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ASOS GUIDE FOR PILOTS

INTRODUCTION.............................................................................................................................................

1

NOTICESTOAIRMEN..................................................................................................................................... 1

SYSTEMDESCRIPTION................................................................................................................................... 2

WEATHEROBSERVATIONELEMENTS............................................................................................................ 3

DATANOTAUTOMATICALLYPROVIDEDBYASOS........................................................................................ 7

MISSINGDATAANDNONEVENTDATA........................................................................................................ 7

BACKUPANDAUGMENTATION.................................................................................................................... 8

VOICEMESSAGES.......................................................................................................................................... 8

SENSORDESCRIPTION................................................................................................................................... 8

INTRODUCTION

TheAutomatedSurfaceObservingSystem(ASOS)isasurfaceweatherobservingsystemmanagedbythe

NationalWeatherService(NWS),theFederalAviationAdministration(FAA),andtheDepartmentof

Defense

(DOD).

ASOS

is

designed

to

support

aviation

operations

and

weather

forecasting.

This

guide

is

designedtoprovidebasicASOSinformationtopilotsandotheraviationusers. RefertotheASOSQuick

ReferenceHandbookpublishedbytheNationalWeatherServiceforadditionalinformation.

PleaserefertotheFAA'sAeronauticalInformationManualforoperationalguidanceandtotheFAA's

Airport/FacilityDirectory,aeronauticalchartsandrelatedpublications,forASOSbroadcastfrequency,

dialintelephonenumber,andlocationinformation.

NOTICESTOAIRMEN

AirCarrierandCommercialOperatorsshouldrefertoapplicablepartsoftheFederalAviation

RegulationsandtheircompanyOperationsSpecificationsforuseofautomatedweatherobservations.

ASOSwillprovideminutebyminuteobservationsandperformthebasicobservingfunctionsnecessary

togenerateasurfaceweatherobservationandotheraviationweatherinformation.Pilotsneedto

understandthatautomatedandmanualweatherobservationsaredifferentandhavedifferent

operationalimplications.

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ASOSandthehumanobserverdifferinmethodsofdatacollectionandinterpretation.Forelementssuch

aspressure,airtemperature,dewpointtemperature,wind,andprecipitationaccumulation,boththe

automatedsystemandtheobserveruseafixedlocationandtimeaveragingtechnique.

Forvisualelements(i.e.,skycondition,visibility,andpresentweather),observersuseafixedtime,

spatialaveraging

technique

while

the

ASOS

uses

afixed

location,

time

averaging

technique.

Although

thisisafundamentalchange,themanualandautomatedtechniquesyieldsimilarresultswithinthe

limitsoftheirrespectivecapabilities.

SYSTEMDESCRIPTION

TheASOSateachairportconsistsoffourmaincomponents:IndividualWeatherSensors

DataCollectionPackage(s)(DCP) AcquisitionControlUnit(ACU) PeripheralsandDisplays

EveryASOScontainsthefollowing:

CloudHeightIndicator VisibilitySensor PrecipitationIdentifierSensor PressureSensors Temperature/DewPointSensor Winddirection/speedsensor RainfallAccumulationSensor FreezingRainSensorsincludedatmostsites

ASOSdataoutletsinclude:

Onsiteairportusers Nationalcommunicationsnetworks Computergeneratedvoice(throughFAAradiobroadcastanddialintelephonelines) ASOSreportsavailablethroughtheseoutletsinclude:Surfaceweatherobservations METAR(ScheduledWeatherReport) SPECI(UnscheduledWeatherReport) Nonaviationrelatedreports

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WEATHEROBSERVATIONELEMENTS

TYPEOFREPORT STATIONIDENTIFIER

DATE/TIME

OF

REPORT

REPORTMODIFIER WINDDIRECTIONANDSPEED VISIBILITY RUNWAYVISUALRANGE WEATHERPHENOMENA SKYCONDITION TEMPERATURE/DEWPOINT ALTIMETERSETTING REMARKSIDENTIFIER

REMARKS

Ifanelementorphenomenadoesnotoccur,ismissing,orcannotbeobserved,thecorrespondinggroup

andprecedingspaceareomitted(bodyand/orremarks)fromthatparticularreport.Exception:Sea

LevelPressure(SLPppp).SLPNOshallbereportedinaMETARwhentheSLPisnotavailable.

TYPEOFREPORT:METAR:hourly(scheduled)report;SPECI:special(unscheduled)report.

STATIONIDENTIFIER:Fouralphabeticcharacters;InternationalCivilAviationOrganization(ICAO)

locationidentifier.

DATE/TIMEOFREPORT:AlldatesandtimesinCoordinatedUniversalTime(UTC)usinga24hourclock;

twodigitdateandfourdigittime;letterZappendedtoindicateUTC.

REPORTMODIFIER:AUTOindicatesafullyautomatedreport.Nohumanintervention.CORindicatesa

correctiontoapreviouslydisseminatedreport.

WINDDIRECTIONANDSPEED:Directionintensofdegreesfromtruenorth(firstthreedigits);nexttwo

digits:speedinwholeknots;asneededGusts(character)followedbymaximumobservedspeed;always

followedbyKTtoindicateknots;00000KTforcalm.AVariablewindgroupisreportedwhenthe

directionvariesby60ormorewhentheaveragewindspeedisgreatherthan6knots;orifthewind

speedislessthanorequalto6knots.

Example:21016G24KT180V240=Windsarefrom210degreesat16knotswithguststo24knots.The

winddirectionvariesfrom180to240degrees.Ifwindslessthanorequalto6knots;VRB05KT,etc.,

withoutthedegreesofvariance.

VISIBILITY:Prevailingvisibilityinstatuemilesandfractions(spacebetweenwholemilesandfractions);

alwaysfollowedbySMtoindicatestatutemiles;valueslessthan1/4reportedasM1/4SM.

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RUNWAYVISUALRANGE:TenminuteRVRvalueinhundredsoffeet;reportedifprevailingvisibilityis

onemileorless;orRVR6000feetorless;alwaysfollowedbyFTtoindicatefeet;valueprefixedwithM

orPtoindicatevalueislowerorhigherthanthereportableRVRvalue.Example:P6000FT=greaterthan

6000feet,M1000FT=lessthan1000feet.

WEATHERPHENOMENA:

ASOS

reports

rain

(RA):

liquid

precipitation

that

does

not

freeze;

snow

(SN):

frozenprecipitationprecipitationotherthanhail;Precipitationofunknowntype(UP);intensityprefixed

toprecipitation:light(),moderate(nosign),heavy(+);fog(FG);freezingfog(FRFG temperaturebelow

0C);Mist(BR);Haze(HZ);Squall(SQ);augmentedbyobserver:FunnelCloud/Tornado/Waterspout(FC);

Thunderstorm(TS);Hail(GR);Smallhail(GS;

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WINDSHIFT:AwindshiftiscodedintheformatWSHFT_(hh)mm.Itisdefinedasachangeinwind

directionof45degreesormorewhichtakesplaceinlessthan15minutesandhassustainedwindsof10

knotsormorethroughoutthewindshift.ThecontractionFROPAmaybeenteredafterthetimeifthe

windshiftisaresultofafrontalpassage.Example:WSHFT1715FROPA=awindshiftoccurredat1715Z

asaresultofafrontalpassage.

TOWERORSURFACEVISIBILITY:TowervisibilityiscodedasTWR_VIS_vvvvandisthevisibilityreported

bytowerpersonnel.Forexample,TWRVIS2.SurfacevisibilityiscodedasSFC_VIS_vvvvandisthe

visibilityreportedbyASOS.Forexample,SFCVIS2.Visibilityisreportedinstatutemiles.

VARIABLEPREVAILINGVISIBILITY:VariableprevailingvisibilityiscodedasVIS_vnvnvnvnvnVvxvxvxvxvx,

wherevnvnvnvnvnisthelowestvisibilityevaluatedandvxvxvxvxvxisthehighest.Thisisonlyreportedif

thevisibilityislessthan3milesandrapidlyincreasesordecreasesbymileormoreduringthe

observationperiod.Example:VIS1V2=visibilityvaryingfrom1to2miles.

VISIBILITYATSECONDLOCATION:VisibilityatasecondlocationiscodedasVIS_vvvv[LOC]andis

reportedif

lower

than

the

reported

prevailing

visibility

in

the

body

of

the

report.

Example:

VIS

2RWY

11

=visibility2milesatrunway11.

LIGHTNING:LightningiscodedasFrequency_LTG(type)_[LOC];Whendetectedthefrequency,type,and

locationoflightningisreported.Example:FRQLTGCCNE=frequentcloudtocloudlightningtothe

northeast.

BEGINNINGANDENDINGOFPRECIPITATIONANDTHUNDERSTORMS:Thebeginningandendingof

precipitationiscodedaswwB(hh)mmE(hh)mm;thebeginningandendingofthunderstormsiscoded

asTSB(hh)mmE(hh)mm.Example:RAB07E15meansthatrainbeganat7minutespastthehourand

ended

at

15

minutes

past

the

hour.

VIRGA:Virgaisanaugmentedremarkdescribingprecipitationnotreachingtheground.

VARIABLECEILINGHEIGHT:VariableceilingheightiscodedasCIG_hnhnhnVhxhxhx.Itisreportedifthe

ceilinginthebodyofthereportislessthan3,000feetandvariable.Example:CIG020V030=ceiling

heightvaryingfrom2,000to3,000feet.

CEILINGHEIGHTATSECONDLOCATION:CeilingheightatasecondlocationiscodedasCIG_hhh_[LOC].

Itisreportedifasecondaryceilometersiteisdifferentthantheceilingheightinthebodyofthereport.

Example:CIG030RWY11=ceilingheight3000feetatrunway11.

PRESSURERISING

OR

FALLING

RAPIDLY:

This

is

coded

as

PRESRR(pressure

rising

rapidly)

or

PRESFR(pressurefallingrapidly).Itisariseorfallinstationpressureattherateof0.06inchesormore

perhourwhichtotals0.02inchesormore.

SEALEVELPRESSURE:SealevelpressureiscodedasSLPppp,wherepppisthesealevelpressurein

hectopascals(hPa.).ExampleSLP982=998.2hPa.Ifsealevelpressureisnotavailable,itiscodedas

SLPNO.

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ADDITIVEDATA:Thefollowingelementgroupsarereportedatspecifictimesduringtheday:

HOURLYPRECIPITATIONAMOUNT:Hourlyprecipitation(waterequivalent)sincethelastMETARis

codedasPrrrrandiscodedinhundredthsofaninch.AtraceisP0000.Example:P0009=.09inchesof

precipitation.

3 AND6HOURPRECIPITATIONAMOUNT:3 and6hourprecipitation(waterequivalent)iscodedas

6RRRR.Theprecipitationamountinhundredthsofaninchforthepast3hoursisreportedin03,09,15,

and21UTCobservations.Theprecipitationforthepast6hoursisreportedin00,06,12,and18UTC

observations.Atraceisencodedas60000.Example:60116=1.16inchesofprecipitation.

24HOURPRECIPITATIONAMOUNT:24hourprecipitation(waterequivalent)iscodedas

7R24R24R24R24.Theprecipitationamountinhundredthsofaninchforthepast24hoursisreportedin

the12UTCobservation.

HOURLYTEMPERATUREANDDEWPOINT:Hourlytemperatureanddewpointarecodedas

TsnTaTaTasaTaT

aT

a.

Both

are

measured

in

tenths

of

adegree

Celsius.

sn

is

coded

as

1ifthe

temperatureisbelow0Cand0ifthetemperatureis0Corhigher.Example:T00241011=the

temperatureis2.4Candthedewpointis 1.1C.

6HOURMAXIMUMTEMPERATURE:6hourmaximumtemperatureiscodedas1snTxTxTxandis

measuredintenthsofadegreeCelsius.sniscodedas1ifthetemperatureisbelow0Cand0ifthe

temperatureis0Corhigher.Example:10214=21.4C,11134= 13.4C.

6HOURMINIMUMTEMPERATURE:6hourminimumtemperatureiscodedas2snTnTnTnandis

measuredintenthsofadegreeCelsius.sniscodedas1ifthetemperatureisbelow0Cand0ifthe

temperatureis0Corhigher.

24HOURMAXIMUMANDMINIMUMTEMPERATURE:24hourmaximumandminimumtemperature

arecodedas4snTxTxTxsnTnTnTnandarereportedatmidnightlocalstandardtime.Botharemeasured

intenthsofadegreeCelsius.sniscodedas1ifthetemperatureisbelow0Cand0ifthetemperatureis

0Corhigher.

PRESSURETENDENCY:Thisiscodedas5appp,whereaisthecharacter(trend)ofpressurechangeand

pppisthechangeinpressureintenthsofhPaoverthepastthreehours.Example:58018=pressure

fallingthenfallingrapidly1.8hPa.

ENSORSTATUSINDICATORS:Thesemaybereported:RVRNO:RVRmissing,PWINO:precipitation

identifierinformation

not

available,

PNO:

precipitation

amount

not

available,

FZRANO:

freezing

rain

informationnotavailable,TSNO:thunderstorminformationnotavailable,VISNO[LOC]:visibilityat

secondlocationnotavailable,and;CHINO[LOC]:skyconditionatsecondarylocationnotavailable.

MAINTENANCECHECKINDICATOR:Thesymbol$willappeariftheASOSdetectsthatapreventative

maintenancecheckisneeded.

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DATANOTAUTOMATICALLYPROVIDEDBYASOS

WeatherelementsnotcurrentlyprovidedautomaticallybyASOSarelisted.Elementsmarkedwithan

asterisk(*)maybeaddedbyanobserver(seeBackupandAugmentation).

Tornado*

Snowfall

Funnelcloud Snowdepth

Waterspout Snowwaterequivalent

Thunderstorms* Cloudsabove12,000FT

Hail* Virga*

Icecrystals Distantprecipitation

Snowpellets Distantclouds

Snowgrains Localvariationsin

Icepellets Visibility

Drizzle Towervisibility*

Freezingdrizzle Specificvisibilityvaluesabove10miles

Volcanic

Ash*

Blowing

snow

Dust Blowingsand

Groundfog Smoke

Blowingdust Icefog

Blowingspray loudtypes

NOTE:ThedegreeofaugmentationatservicelevelAorBsitesincludesotherparametersinadditionto

thosewiththeasterisk.

MISSINGDATAANDNONEVENTDATA

Intheeventthattheprecipitationidentifiersensorisnotoperational,theremarkPWINO(present

weatherinformationnotavailable)willbeplacedintheSTATUSREMARKSsectionoftheobservation.In

thiscase,ASOSwillnotbeabletoreportRA,SN,orUP.IftheFreezingRainsensorisnotoperational,

theremarkFZRANO(Freezingraininformationnotavailable)willappearintheSTATUSREMARKS.When

noprecipitationisoccurringandthetemperatureordewpointareinoperative,anobstructiontovision

isreportedashazeifthevisibilityis4or5statutemiles.Ifthevisibilityislessthan4statutemiles,fogor

mistwillbereported.Ifthetemperatureordewpointsensor,andthevisibilitysensorareinoperative

ASOScannotreportanobstructiontovision.TheMaintenanceCheckIndicator($)willappearinthe

STATUSREMARKSiftheASOSrequiresrepairorifroutinepreventativemaintenanceisneeded.Inthe

examplebelow,

wind

speed

and

direction,

RVR,

and

temperature

are

missing,

so

they

are

omitted.

The

MaintenanceCheckIndicator($)attheendoftheobservationshowsthattheASOSrequires

maintenance

METARKABC121755AUTO1SMBKN015A2990$

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BACKUPANDAUGMENTATION

Backupistheprocessofmanuallyeditingspecificelementswithintheobservationorprovidinga

completemanualobservationandalternatemeansofdisseminationincaseoftotalASOSfailure.

Backupincludessubstitutingmanuallyobserveddatafor"missing"orunrepresentativedatatoensure

thatobservationcontentiscorrectandcomplete.

AugmentationistheprocessofaddinginformationtoanASOSobservationthatisnotprovidedor

observedautomatically.Thisinformationisderivedbyanobserver.Anexampleofaugmentationwould

beaddingtheoccurrenceofathunderstormtotheobservation.

VOICEMESSAGES

ASOScomputergeneratedvoicemessagearemadeavailablethroughtelephonedialinaccessandare

broadcastdirectly

to

pilots

via

ground

to

air

radio

(where

installed).

The

information

contained

in

the

groundtoairradiobroadcastmessageandthetelephonedialinmessageareidentical.Thevoice

messageiscomposedofthebodyoftheASOSobservationandselectedremarks.Selectedremarkscan

includevariablevisibility,towervisibility,orDensityAltitudewhen1,000ormorefeetaboveairport

elevation.ControltowerpersonnelmayaddNoticetoAirmen(NOTAM)orotherinformation.Notethat

winddirectioninthevoicemessageisgivenindegreesmagnetic.

SENSORDESCRIPTION

SKYCONDITION:

ASOS

sky

condition

is

determined

by

alaser

ceilometer

referred

to

as

the

Cloud

Height

Indicator(CHI).TheCHIisusedtodetectthepresenceofcloudsdirectlyoverheadupto12,000feet

AGL.

VISIBILITY:ASOSvisibilityisbasedonlightscattering.Thevisibilitysensorprojectsabeamoflightover

averyshortdistance,andthelightthatisscatteredisdetectedbyareceiver.Theamountoflight

scatteredandthenreceivedbythesensorisconvertedintoavisibilityvalue.

PRESENTWEATHERANDOBSTRUCTIONSTOVISION:TherearetwoASOSpresentweathersensors.The

PrecipitationIdentifier(PI)sensordiscriminatesbetweenrain(RA)andsnow(SN).TheFreezingRain

(FZRA)

sensor

detects

freezing

rain.

ASOS

evaluates

multiple

sensor

data

and

infers

the

presence

of

obstructionstovision.

ThePIsensorhasthecapabilitytodetectandreport RA,RA,+RA, SN,SN,+SN.Whenrainandsnow

aremixedandtheprevailingprecipitationtypecannotbedetermined,ASOSwillreportUP.

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TheFreezingRainsensormeasuresaccumulationratesaslowas0.01inchesperhour.Iffreezingrainis

detectedandthePIsensorindicatesnoprecipitationorrain,thenfreezingrainisreported.Iffreezing

rainisdetectedwhenthePIindicatessnow,thensnowisreported.

ObstructionstovisionarenotdirectlymeasuredbyASOS,butinferredfrommeasurementsofvisibility,

temperature,and

dew

point.

There

are

only

two

reported

by

ASOS:

Fog

(FG)

and

Haze

(HZ),

and

only

whenthevisibilityisbelow7statutemiles.

TEMPERATUREANDDEWPOINT:TheASOStemperatureanddewpointsensorsdirectlymeasurethe

airanddewpointtemperatures.

WIND:ASOSsenseswindspeedanddirectionusingarotatingcupanemometerandwindvane.Wind

characterandpeakwindareobtainedbycomparingaverageandmaximumwindspeeds.

PRESSURE:Becausepressuremeasurementiscritical,threeseparateandindependentpressuresensors

areusedatlargerairports.Twopressuresensorsareusedatotherlocations.

PRECIPITATIONACCUMULATION:ASOSusesaHeatedTippingBucketprecipitationgauge.