPa, the altitude is reported to the nearest decameter (tens of meters) with the
ten‐thousand value deleted. Refer to table 2‐15, (Standard Pressure Surfaces) to determine the
standard altitudes of the mandatory levels from the 1,000‐ to 10‐hPa levels. To decode a reported altitude
in Part A of 10711, the first two digits, 10, indicate the 100‐hPa level. The 711 is the altitude in decameters,
or "something‐7,110 meters." Since the standard altitude of the 100‐hPa level is approximately 16,180
meters (with the ten‐thousands value of 1), one could correctly decode the reported altitude as 17,110
meters.
Table 2‐15.—Standard Pressure Surfaces.
2.8.11.4 Temperature/Dew Point Depression
Following the surface‐pressure group and the pressure‐level/altitude groups, the next group contains
the coded temperature and dew‐point depression. The temperature is reported by TTTa where TT is the
tens and units value of the temperature, in degrees Celsius, at the surface or the pressure level. The tenths
value, Ta, of the temperature is used indicate whether the reported temperature is positive or negative. If
Ta is an even number, the temperatures listed are positive. If Ta is an odd number, the temperatures
listed are negative. When encoding, the tenths value is dropped to the next lower tenths value, if
necessary, to indicate the proper temperature sign. For example, a temperature of ‐23.8°C is
encoded 237, while a temperature of+23.9°C is encoded 238.
The radiosonde instrument measures temperature and relative humidity, and the receiving system,
commonly the Mini‐RawinSonde System (MRS) calculates the difference between the two
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instrument‐reported readings when determining the dew‐point temperature. Only the dew‐point
depression, or the absolute difference between the air temperature and the dew‐point temperature
(with respect to liquid water), is reported in the TEMP code by DD, a coded figure. Dew‐point
depression (always an absolute value), is normally calculated to the nearest tenth of a degree Celsius,
and encoded using WMO code table 0777. Code figures 00 through 50 report dew‐point depressions
from 0.1 °C through 5.0°C, respectively while code figures 56 through 99 represent dew‐point
depressions rounded off to the nearest whole degree from 06°C through 49°C (subtracting 50 from the
coded figure yields the dew‐point depression in whole degrees).
2.8.11.5 Winds
The group ddfff is used to report wind direction and wind speed. The dd is the true direction in tens of
degrees from which the wind is blowing. Observed wind directions are rounded off and reported to the
nearest 5 degrees, as specified by WMO regulations. The fff is the wind speed in hundreds, tens, and
units. For example, a wind from 275° true at 159 knots is encoded 27659 (a 1 is added to the wind
direction); a wind of 275° at 25 knots is encoded 27525, and a wind of 270° at 25 knots is encoded 27025.
The units of speed are specified in the Identification Data section.
2.8.11.6 Tropopause Data
From Table 2‐13, tropopause data is contained in section 3 of message Part A, and may also be contained in
Part C in the identical format. Tropopause data is only reported in the part of the message (A or C) that
pertains to the level of the atmosphere in which the tropopause is located. The tropopause level is
selected by the MRS and is the base of the layer in which temperature stops decreasing with height or
decreases very slowly with height, normally between the 250 hPa and 200 hPa level. Criterion on which
the MRS basis this selection is contained in the Federal Meteorological Handbook Number 3. In some
cases, there may be more than one tropopause, one below the 100‐hPa level and the other above the
100‐hPa level. In this case, both Parts A and C may report tropopause data.
Following the mandatory level data, three groups, 88PPP, TTTaDD, and ddfff, contain information very
similar to the mandatory level information that pertains to the tropopause level. If the sounding did not
locate a tropopause, the group 88999 is used in place of the data groups.
The indicator for the tropopause group is 88. The pressure level at which the tropopause is identified is PPP
and is encoded to the nearest hPa.
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The temperature and dew‐point depression follow the 88 indicator, are represented by the code
TTTaDD, and are encoded in the same manner as the mandatory level data.
The wind direction and speed, ddfff, are encoded in the same manner as the mandatory levels.
Table 2‐13, section 4 contains maximum wind information. Information on the highest winds observed
between the 500‐hPa level and the 100‐hPa level, in excess of 60 knots, is contained in the Maximum
Wind data group. Maximum winds located above the 100‐hPa level are reported in an identical section
in message Part C. Maximum wind data is reported in two or three groups. The first group, 77, is the
indicator for maximum wind, followed by PPP, the pressure level of the maximum wind to the nearest
hectopascal. The second group, ddfff, contains the wind direction and speed, as previously described.
The third group, 4vbvbvava, is optional and used to report the absolute value of the vertical wind shear. The
vbvb reports the vertical wind shear difference between the level of maximum wind and the winds 3,000
feet below the level of maximum wind, while the vava reports the vector difference between the level of
maximum winds and the winds 3,000 feet above the level of maximum wind. The vertical wind shear
values are important indicators for the identification of clear air turbulence (CAT). The procedure
to calculate vertical wind shear is presented in the FMH‐3.
When no winds in excess of 60 knots are observed between the 500‐hPa level and the 100‐hPa level, the
group 77999 is reported. If two winds with identical wind speeds satisfy the criteria for a maximum
wind, the levels will be encoded successively, beginning with the lowest altitude.
2.8.12 PART B ‐ LOWER SIGNIFICANT LEVELS
The second part, Part B of the TEMP coded messages, contains data on levels that are considered
significant because of distinct changes noted in the temperature, humidity, or wind data. Although the
significant levels are selected by the MRS, you must verify the selection of significant levels. When the
MRS processor selects levels, it first considers the mandatory significant level criteria, followed by
the "supplemental" significant level criteria, then, the MRS automatically encodes Part B.
Remember some stations do not report significant wind levels.
Proper evaluation of an upper‐air sounding requires that the operator select significant levels when a
sounding is conducted using manual equipment. The MRS automatically searches for and encodes
significant levels but MRS operators must review and verify the computer‐selected levels. In general,
significant levels are selected with respect to temperature, humidity, and wind changes.
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2.8.12.1 Symbolic Form of Part B
Part B (Table 2‐16) consists of several sections of data. It starts with an Identification Data section
(section 1), followed by section 5data for each significant level selected with respect to temperature or
humidity changes; section 6, data for significant levels selected with respect to changes in the wind
direction or speed; section 7, sounding system data and observation time; section 8, observed cloud data;
and ends with sections 9 and 10, regional and nationally coded data groups. Ship observations also
report the sea surface temperature data in section 7.
2.8.12.2 Significant Temperature/Humidity Levels
Section 5 (Table 2‐16) contains data for each level selected as significant for either temperature or
humidity. Data for each significant level is contained in two five‐digit groups, nnPPP and TTTDD, which are
repeated for each significant level selected. The first group contains a level identifier, nn, and the
pressure, PPP, of the level in hundreds, tens, and units of hectopascals. The level identifier is a two‐digit
number with the surface always identified as 00. All remaining significant levels are identified, from lower
to higher, as 11,22,33,..., 99,11,22, and so forth. If a level previously reported in Part A also fits the
criteria for a significant level, it is reported again in this section as a significant level. The second group,
TTTDD, is the temperature and dew‐point depression, exactly as reported for the mandatory levels.
Winds are not reported for significant levels.
If temperature or humidity values are missing at points during ascent, the top and bottom boundaries of
the missing data layer are significant levels. At least one additional level must be selected within the
layer of missing data to indicate the missing data. The missing data is encoded with slants, for example,
"55745 01522 66680 061// 77650 08310" identifies three significant levels. The base of a missing humidity
layer, level 55, is at 745 hPa, with a temperature ‐1.5°C and dew‐point depression of 2.2°C. The top of
the layer, level 77, is at 650 hPa, with a temperature of ‐8.3°C and dew‐point depression of 1.0°C.
The fact that the humidity data is missing in this layer is revealed by significant level 66 at 680 hPa, with a
temperature of ‐6.1°C and"//" encoded in place of the dew‐point depression.
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Table 2‐16.—Part "B" TEMP Coded Upper‐Air Report (Surface to 100‐hPa Level Significant Reporting Levels)
SEC SYMBOLIC FORMAT CONTENTS
1 MiMiMjMj YYGGa4 IIiii or Example: (1) TTBB 64120 72306 MiMiMjMj D....D YYGGa4 99LaLaLa QcLoLoLoLo MMMULaULo (hohohohoim) Examples: (2) IIBB 64120 99352 70787 11658 01002 (3) UUBB NSHP 64120 99311 70721 11612 (4) XXBB 14123 99311 70721 11612
Identification Data (Land station) Identification Data (Mobile‐land station) (Ship) (Aircraft)
5 nnnnPnPnPn TnTnTanDnDn Example: 00030 05050 11930 06040 22847 00043 33770 02920 44650 10100 55600 14740 66435 29769 77358 38170 etc.
Significant temperature and humidity levels
6 21212 nnnnPnPnPn dndnfnfnfn Example: 21212 00030 13015 11990 17022 22985 17035 33972 17015 44925 18005 55860 19015 66700 20025 77550 22040 88320 23050 99300 23070 11260 23112 22220 23090 33101 24060
Significant wind levels
7 31313 srrarasasa 8GGgg 9snTwTwTw Example: 31313 46105 81135 90156
System status, time of launch, and Sea‐water temperature
8 41414 NhCLhCMCH Example: 41414 43322 Cloud data
9 10 code groups following indicator groups 51515, 52525, through 59595 code groups following indicator groups 61616,62626,
Regional codes National codes
2.8.12.3 Significant Wind Levels
The beginning of section 6 may be identified in the coded message by the indicator group 21212. This
section is used to report winds at "significant wind levels." All Navy and Marine Corps METOC
professionals operating outside of WMO Region IV report significant level winds in this section, and need
not report any information in the PILOT reporting code. Synoptic stations and other land or mobile land
stations within WMO Region IV do not include this group in the TEMP coded report, instead, the Fixed
Regional Level Winds are reported using the appropriate PILOT code message Parts B and D.
Data for each significant wind level is contained in two groups of five‐digit numbers, nnPPP and ddfff,
which are repeated for each level selected. The first group for each level is used the same as in the
significant temperature levels, and contains the level identifier nn and the pressure level PPP. The
second group, ddfff, is the wind direction and speed which is encoded the same as in part A of the code.
Levels of missing wind data are reported similarly to missing significant levels of temperature and
humidity.
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2.8.12.4 System Status and Seawater Temperature
Section 7 (Table 2‐14), 31313 srrarasasa 8GGgg 9snTwTwTw, contains information on the rawinsonde system
used for the observation, the actual launch time of the instrument, and the seawater temperature.
31313 is the section identifier.
In the second group, srrarasasa, the sr is the solar and IR radiation correction, found from WMO Code
Table 3849. For an MRS use code figure 4 which indicates the solar and IR radiation is corrected
automatically by system. The rara is the code for the rawinsonde system used and is obtained from
WMO Code Table 3685. The current MRS is reported by code 61 for the Vaisala Radiosonde RS92. The
sasa is the tracking technique and system status, from WMO Code Table 3872. Code 08 is used to indicate
MRS equipment with GPS tracking systems.
The actual UTC time of the radiosonde release is entered in the fourth group following the 8 indicator. If
the radiosonde instrument is released at 1120Z, the group would read 81120.
The seawater temperature group is only reported by ships and begins with the indicator 9. This group is
omitted in reports from stations other than ships.
The sn, the sign of the temperature (0 for positive and 1 for negative), is followed by the water
temperature (TwTwTw) in tens, units and tenths of a degree Celsius.
2.8.12.5 Cloud Data
Section 8 (Table 2‐16) reports cloud information in one group following the 41414 indicator. The
NhCLhCMCH is the cloud group where Nh is the sum of all the low‐etage clouds present, or if no low‐etage
clouds are present, the sum of all the mid‐etage clouds present, in oktas (WMO Code Table 2700). h is the
coded height above the surface of the lowest cloud layer (WMO Code Table 1600) and CLh, CM, and CH
represent, respectively, the predominant type of low‐, mid‐, and high‐etage clouds from WMO Code
Tables 0513, 0515, and 0509.
2.8.12.6 Regional Codes
Regional codes are added to the international code following the regional code indicator groups. In WMO
Region IV, the required regional codes are specified in the FMH‐3. In Region IV, all regional data is reported
in "additional" data groups, commonly called the 101‐groups, following the 51515 Regional code
indicator. Other countries may use different regional codes following any of the other regional code
indicator groups 52525, 53535, . . ., 59595, and national codes 61616, 62626, . . ., or 69696. Regional
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and national codes for other countries are found in WMO Publication 306, Manual On Codes,
Volume II, Regional Codes and National Coding Practices.
The 101‐groups are five‐digit groups following the format 101AdfAdf. AdfAdf. indicates the type of data
being reported. Actual data may follow a "101‐group" in additional code figure groups. Only data
pertaining to the sounding below the 100‐hPa level is reported with 101‐groups in Part B. These groups
can be used to report doubtful data, corrected data, or early transmission data. If the sounding
terminates below the 100‐hPa level, the reason for termination is also entered in this section.
2.8.13 PART C ‐ UPPER MANDATORY LEVELS
Part C of the TEMP codes contains reports for mandatory levels above the 100‐hPa level. The
mandatory levels reported in this section are the 70‐hPa, 50‐hPa, 30‐hPa, 20‐hPa, and 10‐hPa levels.
This section uses the same format as Part A of the TEMP code message, including identification data,
mandatory level data for the levels above 100 hPa, tropopause data (if located higher than 100 hPa), and
maximum wind data (if located higher than 100 hPa).
If the upper‐air sounding terminated below the 100‐hPa level, Part C of the message may be encoded
and transmitted including only the appropriate identification data followed by the code 51515 and
the reason for termination code.
2.8.14. PART D ‐ UPPER SIGNIFICANT LEVELS
Part D is used to report significant temperature and humidity levels, significant wind levels, and regional
codes in the same manner as reported in Part B. Section 7, for sea‐water temperature and the rawinsonde
system information, and section 8, for cloud information, are never included in Part D. Coded regional
information, such as the 101‐groups, are included as appropriate for any levels above 100 hPa.
2.8.15 EARLY TRANSMISSION MESSAGES
Early Transmission messages are brief reports of certain observed upper‐air data, which are sent as soon
as possible after the radiosonde measures the 500‐hPa level. Normally, these messages are manually
encoded by all ships and designated synoptic land stations while the MRS continues to receive and process
data. These messages contain only the appropriate Part B identification data, followed by the code
groups 51515 10196 and data for the 85O‐, 700‐, and 500‐hPa levels (as normally transmitted in Part A).
Land stations may also include the stability index and the low‐level mean winds. The 10196 group
identifies the data as an "early report."
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In addition to encoding Parts A through D of the TEMP code, certain stations must encode some data in
the PILOT code, which is discussed in the following text.
2.8.16 PILOT CODE
The PILOT code is primarily used throughout the world to report PIBAL‐observed wind directions and
speeds. In the United States it is also used to report fixed regional level winds observed during a
Rawinsonde observation.
Like the TEMP code, the PILOT code is also separated into four parts to ease handling and speed
transmission. Parts A and C include winds observed at the standard altitudes for the mandatory pressure
levels. Parts B and D include winds for the significant wind levels. Parts A and B are for levels from the
surface to 100‐hPa (about 53,000 feet), while Parts C and D are for levels above the standard 100‐hPa
level. Each part begins with an Identification Data section.
There are three forms of the PILOT code prescribed for use by the WMO. PILOT is used by designated
shore stations to report upper‐air observations of wind information. The code identifiers PPAA, PPBB,
PPCC, and PPDD are used to identify this code form. PILOT MOBIL is used by mobile sites ashore to
report atmospheric wind observations. The code identifiers EEAA, EEBB, EECC, and EEDD are used to
identify this code. At sea, shipboard upper‐wind observations are reported in PILOT SHIP using the
identifiers QQAA, QQBB, QQCC, and QQDD. Each code form is nearly identical in format except for the
identification information contained in the first line of each message part.
2.8.17 IDENTIFICATION DATA
The identification data contains the data type, the location identifier and/or location, and the date‐time
group. The format is identical to the TEMP code, Part B: MiMiMjMj YYGGa4 IIiii is used with the PILOT
code, while PILOT SHIP and PILOT MOBIL use MiMiMjMj D....D YYGGa4 99LaLaLa QcLoLoLoLo MMMULaULo
hohohohoim. The same identification data format is used in all four parts of the report.
2.8.17.1 Part A ‐ Lower Mandatory Levels
Part A of the message contains identification data, mandatory level winds, and the maximum wind and
wind shear values.
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2.8.17.2 Mandatory Level Winds
After the identification data, the first section of the PILOT code Part A is the winds at the mandatory levels
in the format 44nP1P1 or 55nP1P1 ddfff ddfff ddfff. The 55nP1P1 group is used only when the altitudes of the
pressure levels are based on standard altitudes above mean sea level. The 44nP1P1 group is used when
the altitudes are obtained from pressure equipment, such as a radiosonde. This cluster of four five‐digit
groups, reporting winds at three mandatory levels, is repeated four times to include all the mandatory
levels through the 100‐hPa level.
In the first group, the n indicates the number of standard levels reported in the section and the number of
ddfff groups that follow. This figure is usually a 3, but may be a 1 or 2 in the last repetition. The P1P1 is
the hundreds and tens value of the first pressure level reported.
The ddfff group reports wind directions (dd) and wind speeds (fff). As in the TEMP code, the units of
wind speed are meters per second if the date, YY, in the identification data is simply the UTC date. When
the wind speed units are reported in knots, 50 is added to the date. All wind directions in the PILOT code
are reported to the nearest 5 degrees.
For example, the coded groups 44300 09535 08058 06601 indicate winds at three pressure levels (from a
radiosonde), starting at the 1,000 hPa level ("00"), 095° at 35 knots; the 925 hPa level, 080° at 58 knots; and
the 850 hPa level, 065° at 101 knots.
2.8.17.3 Maximum Wind
The following five different formats are used as indicator groups for the level of maximum wind or a
secondary level of maximum wind:
66PmPmPm—maximum wind at top of sounding, measured pressure level reported.
6HmHmHmHm—maximum wind at top of sounding, standard altitude reported in meters.
77PmPmPm—maximum wind within sounding, measured pressure level reported.
7HmHmHmHm—maximum wind within sounding, standard altitude reported in meters.
77999—no maximum wind observed.
The PmPmPm is the measured pressure level in hectopascals and HmHmHmHm is the altitude in decameters
(units rounded off, hundred‐thousands value not reported). A maximum wind level must have a wind
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speed in excess of 60 knots and occur above the 500‐hPa level. A secondary maximum wind level may
also be reported.
Following the maximum wind indicator group, the wind is reported in the format ddfff, and the optional
vertical wind‐shear group, 4vbvbvava, may be reported the same as in the TEMP code.
2.8.17.4 Part B ‐ Lower Significant Levels
This part of the PILOT code message contains identification data, reports of winds at significant levels, and
regional and national coded information for the levels up through 100 hPa. In WMO Region IV, the fixed
regional levels (PPBB) replace any significant levels (section 6, Part B of TEMP Code).
2.8.17.5 Significant Level Winds
When only significant levels are reported, as indicated by the identifier group 21212, each level is
encoded in two five‐digit groups in the format nnPPP ddfff. The nn indicates the level (number 00 for
surface, and then upward from 11 through 99, and repeating as necessary). The PPP is the pressure for
the level. The ddfff is the wind direction and speed, just as reported for the mandatory levels.
2.8.17.6 Fixed Regional Level Winds
When this section is used to report winds at fixed regional levels, a slightly different format is used and
the 21212 indicator group is not included. Winds are encoded in sets of three fixed levels, from lower
to higher. Each set is preceded by an identifier group 9tnu1u2u3 or 8tnu1u2u3. Identifier groups beginning
with a 9 are used when the fixed levels are separated by 300 meter (1,000 foot) increments, as used in
WMO Region IV. An indicator "1" replaces the indicator "9" when the heights exceed 30,000 meters
(100,000 feet). The "8" indicator means that the fixed levels are separated by 500‐meter increments.
The tn is the tens digit of the first altitude reported in the set; the u1, u2, and u3 are the units digits of the
level number. Essentially, as used by the United States, tn is the ten‐thousands value of the altitude in feet,
and the u1, u2, and u3 are the thousands value of the altitude in feet.
For example, "91246 27575 27090 26606" indicates winds for the 12,000 foot (MSL), 14,000‐foot (MSL),
and the 16,000‐foot (MSL) fixed regional levels, respectively, as 275° at 75 knots, 270° at 90 knots, and
265° at 106 knots.
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2.8.17.7 Regional Codes
Regional codes may be added to the report following the "51515" through "59595" group and
national codes from "61616" through "69696" indicator group, as appropriate. In WMO Region IV, only
the 51515 group is used. The Additional Data Codes or 101‐groups, as discussed previously, may be
added when encoding a Pibal observation. The indicator and the 101‐groups are not included when using
the PILOT code Part B (or Part D) to report fixed regional level winds observed during a rawinsonde
observation since this would duplicate information previously transmitted.
2.8.17.8 Part C ‐ Upper Mandatory Levels
Part C of the PILOT code is formatted exactly as Part A. Only the mandatory levels above the standard
altitude of the 100 hPa level are reported in Part C.
2.8.17.9 Part D ‐ Upper Significant Levels
Significant level winds or fixed regional level winds for the levels higher than the 100‐hPa level are reported
in Part D. The format is exactly the same as Part B.
2.8.18 OBSERVATION RECORDS
The Department of the Navy Records Management Program Manual, identifies meteorological records,
such as upper‐air observations (except those conducted only for training), as permanent official records
of the U.S. Government. Duplicate copies of sounding records are temporary records that may be
retained on board as long as they are useful, normally 1 year, and then destroyed.
2.9 SKEW‐T
Learning Objective
Identify the scales used to plot reported information on the Skew T, Log P diagram.
Determine temperature parameters and condensation levels.
Describe how reported information is plotted on the diagram.
Explain how the temperature, dew‐point temperature, and pressure altitude curves are completed by
using plotted information.
Describe the atmosphere as Absolutely Stable, Absolutely Unstable, or Conditionally Unstable.
Explain the indications of various stability indexes derived from sounding data.
2‐84
One of the most useful products for analysis of the state of the atmosphere over a single location is the
Skew T, Log P diagram (Figure 2‐33). The standard Skew T, Log P diagram is one of the few products that
provide a complete profile of the atmosphere in the vertical, from the surface to as high
as 25 hectopascals.
2.9.1 Skew‐T Log P Diagram Familiarization
This section presents the individual parts of a Skew‐T Log P Diagram. Each line of a Skew‐T represents
some value of a meteorological parameter that, when known, provides invaluable data that can be used
to analyze cloud heights, fog depth, atmospheric stability, wind shears, icing, and temperatures
variables. A well‐analyzed Skew‐T also provides that data necessary to make very accurate forecast
determinations on meteorological phenomena such as thunderstorms and thunderstorm intensity, hail
probability and hail size, maximum winds and temperature, fog dissipation rates, and much more.
Figure 2‐33. Skew T, Log P diagram. (Source: COMET)
2‐85
In order to unlock all the data available from an
upper air sounding and the Skew‐T, it is first
important to have a thorough understanding of
each line on the diagram.
2.9.1.1 Isobars
Isobars are lines of constant pressure
represented by horizontal, solid brown lines
logarithmically‐spaced (10mb intervals).
Pressure levels are labeled every 50mb at both
ends and the center of each line. The area
from 400mb‐100mb is also used for analyzing data from 100mb‐25mb. The labels are in brackets at the
end of appropriate isobars. International Civil Aviation Organization (ICAO) standard heights are printed
under the corresponding isobar on the left side of the chart in feet and meters. Note the 800‐millibar
isobar is highlighted in Figure 2‐34.
2.9.1.2 Isotherms
Isotherms are lines of equal temperature
represented by straight, solid brown lines sloping
from the lower left to the upper right of the chart.
The lines are printed in 1 degree intervals and
labeled in brown every 5 degrees celsius. A
Fahrenheit scale is printed along the bottom edge
of the chart in black for easy conversion. The 10° C
isotherm is highlighted in Figure 2‐35.
Figure 2‐35. Isotherms. (Source: PDC)
Figure 2‐34. Isobars. (Source PDC)
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2.9.1.3 Dry Adiabats
Dry adiabats are lines of constant potential temperature.
Dry adiabats are slightly curved, solid brown lines sloping
from the lower right to the upper left of the chart. They
represent the temperature change of a parcel of dry air
(unsaturated) as it rises through the atmosphere and
cools due to expansion, or descends through the
atmosphere and warms due to compression. The dry
adiabats represent a temperature change of 1° C per 100
meters and 5.5° F per 1,000 feet. They are printed at 2
degree intervals and labeled every 10 degrees along the 1030mb isobar. A dry adiabat is highlighted in
Figure 2‐36
2.9.1.4 Saturation Adiabats
Saturation Adiabats represent the rate of change in
the temperature of a saturated parcel of air rising or
sinking through the atmosphere. They appear as
slightly curved, solid green lines sloping from the
lower right to the upper left side of the chart and
represent a temperature change (warming [sinking]
or cooling [rising]) of .55° C per 100 meters and 2° to
3° F per 1,000 feet. Near 200mb, the dry adiabats parallel the dry adiabats where moisture content is
low and temperatures are cold. They are printed in 2° C
intervals and are labeled every 2° C at 530mb and 200mb.
Figure 2‐37 highlights a saturation adiabat.
2.9.1.5 Saturation Mixing Ratio
The saturation mixing ratio is represented as slightly curved,
dashed green lines sloping from the lower left to the upper right
of the chart, with numerical values printed at 975mb.
Saturation mixing ratio lines are labeled in grams per kilogram,
i.e., parts of water per 1000‐parts of dry air. The spacing
Figure 2‐36. Dry Adiabats. (Source: PDC)
Figure 2‐37. Saturation Adiabats. (Source: PDC)
Figure 2‐38. Saturation Mixing Ratio.
(Source: PDC)
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between the saturation mixing ratio lines decreases as the value increases because warm air can hold
more water vapor than cold air. Figure 2‐38 highlights a saturation mixing ratio line.
2.9.1.6 Thickness
Thickness is defined as the distance between two pressure
levels and is a function of temperature and moisture
content. Ten thickness scales are printed on the chart as
horizontal, graduated black lines with boundary pressures
labeled on left side of scale. The top of the scale is labeled in
hectofeet and the bottom in hectometers. The 1000/700
Thickness scale is represented in Figure 2‐40 just above the
850 millibar isobar.
2.9.1.7 ICAO Standard Atmosphere
The ICAO standard atmosphere lapse rate is shown as a heavy brown line in the center of the chart
starting at 1013mb and 15° C. Standard atmosphere is used to compare the actual atmospheric
conditions to standard atmosphere and is highlighted in Figure 2‐39 by a heavy brown line.
2.9.1.8 1000‐mb Height Nomogram
Consists of three black scales located at the top
left of the Skew‐T chart. (Refer to Figure 2‐40)
The temperature scale is printed horizontally
along the top of the chart and is labeled in both
°F and °C. The pressure scale is along the upper
left of the chart and represents the Mean Sea
Level (MSL) pressure of a station. The height
scale is to the left of the pressure scale and gives
the height of the 1000‐mb SFC Above Ground
Level (AGL) (to calculate this height you must
subtract the station elevation).
Figure 2‐39. Thickness and Standard
Atmosphere. (Source: USAF)
Figure 2‐40. 1000 Millibar Height Nomogram.
(Source: USAF)
2‐88
2.9.1.9 Standard Heights
The heights of pressure surfaces in the standard atmosphere are indicated by a vertical scale along the
right side of the chart. The left side of the scale is in kilometers and the right side is in kilo‐feet. The
heights of standard‐pressure surfaces are also printed at the left margin of the chart beneath each
pressure value. All heights determined using the SKEW‐T are geo‐potential heights which are the
measure of potential energy at a certain level above the earth's surface due to gravitational pull. The
actual height will be slightly greater than the geo‐potential height and will increase with an increase in
height (altitude). The small difference between geo‐potential height and actual height has no
consequence in day‐to‐day weather forecasting and geo‐potential height is normally used as the actual
height.
2.9.1.10 Coded Heights
Heights are reported in 3 digits (YXX from radiosonde balloon observations) with a prefix, suffix or prefix
and suffix added to decode the actual height value. The coded heights can be decoded using Table 2‐17.
Table 2‐17. Decoding 3 Digit Code (MSL).
Level Code Decoded Comments
SFC YXX YXX No decoding required.
850mb YXX 1YXX Prefix with "1."
700mb YXX 2YXX If the value of Y is = 5 prefix with "2."
700mb YXX 3YXX If the value of Y is < 5 prefix with "3."
500mb to 300mb YXX YXX0 Suffix with "0."
250mb to 100mb YXX 1YXX0 Prefix with "1" and suffix with “0.”
2.9.1.11 Wind Scale
Wind scales are represented as vertical black lines along the right side of the chart. Open circles
represent wind data for mandatory pressure surfaces. Closed circles represent heights where winds are
usually recorded from significant pressure surfaces. They are marked every 1000 feet up to 10,000 ft;
every 2,000 feet from 10,000 to 20,000 ft; and every 5,000 feet from 20,000 ft on up.
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2.9.1.12 Contrail‐Formation Curves
Contrail‐Formation Curves represent the critical RH% values necessary for the formation of contrails.
They are overprinted, thin black lines at the top of the chart from 500mb ‐ 100mb, and overprinted, thin
dashed lines from 100mb‐40mb.
2.9.2 PLOTTING SOUNDING DATA ON THE SKEW‐T LOG P DIAGRAM
This section presents the parameters routinely plotted on a SKEW‐T and how they are used to
determine the values of other atmospheric variables.
2.9.2.1 Free Air Temperature Curve
The free air temperature is the actual temperature of the atmosphere at a given level. The points to be
plotted on the chart are located by reference to the pressure and temperature scales of the Skew‐T.
The plotted temperature curve is always represented by a solid line.
2.9.2.2 Dew Point Temperature (Td)
Dew Point Temperature is the temperature to which a parcel of air must be cooled at constant pressure
and moisture content, in order for saturation to occur. The dew point curve is plotted on the Skew‐T
and is decoded by translating the dew point depression from the upper air code into a dew point
temperature.
2.9.2.3 Pressure Altitude (PA) Curve
The PA curve is used to determine the actual height of
pressure levels for atmospheric soundings that have a higher
or lower virtual temperature than the ICAO standard
atmosphere. The height scale located along the right side of
the SKEW‐T is based on ICAO Standard Atmosphere, for
example, the standard geo‐potential height of 600mb is 4206
meters above MSL. This height is computed using a standard
temperature lapse rate and assumes no moisture is present.
If the actual virtual temperature from MSL to 600mb were
colder than the standard atmosphere, the actual geo‐
potential height of 600mb would be less than 4206 meters.
Inversely, if the actual virtual temperature from MSL to 600mb were warmer than the standard
Figure 2‐41. Pressure Altitude Curve.
(Source: U.S. Air Force)
2‐90
atmosphere, the actual geo‐potential height would be greater than 4206 meters. To account for this
difference in height with a nonstandard atmosphere, a pressure altitude is constructed to determine the
actual pressure surface height above or below MSL. Figure 2‐41 displays a plotted Pressure Altitude
Curve.
2.9.3 UNREPORTED METEOROLOGICAL QUANTITIES
2.9.3.1 Mixing Ratio (w)
The Mixing Ratio is the ratio of the mass of water vapor to the mass of dry air and is expressed in parts
per thousand, usually grams of water vapor to kilograms of dry air.
2.9.3.2 Saturation Mixing Ratio
The saturation mixing ratio is the mixing ratio a sample of air would have if saturated.
2.9.3.3 Relative Humidity (RH%)
Relative Humidity is the ratio (in %) of the amount of water vapor in a given volume of air, to the
amount of water vapor that volume would hold if the air were saturated.
2.9.3.4 Vapor Pressure
The Vapor Pressure is that part of the atmospheric pressure which water vapor contributes to the total
atmospheric pressure and is expressed in millibars.
2.9.3.5 Saturation Vapor Pressure
The Saturation Vapor Pressure is the partial pressure which water vapor would contribute to the total
atmospheric pressure if the air were saturated.
2.9.3.6 Aircraft Condensation Trails (Contrails)
Contrails are elongated clouds composed of water droplets or ice crystals that form behind the aircraft
when the wake becomes supersaturated with respect to water. There are two main types of contrails
Engine‐exhaust trails form when the water vapor in the exhaust gas mixes and saturates the air in the
wake of the aircraft. The injected water vapor adds moisture into the air while the heat raises the air
temperature. Therefore, contrail formation is dependent on the initial pressure, temperature, and
relative humidity of the environment.
2‐91
Aerodynamic trails are caused by the momentary reduction in pressure of air flowing around the airfoil.
The pressure drop causes sufficient adiabatic cooling of the air to raise the relative humidity of the
affected environment to saturation. Engine‐exhaust trails account for almost all of the actual contrails
which form in the atmosphere.
2.9.3.7 Potential Temperature ()
The potential temperature is the temperature that a sample of air would have if it were brought dry
adiabatically to the 1000 millibar level. Potential temperature is expressed in degrees K.)
2.9.3.8 Wet Bulb Temperature (Tw)
The Wet Bulb Temperature is the lowest temperature to which a volume of air at a constant pressure
can be cooled by evaporating water into it. This assumes the heat required for evaporation is drawn
directly from the air itself, thus cooling the air. The wet bulb temperature is very useful when used to
determine a rain or snow precipitation decision. Air will undergo evaporative cooling with the onset of
precipitation. The wet bulb temperature reveals the coldest possible temperature the air can reach as
precipitation and evaporation of that precipitation occurs and whether it will cool to a temperature low
enough to support snow fall or other frozen precipitation.
2.9.3.9 Wet Bulb Potential Temperature (w)
The Wet Bulb Potential Temperature is the wet bulb temperature a parcel of air would have if brought
saturation‐adiabatically down to the 1000 millibar level. This temperature is used to determine the
downdraft temperature of thunderstorms that directly relate to gust potential. It is also used is some
severe weather index calculations
2.9.3.10 Equivalent Temperature (TE)
The Equivalent Temperature is the temperature a parcel of air would have if all of its moisture was
condensed out by the pseudo‐adiabatic process, and then returned dry adiabatically back to the original
pressure level. The latent heat of condensation is being used to heat the parcel, and shows the
tremendous energy that the condensation of water vapor contributes to the atmosphere. The
equivalent temperature is used to trace parcels to their origin in thunderstorm studies. The equivalent
temperature is sometimes termed the adiabatic equivalent temperature and should not be confused
with the isobaric equivalent temperature which is always slightly lower.
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2.9.3.11 Equivalent Potential Temperature
The Equivalent Potential Temperature is the temperature a sample of air would have if all its moisture
was condensed out by the pseudo‐adiabatic process then returned dry adiabatically to the 1000 millibar
level.
2.9.3.12 Virtual Temperature (Tv)
The Virtual Temperature of a moist parcel of air is defined as the temperature at which dry air at the
same pressure would have the same density as the moist air parcel. This temperature is used to
calculate thickness values between standard pressure surfaces and does incorporate moisture that is
present in the air.
2.9.3.13 Thickness (z)
The Thickness is the distance between two constant pressure surfaces and is directly proportional to the
mean virtual temperature of a layer. The higher the virtual temperature of a layer, the thicker the layer
is (resulting in higher than standard pressure surfaces) and the lower the virtual temperature of a layer,
the thinner the layer is (resulting in lower than standard pressure surfaces.) The thickness between two
constant pressure surfaces is calculated by using the Hypsometric Equation or:
h = z - z = 2 1
LnRTg
PP
2
1
Thickness is used to determine the intensity of temperature advection in the horizontal in the lower
troposphere, and is very useful in making a rain vs. snow determination.
2.9.3.14 Convective Temperature (Heated Method) (Tcp)
The Convective Temperature (Heated Method) is the temperature of the air the surface must reach in
order for convection to begin.
2.9.3.15 Convective Temperature (Moist Layer Method) (Tcml)
The Convective Temperature (Moist Layer Method) is the temperature at which convection would begin
when using the moist layer method.
2‐93
2.9.4 CONDENSATION LEVELS
Cloudiness develops if air rises, or is forced to rise through the atmosphere. The Skew‐T is used to
determine the height at which clouds will form if the air is lifted by various means. The base of clouds
expected to develop at some point in the future is very important to aviation forecasting.
2.9.4.1 Lifting Condensation Level (LCL)
The Lifting Condensation Level is the height at which a parcel of air would become saturated if lifted dry
adiabatically. The LCL is the saturation point of a parcel lifted mechanically (e.g., frontal or orographic).
2.9.4.2 Convection Condensation Level (Heated Method) (CCLp)
The Convection Condensation Level is the height to which a parcel of air, heated from below, will rise
adiabatically until saturation occurs. (The potential cloud bases of cumuliform clouds formed due to
surface heating.)
2.9.4.3 Convection Condensation Level (Moist Layer Method) (CCLml)
The Convection Condensation Level (Moist Layer Method) is used to determine the potential cloud
bases when there is a highly variable moisture content in the lower layers of the troposphere (near
surface). This method can only be used when surface dew point depression is 6 C degrees or less.
2.9.4.4 Mixing Condensation Level (MCL)
The Mixing Condensation Level is the lowest height in a layer, mixed by turbulence, at which saturation
occurs after complete mixing of the layer. The MCL is used for forecasting cloud bases in areas of strong
mechanical mixing.
2.9.5 EVALUATING ENERGY AREAS AND STABILITY
On a thermodynamic diagram, such as the Skew‐T, a given area of the vertical sounding can be
considered proportional to a certain amount of kinetic energy of a vertically and adiabatically moving air
parcel. There are areas of positive energy in which a parcel of air will rise freely because it is warmer
than the surrounding environment, and areas of negative energy in which energy must be supplied in
order for the parcel to ascend through the atmosphere. The type and size of these energy areas, as well
as the stability of the vertical column often determine the type of weather that will occur over a region.
In order to fully understand positive energy areas, negative energy areas, and the three conditions of
stability, it is first important to understand how to understand how to manipulate a parcel of air through
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the vertical. The manipulation of air parcels on a Skew‐T Log P Diagram is mostly conducted using the
Saturation Mixing Ratio, Dry Adiabat, and Saturation adiabat curves.
2.9.5.1 Level of Free Convection
The Level of Free Convection (LFC) is the height at which a parcel of air lifted dry adiabatically until
saturated, then saturation adiabatically once saturated (temperature and dew point are equal) first
becomes warmer (less dense) than the surrounding environment. The parcel, once warmer than the
surrounding environment, will continue to rise at the LFC until it becomes colder (more dense) than the
surrounding environment.
Figure 2‐43 represents the process to find the LFC and positive and negative energy areas if the parcel is
expected to be lifted mechanically.
The first step in the process is to locate the LCL. From the surface dew point, extend a line upward,
parallel to the nearest saturation mixing ratio. From the surface temperature, extend a line upward,
parallel to the nearest dry adiabat until that line intersects the first line. The height at which the two
lines intersect is LCL, or lifting condensation level. Since the parcel is saturated at the LCL, from the LCL,
draw a line upward, parallel to the nearest saturation adiabat, to a point that intersects the temperature
line at the top of the sounding.
All points below the LFC, where the temperature line is to the left, or cold side of the sounding, outline
the near surface negative energy area. Note the near surface, blue shaded area on Figure 2‐42.
All points above the LFC, where the temperature line is to the right, or warm side of the sounding,
outline areas of positive energy through the sounding. Note the red shaded area on figure 2‐42.
2‐95
Near the tropopause, at the point where the temperature line again crosses to the left of the
temperature line is the Equilibrium Level. The Equilibrium Level is defined as the height where the
temperature of a buoyantly rising parcel again becomes equal to the temperature of the environment.
Note the blue shaded area near the top of the sounding in Figure 2‐42.
Note: If the expected lift is due to heating alone, the LFC is found by first locating the CCL.
Nearly all the procedures routinely used to analyze the stability of the atmosphere employ the “parcel”
method. The stability is determined by lifting or lowering a hypothetical air parcel and comparing the
resulting parcel conditions to the conditions of the surrounding environment. As a parcel is lifted, or as
it descends through the atmosphere it will cool (ascend) or warm (descend) adiabatically. Therefore,
the dry and saturation adiabats are used to represent the rate of cooling or warming of a parcel as it
moves through the atmosphere.
2.9.5.2 Positive Energy Area
When a parcel rises freely because it is in a layer where the adiabat it follows (dry or saturation
adiabats) is warmer than the surrounding environment, the area between the adiabat and the sounding
is proportional to the amount of kinetic energy the parcel gains as it rises freely in the atmosphere. In
other words the warmer the parcel is relative to the sounding trace, the more potential kinetic energy.
LCL
LFC
EL
Figure 2‐42. Level of Free Convection. (Source PDC)
2‐96
Through this layer, the displaced parcel is warmer than the environment and will continue to rise until
its temperature equals, or becomes colder, than that of the surrounding environment.
2.9.5.3 Negative Energy Area
When a parcel on a Skew‐T lies within a stable layer, energy must be supplied in order to force the
parcel vertically, upward or downward, through the environment. The area of the Skew‐T proportional
to the amount of kinetic energy needed to move a parcel vertically. Through this layer, the displaced
parcel will be colder than the environment. By theory, the parcel is in a stable environment and will
return to its original position.
2.9.5.4 Absolute Stability
Absolute Stability exists when the temperature of air above a level
is warmer than the saturation adiabatic lapse rate. In this
condition, the parcel, whether saturated or unsaturated, will return
to its original position. In other words, to lift a parcel from
anywhere in the sounding, the parcel, when lifted, remains colder
relative to the actual environmental temperature represented by
the temperature trace. In Figure 2‐43, a parcel is lifted both dry
and saturation adiabatically from the surface. At all points the
lifted parcel is colder than the actual environment. Once the lifting
force is removed from this parcel, it will return to its original level
where the parcel temperature is again equal to that of the environment.
`2.9.5.5 Absolute Instability
Absolute Instability exists when the temperature of the air
above a level is colder than both the saturation and dry
adiabatic lapse rates. Here the parcel will be warmer than
the environment and, whether saturated or unsaturated, will
continue to move in the direction in which it has been
displaced. Lifting a parcel from anywhere in the sounding,
the parcel, when lifted, remains warmer relative to the actual
environmental temperature represented by the temperature
Figure 2‐43. Absolutely Stable. (Source PDC)
A
Figure 2‐44. Absolutely Unstable. (Source PDC)
A
2‐97
trace. In Figure 2‐44, a parcel is lifted both dry and saturation adiabatically from the surface. At all
points the lifted parcel is warmer than the actual environment. Once the lifting force is removed from
this parcel, it will continue rising until it meets a layer with an equivalent temperature.
2.9.5.6 Conditionally Unstable
The atmosphere is determined conditionally unstable when the temperature of the air above a level lies
between the saturation and dry adiabatic lapse rate. If the
parcel is saturated, it will continue to move in the direction
displaced, or be unstable. If the parcel is unsaturated, it will
return to its original position, or stable. The condition in the
unconditionally unstable state is that the parcel must be
saturated in order for the condition to be unstable. In figure 2‐
45, if a parcel is lifted dry adiabatically it will become colder
than the surrounding environment and will therefore return to
its original position once the lifting force is removed. If a
parcel is lifted saturation adiabatically, however, the parcel
becomes warmer than the surrounding environment and will therefore continue to rise once the lifting
force is removed.
2.9.6 STABILITY INDEXES
The overall stability or instability of a sounding is sometimes conveniently expressed in the form of a
single numerical value called a stability index. Such indexes have been introduced mainly as aids to
determine the potential of the atmosphere to produce a variety of meteorological phenomena such as
thunderstorms, hail, tornadoes, etc. Used alone, stability indexes are less useful than when used in
combination with analysis of the complete upper air sounding. The greatest value of a stability index
lies in alerting the forecaster to those soundings, routes, or areas which should be more closely
examined by applying more in depth forecasting techniques.
Several indexes are in existence to date and are readily available right along with the computer plotted
Skew‐T from a number of web based outlets. This section will not provide analyzation techniques, but
does provide an understanding of what the individual index values indicate so proper application of the
computer derived values can be accomplished.
Figure 2‐45. Conditionally Unstable. (Source PDC)
A
2‐98
2.9.6.1 Showalter Index
The Showalter Index (SHOW, SWI, or SI) is a measure of the local static stability of the atmosphere
expressed as a numerical index. This index is determined by raising an air parcel from the 850‐mb level,
dry adiabatically to the point of saturation, then saturation adiabatically to the 500‐mb level. The
temperature of the lifted parcel is then compared to the temperature at 500‐mb, the difference being
the SI. If the parcel is colder than the 500‐mb, the index is positive, if the parcel is warmer than the 500
mb temperature, the value is negative.
When the SI is +3 or less, showers are probable. The probability of thunderstorms increases rapidly
when the SI is between +1 and ‐2. SI values of ‐3 or less indicate the potential for severe thunderstorms.
If the SI is less than ‐6, the possibility of tornadic activity exists.
2.9.6.2 Lifted Index
The Lifted Index (LIFT, or LI) is a modification to the SI. To evaluate the LI, the mean mixing ratio and the
mean potential temperature for the time of convection are first determined for the lowest 3,000 feet of
the sounding. From the mean values the LCL is located, and then a parcel is extended upward through
the sounding, using the saturation adiabat, to the 500‐mb level. The index is then determined in the
same manner as the SI by calculating the difference of the lifted parcel to that of the 500‐mb
temperature.
Lifted Index values of 0 or greater indicate stable to mostly stable conditions with thunderstorms being
unlikely. From 0 to ‐2 thunderstorms become possible, ‐3 to ‐5 thunderstorms become probable, less
than ‐5 severe thunderstorms become possible, and less than ‐7 violent thunderstorms with tornadoes
become possible.
2.9.6.3 Cross Totals
Cross Totals (CTOT, or CT) combines a measure of the low‐level moisture with temperatures aloft. Moist
850 mb dew points and cold 500 mb temperatures yield high CT values. Although a CT of 18 is normally
required before thunderstorms can occur, a value over 18 does not guarantee thunderstorm
development. A trigger mechanism must also exist.
2.9.6.4 Vertical Totals
Vertical Totals measures the vertical lapse rate while CT incorporates low level moisture. Although VT
may be a better indicator of thunderstorm activity in one region over another, or in certain synoptic
2‐99
situations, and although CT may carry more weight in certain regions and situations, a combination of CT
and VT to determine Total Totals has proven a more reliable indicator of severe weather activity.
2.9.6.5 Total Totals
The Total Totals (TOTL, or TT) index is used to identify areas of potential thunderstorm development and
is the sum of the Cross Totals and Vertical Totals.
The general TT convection threshold is 44. Weak potential for heavy thunderstorms is 50; the potential
for moderate thunderstorms occurs with a TT of 50 to 55, and strong thunderstorms with a TT greater
than 55.
2.9.6.6 SWEAT Index
The SWEAT Index (SWET, or SW), is the Severe Weather Threat Index and is used to estimate the severe
weather potential of an air mass. SW is computed from five terms that contribute to severe weather
potential. Those terms are:
Low‐level moisture (850 mb dew point)
Instability (Total Totals value)
Low‐level jet (850 mb wind speed)
Upper‐level jet (500 mb wind speed)
Warm advection (wind veering between 850 and 500 mb)
Commonly accepted SWEAT index threshold values used by the Air Force, who developed SWEAT, are
300 for severe thunderstorms and 400 for tornadoes. Keep in mind that the SWEAT index is an indicator
of the potential for severe weather; other forecasting techniques must be applied in order to confirm a
triggering mechanism.
2.9.6.7 K Index
The K Index (KINX, or K) is a measure of thunderstorm potential based on the vertical temperature lapse
rate, moisture content of the lower troposphere, and the vertical extent of the moist layer. The K Index
is derived arithmetically and does not require a plotted sounding. The temperature difference between
850 and 500‐mb is used to determine the vertical temperature lapse rate. The 850‐mb dew point
provides the value for moisture content and the vertical extent of the moisture is determined from the
700‐mb dew point depression.
2‐100
K Index values vary depending on the geographical location relative to the Rocky Mountains. East of the
Rocky Mountains a K Index less than 20 indicates no potential for thunderstorm development, 20 to 25
indicates isolated thunderstorm activity is possible, 26 to 30 indicates widely scattered thunderstorms
possible, 31 to 35 indicates scattered, and greater than 35 indicates numerous thunderstorms are
possible.
2.9.6.8 Convective Available Potential Energy
Convective Available Potential Energy, or CAPE, is used to determine the potential for severe weather,
and if the potential exists, how intense the storms may become based on the potential energy as
determined by the size of the positive energy area.
CAPE is expressed as Joules per Kilogram and represents the amount of buoyant energy available to
accelerate the upward vertical motion of a parcel. Higher CAPE values, indicating more energy available
in the positive energy area, indicate a stronger thunderstorm updraft.
CAPE values less than 1,000 indicate marginal instability. Values from 1,000 to 2,500 indicate moderate
instability. Values from 2,500 to 3,500 indicate a very unstable environment and values greater than
3,500 indicate the potential for explosive thunderstorm development.
2.9.6.9 Bulk Richardson Number
The Bulk Richardson Number (BRCH or R) is the ratio of positive buoyant energy within the layer of free
convection to one‐half the square of the shear vector in the first 6 km above the surface. The
calculation for R is complex and includes the CAPE value. Values of R less than 50 indicate the potential
for severe storm development. Values greater than 50 are associated with weaker, multi‐celled
thunderstorms.
2.9.7 INVERSIONS
A layer in the atmosphere where the temperature is either isothermal or increases with an increase in
height (altitude). The lapse rate in the inversion is "NEGATIVE" and represents a stable layer. The base
of the inversion is the point at which the temperature first becomes isothermal or increases. The top of
the inversion is where the temperature begins to decrease again with height.
2‐101
2.9.7.1 Radiation Inversion
A Radiation Inversion is a nocturnal or polar, surface‐based inversion
formed when the lowest layers of air contact the radiationally
cooled surface of the earth. Radiation inversions occur in times of
maximum radiational cooling, normally just before and after sunrise.
The depth and lapse rate of the radiation inversion depend on the
wind speed, amount of cloud cover, type of surface, and the number
of hours of darkness. Figure 2‐46 depicts the sounding during a
radiation inversion event.
2.9.7.2 Subsidence Inversion
A Subsidence Inversion is a mechanically produced inversion
formed by the adiabatic warming of sinking air. The dew point
will rapidly decrease at the base of the inversion as the sinking
air warms and dries during its descent through the air column.
Subsidence inversions are commonly found in areas of high
pressure and significantly suppress convective activity. Figure 2‐
47 depicts a sounding during a subsidence inversion.
2.9.7.3 Frontal Inversion
Frontal Inversions are found in the transition zone between a
surface based cold air mass and the warmer air mass above it.
Frontal inversions and are used to calculate the slope and intensity
of fronts and are depicted as a layer of gradual warming with an
increasing dew point through the inversion. The winds will back
through the inversion with a cold frontal transition zone, and will
veer through the inversion with a warm frontal transition zone.
Figure 2‐48 depicts a frontal inversion.
Figure 2‐46. Radiation Inversion. (Source PDC)
Figure 2‐47. Subsidence Inversion. (Source PDC)
Figure 2‐48. Frontal Inversion. (Source PDC)
2‐102
2.10 SUMMARY
In this chapter, the TAF code and its importance to aviation safety was presented in detail. Also
explained were the procedures used to record and transmit PIREPs, the various types of upper‐air
observations and their application in the Navy and Marine Corps, and the basic procedures for
conducting Rawinsonde and Pibal observations. Then, the two primary code forms used to report
Rawinsonde and Pibal‐observed information, other upper‐air reporting code forms in international
use, and how, by national practice, the United States reports fixed regional level winds in the PILOT code
in addition to reporting the usual information in the TEMP code were presented. Finally, the
disposition of upper‐air observation records followed by an in‐depth review of the Skew‐t and its
application to aviation, as well as stability conditions of the atmosphere were presented.
2‐104
METOC‐045‐841‐609‐001
Chapter Two
Aviation Weather
Chapter Review Questions
2‐106
For questions 1 through 5, refer to the below TAF for Meridian, MS.
KNMM 2115/2215 VRB05KT 9999 VCTS BKN005 OVC011CB QNH2997INS TEMPO 2116/2121 VRB20G30KT 4800 ‐TSRA BKN003 OVC011CB BECMG 2121/2123 12008KT 9000 ‐SHRA BR VCTS BKN030CB BKN100 BKN200 QNH2994INS BECMG 2123/2201 09009KT 9999 NSW SCT015 SCT030 BKN120 QNH2992INS BECMG 2203/2205 VRB04KT 9000 BR SCT007 SCT030 BKN060 OVC120 QNH2996INS TEMPO 2205/2211 1600 BR BKN005 BKN015 OVC060 BECMG 2211/2213 04008KT FEW010 SCT015 SCT080 QNH2990INS T29/2120Z T23/2210Z COR 211513Z
1. A pilot files a military flight plan with an ETA of 2100Z into KNMM. What are the forecasted weather conditions? (Section 2.2.1, pages 2‐2 through 2‐12)
a. 500 Broken, 1,100 Overcast; visibility unrestricted; variable winds of 5 knots; lowest altimeter setting of 29.97 inches; thunderstorms in the vicinity
b. 500 Broken, 1,100 Overcast; visibility unrestricted; variable winds of 5 knots; highest altimeter setting of 29.97 inches; thunderstorms in the vicinity
c. 500 Broken, 1,100 Overcast; visibility unrestricted; variable winds of 5 knots; lowest altimeter setting of 29.97 inches; thunderstorms in the vicinity, with temporary conditions of 300 Broken; 1,100 overcast; visibility 3 miles in light thunderstorms with rain; winds variable 20 knots, gusts to 30 knots
d. 3,000 Broken, 10,000 Broken, 20,000 Broken; visibility 6 miles in light showers of rain and fog; winds southeast at 8 knots; lowest altimeter setting of 29.94 inches; thunderstorms in the vicinity
2. The published field minimums at NAS Meridian are 700 foot ceilings and 1 ½ miles visibility. What is the requirement for an alternate airfield if the ETA is 220800Z? (Section 2.2.1, pages 2‐2 through 2‐11)
a. Ceiling greater than or equal to 1,000 feet and visibility greater than or equal to 2 miles
b. Ceiling greater than or equal to 3,000 feet and visibility greater than or equal to 3 miles
c. Ceiling and or visibility must be greater than or equal to the alternate airfield published minimums
d. NATOPS does not publish requirements for alternate airfield weather conditions
3. What is the total amount of time Meridian will experience visibility 1 mile in fog between 220500Z and 221100Z? (Section 2.2.1, pages 2‐2 through 2‐11)
a. 6 hours
b. 6 hours as long as the visibility does not lower to 1 mile for longer than 3 hours each occurrence
c. The visibility is not expected to lower to 1 mile for longer than 1 hour each occurrence, and not more than 3 hours in the aggregate
d. 3 hours, so long as the visibility does not lower to 1 mile longer than 1 hour each occurrence
2‐107
4. Is an alternate airfield required with an ETA of 220600Z? (Section 2.2.1, pages 2‐2 through 2‐11)
a. Yes, because weather conditions are expected to temporary lower to 500 foot ceiling and 1‐mile visibility
b. No, because the predominate weather conditions are 6,000 foot ceiling and 6 miles visibility in fog
c. No, because the temporary conditions of 500 foot ceiling and 1 mile visibility will not last for more than 1 hour each occurrence so chances are slim conditions will lower at the 220600Z ETA
d. No, because a 500 foot ceiling does not require an alternate airfield
5. Which line or lines of the TAF are valid at 212100Z? (Section 2.2.1, pages 2‐2 through 2‐11)
a. TEMPO 2116/2121
b. TEMPO 2116/2121 and BECMG 2121/2123
c. BECMG 2121/2123
d. KNMM 2115/2215 and TEMPO 2116/2121
For questions 6 through 8, refer to the below TAF for Springfield, MO. KSGF 211757Z 2118/2218 16014G20KT P6SM SCT030 BKN080 FM212100 16012KT P6SM VCTS SCT040CB BKN080 TEMPO 2122/2202 VRB15G25KT 3SM TSRA BKN040CB FM220200 15010KT 4SM SHRA VCTS BKN020 OVC050CB FM220600 33008KT 3SM SHRA VCTS OVC010CB FM221200 32008KT 3SM SHRA OVC008
6. What is the valid time of the FM212100 change line? (Section 2.2.1, pages 2‐2 through 2‐11)
a. 212300Z, because the FM line indicates a change that occurs at a regular or irregular rate beginning at 2100Z and lasting no longer than 2 hours
b. From 212100Z until 220200Z
c. FM211900Z, because the FM line indicates a change that occurs at a regular or irregular rate beginning 2 hours prior to the time of the FM line
d. From 212100Z until 212200Z
7. What is the forecasted ceiling at 220100Z? (Section 2.2.1, pages 2‐2 through 2‐11)
a. 4,000 Broken
b. 8,000 Broken
c. 8,000 Broken, temporarily 4,000 Broken
d. 1,000 Broken
2‐108
8. What is the forecasted visibility at 212130Z? (Section 2.2.2, page 2‐16)
a. 6 Statue Miles
b. Greater than 6 Statute Miles
c. 6 Statute Miles, temporarily decreasing to 3 Statute Miles
d. 3 Statute Miles 9. The FMH‐12 and NAVMETOCCOMINST 3142.1 outline procedures that govern the proper encoding
and dissemination of pilot weather reports (PIREP). (Section 2.3, par 1, page 2‐19)
a. True
b. False
10. A PIREP shall be submitted by a pilot when they encounter weather during the following event(s)? (Section 2.3.1, par 1, page 2‐19)
a. During take‐off
b. Climbing to flight level
c. At flight level
d. All of the above
11. The minimum information required with any PIREP include(s)? (Section 2.3.1, par 3, page 2‐20)
a. The flight level of the aircraft
b. The location of the aircraft
c. At least one meteorological element observed, with time occurrence
d. All of the above
12. The abbreviation used in a PIREP to encode isolated weather phenomena, in accordance with FAA order 7340.1 contractions, is? (Table 2‐8, page 2‐22)
a. ISOD
b. ISOLD
c. ISOL
d. None of the above
13. NMRS is the correct abbreviation for numerous in accordance with FAA order 7340.1 contractions. (Table 2‐8, page 2‐22)
a. True
b. False
2‐109
Match the following Text Element Indicators (TEI) abbreviations in column (a) with the appropriate term in column (b).
(Table 2‐9, page 2‐24)
Column (a) Column (b)
14. /OV a. Temperature (outside air)
15. /TM b. Aircraft Type
16. /TP c. Time (UTC)
17. /TA d. Over Location
18. A PIREP is disseminated if it already reports the sky condition that has been incorporated in a METAR or SPECI observation. (Section 2.3.3, page 2‐25)
a. True
b. False
19. When shall a DD 175‐1 weather briefing be provided to a pilot? (Section 2.4, page 2‐30)
a. Whenever the pilot files an IFR flight plan
b. Whenever the pilot files a combination IFR/VFR flight plan
c. When the pilot requests a VFR certification stamp, but the forecaster expects isolated IMC conditions along the route
d. All the above
20. In order to use a Visual Flight Rule (VFR) certification stamp, the pilot must complete the following? (Section 2.4, page 2‐30)
a. Pilot must file VFR for the entire planned route
b. Pilot must request the stamp
c. Pilot must be flying during daylight
d. Both a and b
Match the following parts of the DD 175‐1 flight weather brief in column (a) with the appropriate term
in column (b). (Figure 2‐10, page 2‐31)
Column (a) Column (b)
21. Part I a. Comments/Remarks
22. Part II b. Takeoff Data
23. Part III c. Enroute & Mission Data
24. Part IV d. Aerodrome Forecasts
2‐110
25. The ETA of a flight is 1530Z and the weather briefed time is 1415Z. What is the Void Time of the DD 175‐1 weather brief? (Section 2.4.5, page 2‐44)
a. 1545Z
b. 1445Z
c. 1600Z
d. 1800Z
26. The ETA of a flight is 1830Z and the weather briefed time is 1600Z. What is the Void Time of the DD 175‐1 weather brief? (Section 2.4.5, page 2‐44)
a. 1900Z
b. 1830Z
c. 1845Z
d. 1630Z
27. When is it authorized for a pilot to file through an issued Weather Watch? (Section 2.4.2, page 2‐35)
a. Operational necessity or emergency situations
b. The forecaster deems the storms are not developing as forecast
c. The performance of the aircraft permits a flight level above the maximum thunderstorm tops
d. All the above
28. From which agencies can you obtain Convective SIGMET information? (Section 2.4.2, page 2‐35)
a. Naval Aviation Forecast Center
b. NOAA Aviation Digital Data Service
c. Fleet Numerical Meteorology and Oceanography Center
d. Both a and b
29. Blocks 23 and 24 are to be used to indicate all turbulence and icing along the route of flight, even turbulence and icing associated with thunderstorms. (Section 2.4.2, pages 2‐37 through 2‐41)
a. True
b. False
30. Which intensity of turbulence causes occupants to feel a definite strain against seat restraints and dislodges unsecured objects in the aircraft? (Section 2.4.2, pages 2‐37 and 2‐38)
a. Light
b. Moderate
c. Severe
d. Extreme
2‐111
31. What factor(s) will increase the rate of ice accumulation on an aircraft in flight? (Section 2.4.2, pages 2‐40 and 2‐41)
a. Slow air speeds
b. Very small super‐cooled water droplets
c. High air speeds, especially air speeds in excess of 575 knots
d. Highly streamlined aircraft
32. Which of the following ceiling and visibility (CIG/VIS) combination(s) require an alternate airfield to be briefed on the DD 175‐1? (Section 2.4.3, page 2‐42)
a. 3,000/3
b. 4,000/5
c. 2,500/7
d. 3,500/4
33. The following module(s) are available within Flight Weather Briefer (FWB)? (Section 2.6.1, page 2‐51)?
a. Pilot
b. Forecaster
c. AIROPS
d. All of the above
34. This tab on FWB provides an alert/messaging capability for the user? (Section 2.6.1, page 2‐52)
a. Home
b. Consoles
c. WX Functions
d. Archive Briefs
35. This tab on FWB allows the FDO to manage canned routes, WX graphic links and access METAR/TAF information? (Section 2.6.1, page 2‐53)
a. Console
b. Archive Briefs
c. WX Function
d. Home
36. FWB does not relieve the pilot of his or her responsibilities to contact a local or remote weather forecast office to ensure receipt of the DD‐175 for the flight of interest. (Section 2.6.1, page 2‐53)
a. True
b. False
2‐112
37. What does the acronym OPARS stand for? (Section 2.7, page 2‐55)
a. Optimum Plane and Aircraft Radar System
b. Optimum Plane and Aircraft Routing System
c. Optimum Path Aviation Route System
d. Optimum Path Aircraft Routing System
38. How many subsystems are contained in the OPARS program? (Section 2.7.1, page 2‐56)
a. 2
b. 3
c. 4
d. 5
39. The OPARS Aeronautical Database is updated at what time interval? (Section 2.7.1.3, page 2‐56)
a. Every 14 days
b. Every 28 days
c. Every 2 months
d. Every 6 months
40. The OPARS Environmental Database consists of wind and temperature fields for what range of flight levels? (Section 2.7.1.4, par 1, page 2‐56)
a. 100 ft through 10,000 ft
b. 1,000 ft through 65,000 ft
c. 100 ft through 65,000 ft
d. 1,000 ft through 100,000 ft
41. Environmental fields that are produced four times daily are derived from what forecast model? (Section 2.7.1.4, page 2‐56)
a. GFS
b. NOGAPS
c. COAMPS
d. UKMET
42. The Optimum Path Aircraft Routing System User's Manual that provides detailed information for processing OPARS flight plans is published by what command? (Section 2.7.2, page 2‐57)
a. Fleet Numerical Meteorology and Oceanography Center (FNMOC)
b. Commander Naval Meteorology and Oceanography Command (CNMOC)
c. Naval Meteorology and Oceanography Professional Development Center (NMOPDC)
d. Naval Aviation Forecast Center (NAFC)
2‐113
43. An upper‐air sounding measures atmospheric elements in what two layers of the atmosphere? (Section 2.8, page 2‐58)
a. Stratosphere and Exosphere
b. Troposphere and Stratosphere
c. Stratosphere and Mesosphere
d. Troposphere and Mesosphere
44. Which type of upper‐air observation is tracked with an optical theodolite (or radar) to determine the low‐level wind speed and direction? (Section 2.8.2.4, page 2‐62)
a. RABAL Observations (Radiosonde Balloon)
b. PIBAL Observations (Pilot Balloon)
c. Rocketsonde Observations
d. Dropsonde Observations
45. Which publication governs upper‐air procedures? (Section 2.8.4, page 2‐63)
a. Federal Meteorological Handbook No. 4 (FMH‐4)
b. WMO publication number 306, Manual on Codes, Volume 1
c. Federal Meteorological Handbook No. 3 (FMH‐3)
d. Federal Meteorological Handbook No. 6 (FMH‐6)
46. What are the standard upper‐air observation times? (Section 2.8.7, page 2‐65)
a. 0000Z, 0300Z, 0900Z, and 2100Z
b. 0300Z, 0900Z, 1500Z, and 2100Z
c. 0000Z, 0600Z, 1200Z, and 1800Z
d. 0300Z, 1200Z, 1800Z, and 0000Z
47. Of the four message parts in the TEMP code, which two parts contain data pertinent to the significant levels? (Section 2.8.9.1, page 2‐68)
a. A and B
b. B and D
c. C and D
d. A and C
48. Which of the following condensation levels is the height at which a parcel of air would become saturated if lifted dry adiabatically? (Section 2.9.4.1, page 2‐90)
a. Convection condensation level (Heated method)
b. Convection condensation level (Moist layer method)
c. Lifting condensation level
d. Mixing condensation level
2‐114
49. Which of the following temperature parameters is used for rain/snow precipitation (PCPN) decisions? (Section 2.9.3.7, page 2‐89)
a. Potential temperature
b. Wet‐bulb temperature
c. Equivalent temperature
d. Wet‐bulb potential temperature
50. Refer to the image to the right. A parcel that is lifted through this atmosphere will ______________ once the lifting force is removed. (Section 2.9.5.4, 2.9.5.5, 2.9.5.6, pages 2‐93 and 2‐94)
a. Continue to rise to the Equilibrium Level whether dry or saturated
b. Return to its original position whether dry or saturated
c. Continue to rise only if saturated
d. Return to is original position only if saturated
A
(Source PDC)