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CHAPTERFOUR
INTRODUCTIONTOENVIRONMENTALSATELLITES
4.1 INTRODUCTION
Satelliteimages,orpictorialrepresentationsofsatellitesensedinformation,aresomeofthemost
frequentlyusedtoolsinthefieldsofmeteorologyandoceanography. SincethelaunchofTIROS1on
April1,
1960,
numerous
satellites
with
ever
increasing
capabilities
and
sophistication
have
been
deployedintospace,revolutionizingtheunderstandingandaccuracyofmeteorologicaland
oceanographicprocessesandpredictionswiththeadventofeachnewsatellite.AsaNavyorMarine
Corpsforecaster,orassistantforecaster,itisimportantforyoutoknowhowobtainrequiredsatellite
imagery,andbeabletointerpretitinordertoidentifyspecificdetailsimportanttothesuccessand
safetyof
Naval
operations.
The
use
of
satellite
imagery
is
one
of
the
most
important
sources
of
informationwhenpreparingmeteorologicalandoceanographicforecasts.
Webeginwithanexplanationofremotesensingandhowelectromagneticradiationisusedtodevelopa
satelliteimage. Next,weintroducethevarioustypesofsatelliteimageryanddescribethevarious
considerationsto takeintoaccountwhenviewingit. Wethendiscussthedifferenttypesofsatellite
vehiclesused
in
obtaining
imagery
and
complete
the
chapter
by
discussing
analysis
techniques
used
in
identifyingcloudformations,noncloudfeatures,andcertainmeteorologicalfeatures.
4.2 WEATHERSATELLITEPRINCIPLES
LearningObjective
Identifythe
advantages
and
disadvantages
of
satellite
imagery
4.2.1 AdvantagesandDisadvantagesofSatelliteImagery
Theuseofsatelliteimageryhassomedistinctadvantagesanddisadvantages. Twomajoradvantagesare
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providestheforecasterwiththeabilitytotrackdevelopingmesoscalefeaturesthatwouldotherwisebe
missedinthelargesynopticanalysis.
Thebiggestobstacletotheuseofsatelliteimageryisinterpretation. Drawingconclusionsaboutthe
meteorologicalprocessesatworkonsatelliteimageryismuchmoredifficultthanreachingthesame
conclusionsfromconventionalconstantpressurecharts. Ittakestraining,timeandpracticeto
effectivelyutilizeasatelliteimage. Additionally,satelliteimageryonlygivesinformationfromthetop
down. Itisnormallyimpossibletogarnerdetailedinformationaboutsurfaceweatherconditionsata
specificpoint
with
just
asatellite
image.
Surface
conditions
can
be
deduced
through
training,
but
the
sameinformationcanbegainedwithclaritythroughtheobservationnetwork,whereestablished,which
providesinformationfromthebottomup. Hence,whenforecastingorbriefing,itisimportanttoutilize
bothsatelliteimageryandsurfaceweatherobservationstogethertoaccuratelyportraythestateofthe
atmosphere.
4.2.2 RemoteSensingofElectromagneticRadiation
LearningObjective
Describethesatellitesensorimagingprocess.Remotesensingisatermusedtodescribethestudyofsomethingwithoutmakingactualcontactwithit.
Satellitetechnologyisanexampleofremotesensing,sincesatellitesensorsaredesignedtostudy
energyreflected,andemittedfromtheEarth. Weathersatellitesensorsworkbygathering
electromagneticradiationfromtheEarthandatmosphere. Youneedabasicknowledgeofradiation
theorytounderstandhowthesensorswork.
Electromagneticradiationismadeupofwaves,soitrequiresanunderstandingofwavemotionbasics.
Holduptheendofalongpieceofropeandquicklymoveyourhandupanddowntoproduceawave
thattravelsdownthelengthoftherope. Ifyoudothisrepeatedlyandregularly,aregularseriesof
waveswilloccur. Howoftenitrepeatsisthefrequencyofthemotion(i.e.,howfrequentlythemotion
repeats). Frequencyisthenumberofwavespassingagivenpointperunitoftime,expressedincycles
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Wedescribetheintensityofawavebymeasuringthewavesheight,oramplitude. Radiationdiffersin
thisrespect. Abeamofelectromagneticradiationresemblesastreamofparticlescalledphotons. Each
photonhasitsownspecificwavelength. The
totallevelofenergyinabeamofradiation(the
beamsradianceorintensity)isjusttheenergy
ofeachphoton,timesthenumberofphotons
inthatbeam. Insatellitemeteorology,the
preferredterm
for
energy
output
is
brightness
temperature,usuallyexpressedindegrees
Kelvin(K). Brightnesstemperatureisaphoton
count.
Electromagneticradiationhasspecific
wavelengthsand
frequencies,
and
resembles
more
common
waves
such
as
water
and
sound
waves.
Whiletherearemanytypesofelectromagneticradiation,theonlyrealdifferencebetweenthemistheir
wavelengthorfrequency. Webringupwavelengthandfrequencybecausebothareusedinsatellite
applications. Mostusersofvisualandinfraredsatelliteimageryprefertoclassifyradiationby
wavelength,usuallymeasuredinmicrons(m),ormillionthsofameter. Usersofmicrowaveimagery
liketo
use
frequency.
Most
microwave
radiation
has
frequencies
measured
in
the
Gigahertz
range
(1
GHzisabillioncycles/second).
Ingeneral,shortwavelengthsandhighfrequenciesarecharacteristicsofhighenergyradiation,usually
comingfromtheSun,andlongwavelengthsandlowfrequenciesarecharacteristicsoflowenergy
radiation,usuallycomingfromtheEarth. Weseparateradiationintoseparatebands,basedonitsuse,
oron
how
familiar
objects
react
to
it.
This
is
known
as
the
electromagnetic
spectrum.
The
electromagneticspectrumisacontinuumofallthetypesofelectromagneticradiation. Figure41
showstherangeoftheelectromagneticspectrum,withitsrespectivedivisions. Thehumaneyedetects
onlyasmallportionoftheelectromagneticspectrum,calledvisiblelight.
Figure41. ElectromagneticSpectrum
(Source:PDC)
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Therearealsoareaswithinthe
electromagneticspectrumwherethe
atmosphereistransparenttospecific
wavelengths. Thesewavelengthbandsare
knownasatmosphericwindows,sincethey
allowtheradiationtoeasilypassthroughthe
atmosphere. Therearespecificatmospheric
windowscorresponding
to
specific
wavelengthsalongtheelectromagnetic
spectrum.Thesensorsonmeteorologicaland
oceanographicsatellitesaredesignedtotake
advantageoftheseatmosphericwindows.
Theseinstruments
measure
received
radiation
inspecific,narrowwavelengthbandsknownas
channels. Bytakingadvantageofatmospheric
windows,thesatellitecansensetheamount
ofelectromagneticradiationreceivedfrom
specificregionsoftheEarth. Thesensorthen
convertstheamountofsensed
electromagneticradiationtoagrayshade,
whichisassignedtoacorrespondingsquare
ontheimagecalledapixel. Allelectronic
images,suchassatelliteimages,consistof
pixels,which
directly
relate
to
the
resolution
of
the
image.
A
computer
assigns
one
of
256
shades
of
gray(rangingfromblacktowhite)toeachpixelontheimagery. Thecontrastsingrayshadeshelpus
interpretcloudandnoncloudphenomenaonthesatelliteimagery. Humaneyescandistinguishonly
about1520grayshades. Becauseofthis,imageryiseithergrayshadeorcolorenhancedtomake
Figure42. GrayscaleEnhancement. (Source:
UniversityofWisconsin)
Figure43. ColorizedEnhancement. (Source:
UniversityofWisconsin)
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4.3 TYPESOFSATELLITEIMAGERY
LearningObjective
Identifyanddescribethedifferenttypesofsatelliteimageryavailableforuse.WeathersatelliteshaveinstrumentscapableofdetectingradiationfromcloudsaswellastheEarths
surface. Theseinstrumentshavesensorsthatdetectboththevisible(VIS)rangeoftheelectromagnetic
spectrumandtheinfrared(IR)range. Thisallowsforbothdaytimeandnighttimeimageryandprovides
theabilitytocomparetheinfraredandvisualimagesoverthesameregion.
Environmentalsatellitesprovidedatathroughseveraldifferentchannels. Eachchannelsensesradiation
ataspecificwavelengthorarangeofwavelengths. Themostcommonlyusedchannelsonweather
satellitesarevisible,infrared,andwatervapor. Speciallydesignedsensorswithspecificchannelsare
usedtopickupmicrowaveradiation. Eachofthesechannelsaresensitivetoenergyinitsparticular
rangeof
frequencies;
therefore,
each
type
provides
adifferent
view
of
the
Earth
and
its
atmosphere.
Meteorologistsrelyonallfourtypesofdataforunderstandingtheinteractionsbetweenthe
atmosphereandtheEarth'ssurface.
4.3.1 VISUALIMAGERY(VIS)(0.40.74M)
Theelectromagneticenergythatyoureyescanseerangesfromawavelengthof0.7mforredlight,
throughthevisiblespectrum(red,orange,yellow,green,blue,indigoandviolet)to0.4mforviolet
light. About44percentofthesun'senergyfallsontheearthintheformoflight. Althoughsomelightis
absorbed,muchofthelightincidentontheearth'satmosphereandsurfaceisreflectedbackintospace.
ThereflectedlightfromtheEarthismeasuredbyachannelinthesensoraboardthesatellitethatis
sensitiveonlytoelectromagneticenergyinthevisualwavelengths.Thesensormeasurestheenergy
receivedineachpixelandassignsitareadingfrom0,fornoenergysensed,to256,forveryhighenergy
sensed. MeasurementsaretransmittedtoEarth,andtheconsecutivepixelsandscanlinesare
processedtocomposeanimage.
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bulb(youareincreasingthe
angleofthelighthittingthe
paperssurface),thedarkerthe
paperisgoingtoappear. The
sunsangleisafunctionofthe
timeofday,theseason,andthe
latitudeofyourlocation. Albedoisdependentontheobject'ssurfacetextureandcolor. Table41
providessome
common
albedos
of
various
surfaces
on
the
Earth.
Invisualimages,areasoflowreflectedlight(lowalbedo),suchaswaterandforestregions,appearblack.
Areasofhighreflectedlight(highalbedo),suchassnow,appearwhite.Whenlookingatcloudsonvisual
imagery,theopticaldepthisahighlyimportantaspecttoconsider. Opticaldepthandreflectivityare
directlyproportional,inotherwords,acloudwithahighopticaldepthishighlyreflective. Theoptical
depthof
an
object
depends
on
four
factors:
thickness,
cloud
density,
cloud
composition,
and
particle
size.
Visibleimageryisveryusefulinbothatmosphericandoceanographicanalysisbecausereflectivityvaries
considerablyamongatmospheric,land,andoceanicfeatures.Duetothereflectivepropertiesofclouds
composedofwaterdroplets,avisualimageisalsoexcellentforidentifyinglowcloudswhencompared
toan
infrared
image.
An
obvious
disadvantage
of
visible
imagery
is
that
it
is
only
available
during
daylighthours. Anotherdisadvantageisthatlow,mid,andhighclouds,whenplacedovereachother,
canbedifficulttoidentifywithouttheuseofaninfraredimage.
4.3.2 INFRAREDIMAGERY(IR)(0.7412M)
Theinfraredsensorsmeasuretheamountofenergyemittedbytheearthandtheatmosphere. Because
thefrequencyofenergyemittedfromtheearthssurfaceandmeteorologicalelementsdependsonthe
temperatureofthesurface,IRimageryisessentiallyapictureofthesurfaceandcloudtoptemperatures
portrayedinblack,white,orgrayshades. Thisinformationcanbeusedtoobservethermalpropertiesof
the Earth and the atmosphere
Table41. CommonEarthAlbedos.
LargeThunderstorm 92% ThinStratus 42%
FreshNew
Snow
88%
Thin
Cirrostratus
32%
ThickCirrostratus 74% Sand,NoFoliage 27%
ThickStratocumulus 68% SandandBrushwood 17%
WhiteSands,NM 60% Coniferous 12%
Snow,37DaysOld 59% WaterSurfaces 9%
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availableforuse24hoursaday. AmajordisadvantageofIRimageryisthedifficultyindisplayinglow
cloudswhentheearthssurfaceandcloudtoptemperaturesarerelativelythesame.
Onmostdisplaysystems,thegrayscaleofanIRimageiscomposedof256grayshadesrangingfrom
white(coldesttemperatures)toblack(warmesttemperatures)andcorrelatessensedtemperaturewith
grayshadesinasimplelinearrelationship. Inaninfraredimage,thehighest(andthereforecoldest)
cloudtopsappearwhite. Lower,warmercloudsappearaslightershadesofgray,andwarmerlandand
watersurfacesappearasdarkershadesofgray. TheIRbandisseparatedintothreechannelsonmost
satellites,theNearIR,FarIR,andFarFarIR.
4.3.2.1 NearInfraredImagery(NIR)(0.741m)
Thiswavelengthisalsoreferredtoasnearvisualbecausewearestilldealingwithreflectedlight. The
wavelengthusedhereisjustoutoftherangeofwhatoureyescansee. Somefeatureshavehigher
reflectivityin
this
wavelength
than
they
do
in
the
visual.
These
include
aerosols
and
land,
especially
withvegetation.
SomefeaturesemitenoughradiationintheNIRwavelengthtobeseen. Citylightsandfiresemitmore
radiationinthisbandthaninvisiblewavelengthsbecausetheiremissionpeakisclosertotheNIR
wavelengthsthanvisualwavelengths. Forthisreason,thistypeofimageryisusedinthedetectionof
andtracking
forest
fires
and
monitoring
of
urban
heat
islands.
4.3.2.2 FarInfraredImagery(FIR)(10.211.2m)
Thisisthegardenvarietyinfraredimagemostweatherforecastersareusedtoviewing. Most
satellitescombinetheFIRandFFIRchannelsintoonebroadbandIRchannel.Thesamefactorsthat
affectopticaldepthinthevisualbandsaffecttheIRwavelengths,butfordifferentreasons. Remember
thatinthevisualbands,cloudswithahighopticaldeptharegoodatreflectingradiation. IntheIRbands,
cloudswithahighopticaldeptharegoodatabsorbingradiation. Byabsorbingradiation,wemeanthat
thecloudisblockingradiationfrombelowandreemittingradiationatitsownphysicaltemperature.
Thin cirrus clouds that are almost invisible in the visual wavelengths can absorb and block some FIR
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4.3.2.3 FarFarInfraredImagery(FFIR)(11.312.5m)
When
comparing
FIR
and
FFIR
imagery,
little
difference
would
be
noticed.
The
only
noticeable
differencewouldbefoundinareascontainingmoistair. Theatmospherehashigherabsorptivity(lower
transmissivity)asenergyapproachestheFFIRwavelengths,mostlyduetoabsorptionbywatervapor. In
moistatmospheres,thesatellitesenseslessFFIRradiationthanFIR. Therefore,moistairappearscooler
andmurkyonanFFIRimage.Dryairwouldlookthesameinbothchannels.
4.3.3
WATERVAPOR
IMAGERY
(WV)
(6.5
TO
7.0
M)
Thistypeofimageryiscreatedbyfocusingthesatellitesensoronaverynarrowbandofwavelengths
wheretheabsorptionofemittedradiationbywatervaporisveryhigh. Theearthabsorbsincoming
solarradiation(shortwaveradiation)andreradiatesthatenergyaslongwaveradiation. Theunique
abilityofwatervaportoabsorbthewavelengthsconcentratedat6.7mprovidesanextremelyuseful
toolwe
use
to
identify
mid
and
upper
level
features
that
may
be
cloudless
and
not
evident
on
visible
or
broadspectrumIRimagery. Wherethesatellitesensesabundantenergyat6.7m,thereisalackof
moistureatthemiddleandupperlevelsoftheatmosphere. Inthiscase,thesatelliteassignsadarker
grayshadetothatpixel. Wherethesatellitesensesminimalenergyat6.7m,thereisabundant
moistureatthemiddleandupperlevelsandthesatelliteassignsalighterorwhitergrayshadetothat
pixel.
Itisimportanttonotehere,thatwatervaporimagerydoesnotindicatemoistureinthelowerlevelsof
theatmosphere,onlythemiddleandupperlevels. Ifmoistureispresentinthemiddleandupperlevels,
energyat6.7misabsorbedandanymoisturepresentinthelowerlevelscannotbedetected. The
satellitewillreceiveminimalamountsofenergyinthismoistureregionandinterpretthelackofenergy
receivedascoldertemperaturesandhighermoisturecontent. Ifmoistureisminimalinthemiddleand
upperlevelsoftheatmosphere,energyemittedfromtheatmosphereslowerlevelsisnotabsorbedby
themiddleandupperlevels. Thesatellitewillreceiveabundantamountsofenergyinthismoisturefree
regionandinterprettheabundanceofenergyreceivedaswarmerandlowermoisturecontentand
assign a dark gray shade
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4.3.4 MICROWAVEIMAGERY
The
Earth
constantly
emits
microwave
radiation.
The
Sun
also
emits
microwave
radiation,
but
at
amountssmallcomparedtothevisualoutput. SolarmicrowavesarenotnormallyreflectedbytheEarth,
butareinsteadabsorbed. Therefore,wecanassumeallradiationbeingreceivedbythesatellitesensor
inthisfrequencyrangeisemittedbyEarth. Theadvantageofmicrowaveimageryisthatdiurnal
changesinradianceoreffectsofsunangleduetotherisingandsettingoftheSundonotimpede
accurateinterpretation.
Microwaveshavelongerwavelengthsandlowerfrequencieswhichareusuallymeasuredinmmorcm.
TheirfrequencyisintheGHz(billioncycles/sec)range. Microwavebandsareusuallycategorizedby
frequency. Mostotherelectromagneticspectrumbandsmeteorologistsusearecategorizedby
wavelength. Becauseofthelongerwavelength,themicrowaveimageryresolutionislowerthanvisual
andinfraredimagery. Typically,resolutionisbetween20and50kmformostproducts. Ontheplusside,
thereisusuallylessattenuationinmostmicrowavechannelsthaninotherbands. Microwaves
penetratecloudsandvegetation,makingthemidealforsurfacemonitoring.
Radianceisusedtodescribemicrowaveradiation. Earthsmicrowaveemissionsshowtremendous
varianceinradiance,moresothantheconventionalbandsusedbysatellitesensors. Therefore,
brightnesstemperaturesareusedtomeasuremicrowaveradiationandtodifferentiatebetween
features. Manysurfaceandatmosphericfeaturescanbeidentifiedbyauniquemicrowaveradiance
signature. Radiancevariablesusedinproductionofmicrowaveimageryincludefrequency,polarization,
emissivity,transmissivity,andspatialuniformity.
4.3.4.1 Activevs.PassiveMicrowaveSensing
Thereare
two
kinds
of
microwave
instruments
in
space:
passive
and
active.
Active
refers
to
asensing
strategyinwhichthesatellitecontinuallysendsmicrowavepulsesofenergytowardthesurfaceofthe
earth. ThemostcommonactiveapplicationforMETOCisscatterometry,whichprovidesoceanwind
speedanddirectiondata(avectorquantity)asseeninFigure45. Inthisregard,activemicrowave
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Passivereferstoasensingstrategyinwhichthesatellitereceivesmicrowaveenergynaturallyemitted
orscatteredfromtheatmosphereandsurface. Theseinstrumentshavenopowersource. As
instruments,
they
are
not
as
capable
as
active
instruments.
For
example,
most
passive
microwave
sensorscanonlyproduceoceanicwindspeed,notdirection,asdisplayedinFigure46. NoticeinFigure
45thesatellitederivedproductindicateswinddirectionandspeed,acapabilityofanactivesensor,
whileinFigure46onlyspeeddataisavailablefromthepassivemicrowavesensor. Anadvantageof
passiveinstrumentsisthattheydonotrequireanonboardpowersupply.
4.4SATELLITE
IMAGERY
VIEWING
CONSIDERATIONS
LearningObjective
Identifyspecificviewingconsiderationsthatmustbeconsideredwheninterpretingsatelliteimagery.
4.4.1 RESOLUTION
Theword
resolution
is
often
used
with
respect
to
satellite
image
data.
Usually,
it
refers
to
spatial
resolution,thesizeofthefootprints,orpixels,thatformanimage. However,therearethreeother
kindsofresolutiontoconsideraswell.
4 4 1 1 S i l R l i
Figure45. ActiveQuikScat. Figure46. PassiveTRMM.
(Source: UniversityofWisconsin)
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Earthssurfacedirectlyunderthesatellitesensor. Thispointiswhereresolutionishighestonasatellite
image. Radiatingoutwardfromthispoint,theresolutiongraduallydecreases(theFOVgradually
increases)duetothecurvatureoftheEarthandthedecreasingangleofincidencefromthesensorto
theearth.
SatellitesensorsdesignedtoproduceimagesofEarth,itsoceans,anditsatmosphereareverydifferent
fromthecamerasusedtotakeaphotograph.Theyaremorelikeavideocamera,onlymuchmore
specialized.Thesescanningsensorsarecalledradiometers,andinsteadoffilm,anelectroniccircuit
sensitiveonlytoasmallrangeofelectromagneticwavelengthsmeasurestheamountofenergythatis
received.Satellitescarryseveraldifferentimagesensors,eachofwhichissensitivetoonlyasmallband
ofenergyataspecificwavelength.
RadiometersscanacrossthesurfaceoftheEarthinconsecutivescanlinesalongapathnormaltothe
directionoftravelofthesatellite.Astheradiometermovesthroughascanline,itveryrapidlymeasures
energylevelsforonlyaverysmallportionoftheEarthatatime. Eachindividualenergymeasurement
willcomposeapixeloftheoverallsatelliteimage. Thesizeofthearea(FOV)scannedbythesensor
determinesthespatialresolutionoftheoverallimage. Thus,thesmallertheareascannedforeachpixel,
thehigherthespatialresolution. Someradiometersmayscananareaassmallas0.5kmacross(high
resolution),whileothersscanareasaslargeas16km(lowresolution). Whencomposedintoanimage,
smallerpixels
allow
the
image
to
be
much
clearer
and
show
greater
detail.
Clouds
and
land
boundaries
appearbetterdefined.Ifobjectsaresmallerthanthesensorresolution,thesensoraveragesthe
brightnessortemperatureoftheobjectwiththebackground. Normally,thesensorsaboardsatellites
areabletoprovidebetterresolutionforvisualimagerythanforinfraredimagery. Also,lowearth
orbitingsatellitesusuallyprovidehigherresolutioncapabilitiesthangeostationarysatellitesduetotheir
closeproximitytotheEarth.
4.4.1.2 SpectralResolution
Spectralresolutionreferstothenumberofbandsintheelectromagneticspectruminwhichthe
instrument can take measurements A greater amount of channels means one can observe an increased
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datasetwithahighernumberofbands. Thedifferentchannelscanbecombinedintoalgorithms,
whicharelikerecipesforderivingtheinformationsought.
4.4.1.3 RadiometricResolution
Radiometricresolutionreferstothesensitivityoftheradiometertosmalldifferencesintheradiation
emittedfromanobservedobject. Thegreatersensitivityaninstrumenthas,themoredetailedthe
imageitcanproduceforusers.
4.4.1.4 TemporalResolution
Temporalresolutionisthefrequencywithwhichasatellitecanrevisitanareaofinterestandacquirea
newimage. Geostationarysatellites,seeingthesameareaasoftenasevery15min,havehigher
temporalresolutionthanlowearthorbiting
satellites,whichviewtheearthperhapstwiceaday.
Thisiswhygeostationarysatelliteimagescanbe
animatedandlowearthorbitingsatelliteimages
cannot.
4.4.2 ATTENUATION
Attenuationisdefinedasthelossofenergydueto
absorptionandscatteringofterrestrialradiationby
atmosphericelements. Longwaveterrestrial
radiationreleasedintotheatmosphereisabsorbed
byboththecloudsandatmosphereitself. Absorptionofterrestrialradiationiscriticaltowarmingour
atmosphereand
sustaining
life
on
Earth.
Clouds
and
suspended
particles
in
the
atmosphere
also
scatter
thisradiation. Thisprocessreducestheamountofenergyreachingthesatellitesensorsocloudtops
appearcolder,andhencehigher,thantheyactuallyare. ThisaffectsIRandWVimageryonlyandisthe
principlebehindwatervaporimagery.
Figure47. Aspectsofattenuation.
(Source:PDC)
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sincetheoblique(shallow)viewingangleincreasestheamountofatmospherethroughwhichthe
energymusttravel.
4.4.3 CONTAMINATION
Contaminationoccurswhenenergyreachesthe
satellitesensorfromtwoormoresources. Thiscan
occurwithvisualorinfraredimagery. Theamountof
contaminationdepends
on
two
things:
the
spacing
of
thecloudelementsandthethicknessofthecloud
layerorlayers,asseeninFigure48. Oneexample
wouldbeinastratocumulusfield,whenthelandor
oceansurfaceisevidentbetweencloudelements.
AnotherwouldbewhentheenergyfromthewarmEarthoralowclouddeckbleedsthroughathin,
upperlevelclouddeckcausingthethinupperlevelcloudlayertoappearwarmerandlower. Likewise,
onavisualimage,whenhighercloudsarethinenough,theEarth'ssurfaceorlowercloudscanbeseen
throughtheclouds. Inallthreeexamples,thesensoraveragesthetemperature/brightnessofthetwo
objects. Thisrendersinaccuratetemperature/brightnessvaluesofbothobjectsandcanmakeclouds
appearlowerthantheyactuallyare.
4.4.4 FORESHORTENING
Foreshorteningisdefinedasalossofresolutioncausedbyanobliqueviewingangle. Thisresultsin
distortionoftheimagepredominatelyneartheedgeoftheEarth'ssurface,butcanoccuronanytypeof
satelliteimagery. Whenthesensorlooksatthe
Earthat
sub
point,
it
is
looking
directly
down
at
thetopofthecloud. However,remember,as
discussedearlier,thatasweexpandoutwardfrom
subpoint,theresolutiongraduallydecreases.
Figure48. Contamination.(Source:PDC)
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Scatteredtobrokenclouddecksappeartobeovercastneartheedgesoftheglobe. Thecloudswillalso
appearfartherawayontheglobalgridthantheyactuallyare. Whennotdirectlylookingdownatthe
topofanobject,thereadingorpositionoftheobjectwillbeofffromitsactualreadingorpositionwhich
iscalledtheerrorofparallaxandisthecauseofforeshorteninginsatelliteimagery,asshowninFigure
49.
4.4.5 SUNANGLE
Often,visual
images
taken
early
or
late
in
the
day
will
include
the
sunrise
sunset
line
known
as
the
terminator. ThisistheactualdividinglinebetweendayandnightontheEarth. Inaddition,thelowsun
anglecanenhancecloudtoptextureduetoshadows. Thiscanbemisleadingininterpretingthe
developmentofconvectiveactivity.
4.4.6 LATITUDE
Typicallyinthetroposphere,iftwofeaturesareatthesamealtitude,butdifferentlatitudes,the
northernfeaturewillappearcolderduetocloserproximitytothepole. Thiscancausedifficultyingray
shadeinterpretationandcorrectidentificationofcloudtypes. Forexample,fogoverMaineusually
appearscolder,hencehigherintheatmosphere,thanfogoverFlorida.
4.4.7
SUN
GLINT
Sunglintoccursinvisualimageryandiscausedbythereflectionofthesun'sraysdirectlyoffawater
surfaceintothesatellitesensor. Sunglintpatternsarecircularinshapeongeostationarysatellite
imageryandlinearinshapeonlowearthorbitingimagery. Strongsunglinttypicallyoccurswhenthe
watersurfaceisrelativelycalmandthewindsarelight. Diffusesunglintpatternscanalsooccur,
indicating
higher
wind
speeds
and
higher
sea
states.
The
larger
and
more
diffuse
the
pattern,
the
higher
thewindspeedandhence,thesea/swellwaves. Sunglintcanalsobeseenoverlandmassesinthe
tropicsassunlightreflectsofftherelativelycalmwatersurfacesofrivers,bays,andarchipelagicwaters.
4.5TYPESOFENVIRONMENTALSATELLITES
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4.5.1 GEOSTATIONARYSATELLITES
GeostationarysatellitesorbittheEarthatanaltitudeofapproximately22,300miles(35,800km)above
theEquatorandtravelatthesameangularvelocityastheEarth. Inordertostayoverthesame
geographicallocation,thesatelliteaxisofrotationneedstobeparalleltotheEarthsaxis. Theonlyway
thesatellitecandothisistoorbitdirectlyovertheequator. Thistypeoforbit,knownasa
geosynchronousorbit,allowsfrequentmonitoringofthesameportionoftheEarth. Asuccessionof
photographsfromthesesatellitescanbeanimatedinsequencetoproduceatimelapsemotionloop
showingcloudmovement. Thisallowsforecasterstomonitortheprogressoflargeweathersystems
suchasfronts,stormcomplexes,andhurricanes. Winddirectionandspeedcanalsobedeterminedby
monitoringcloudmovement.
Someoftheadvantagesofgeostationarysatellitesarespatialandtemporalresolution. Spatial
resolutionmeansthatawideareaoftheearthisbeingviewed. Withtheexceptionofthepoles,
geostationarysatelliteshaveanunmatchedviewoftheEarth. Theimageryfromthesesatellitescovers
140oflongitudeandlatitude,resultinginapproximately120longitudeandlatitudeofusefuldata.
Theother20isconsideredmostlyuselessduetoforeshortening. Thislimitstheareaofeffective
coveragetoaroundonequarteroftheearthssurfacefromnear60Nto60S,and60eastandwestof
subpoint. PolarRegionsarenotcoveredandresolutiondecreasesasyoumoveoutfromsubpoint.
Temporalresolution
refers
to
the
fact
that
these
satellites
view
their
portion
of
the
Earth
continuously.
Mostgeostationarysatellitesproduceanimageeveryhalfhourataminimum. Currently,therearefive
differentgeostationarysatellitemissions(U.S.,Europe,India,Japan,andChina)inspaceprovidingglobal
atmosphericcoverageforspecifiedregionsoftheEarth. Imageryfromeachofthesatellitescovering
thedifferentregionsoftheglobecanbeobtainedontheInternetathttp://www.nesdis.noaa.gov/sat
products.html.
4.5.1.1 GeostationaryOperationalEnvironmentalSatellite(GOES)
GOESsatellitesareamainstayofweatherforecastingintheUnitedStatesandarethebackboneof
short term forecasting The real time weather data gathered by GOES satellites combined with data
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TherearefourGOEScurrentlyinorbit. GOES10,stationedover60west,provides24hourcoverageof
SouthAmerica. GOES11isstationedover135westandistheprimarywesternU.S.satellite. Coverage
fromGOES11extendsfrommiddleAmericawestwardtonearthedatelineinthePacificocean,and
northandsouthtoaround60latitude. GOES12isstationedover75westandprovides24hour
coveragefortheeasternportionoftheU.S.tonearlythewestcoastofAfricaandnorthandsouthto
around60latitude. GOES13isfirstofthenewgenerationofGOESandisstationedover105west.
GOES13currentlyservesasabackuptoGOES11and12.
ImprovementsintechnologyhaveallowedustotakeatmosphericsoundingswiththeGOES11and12
andthecurrentGOESsatelliteshaveaseparateimagerandsounderthatallowthemtocontinuously
scanandsampletheatmospherewithoutoneinterferingwiththeother.Otherimprovementsinclude
threeaxisstabilizationandenhancedsignaltonoisecapability. Threeaxisstabilizationisasignificant
improvementoverthespinscansensors. Threeaxisstabilizationallowsthesatellitetokeepsensors
continuouslyaimed
at
the
earth
instead
of
wasting
time
looking
out
into
space.
The
improved
signal
to
noisefunctionallowsformoreaccuratesensingandimprovedimaging.
4.5.2 LOWEARTHORBITING(LEO)SATELLITES
WhenasatellitecirclesclosetoEarth,wesayitisinLowEarthOrbit(LEO). Unlikegeostationary
satellites,whichstayatoneplacewithrespecttotheEarth,lowearthorbitingsatellitesareplacedin
sunsynchronousorbit,imagingastheygo. LEOsatellitescarrymicrowaveinstrumentsinadditionto
visibleandinfraredimagers. GeostationarysatellitesaretoofarawayfromtheEarthtocarry
microwaveinstrumentsusingtodaystechnology. However,sincelowearthorbitersflyatalowaltitude,
theirspatialresolutionsaregenerallysuperiortogeostationarysatellites. Thus,theyproducedetailed
imagesinamuchnarrowerswath.
Satellitesinlowearthorbitarejust200 500miles(320 800kilometers)high.Becausetheyorbitso
closetoEarth,theytravelveryfastsogravitydoesnotpullthemintotheatmosphere.Theycancircle
theEarthinabouttwohour, allowingthemtoprovideglobalcoverageevery12hours. Thepathwidth
varies with latitude and is about 27 at the equator All of these factors are dependent upon the height
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Sometimesthesesatellitesarecalledpolarorbitingsatellites,butthisissomewhatofamisnomer.
PolarorbitingsatellitescloselyparalleltheEarth'smeridianlines,andpassesoverthenorthandsouth
poleswitheachrevolution. Astheearthrotatestotheeastbeneaththesatellite,eachsatellitepass
monitorsanareatothewestofthepreviouspass. Earthrotatesabout25.4totheeastinthetimeit
takesthesatellitetomakeanorbit. Becauseofthis,eachpasswillbe25.4westofthepreviouspass,
asshowninFigure410. Thesepassescanbepiecedtogethertoproduceapictureofalargerarea.
ManyLEOs
cross
near
the
poles
but
do
not
cross
directly
over
the
poles.
Some
LEOs,
like
TRMM,
cover
onlytropicalareasandnevercomenearthepoles. Whileweusethetermpolarorbiterstoreferto
satellitesorbitingnearthepoles,lowearthorbiting(LEO)isamoregeneraltermthatappliestoalltypes
oflowearthorbitingsatellites.
LEOsatellitesareplacedinorbitswhoseinclinationanglesarenearlyperpendiculartotheearth's
equatorialplane.
Polar
Operational
Environmental
Satellites
(POES)
are
aspecial
case
of
low
earth
orbitingsatellitesastheyorbittheEarthatanaltitudeofabout450nm. Thesesatellitesrotatearound
theEarthinanalmostnorthsouthtrackandcomewithinafewdegreesofthepolesineveryorbit. This
ishowpolarorbitersgottheirname. Theypassoveranyparticularspotontheearth'ssurfaceeither
Figure410. CoverageshiftduetoEarthsrotation.(Source:COMET)
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thesatellitewillpassoverthesamespotonearthaboutthesametimeeveryday. Thisisreferredtoas
asunsynchronousorbit,asshowninFigure411.
Figure411. Sunsynchronousorbit.(Source:NASA)
4.5.2.1 PolarOperationalEnvironmentalSatellite(POES)
NOAATIROSNNationalOceanicandAtmosphericAdministration(NOAA)satellitesaremanagedbythe
NationalEnvironmentalSatellite,DataandInformationService(NESDIS),andformthePolarOperational
EnvironmentalSatellite(POES)system.
BecauseofthepolarorbitingnatureoftheNOAATIROSNsatellites,thesesatellitesareabletocollect
globaldataonadailybasisforavarietyofland,ocean,andatmosphericapplicationsviatheAVHRR,
AdvancedVeryHighResolutionRadiometerimager. TheAVHRRischaracterizedbyaverywidefieldof
observation,nearly2700kmandhasaspatialresolutionof1.1km,andutilizesfivechannelsinthe
visible,nearinfrared,midinfraredandthermalinfraredspectralbands. NOAAsatellitesalsocarrythe
TIROS Operational Vertical Sounder (TOVS) that is designed to study the vertical temperature and
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climateresearchandprediction,globalseasurfacetemperaturemeasurements,atmosphericsoundings
oftemperatureandhumidity,oceandynamicsresearch,volcaniceruptionmonitoring,forestfire
detection,globalvegetationanalysis,searchandrescue,andmanyotherapplications. Thecurrent
setup,amorningandafternoonsatellite,providesglobalcoverageovereachregionoftheearthfour
timesdaily. Polarorbitingsatellitesaredefinedbytheascending(northtosouth)nodetime,whichis
thelocaltimewhenthesatellitecrossestheequator. Therearecurrently6satellitesinorbit: NOAA15
and16serveastheAMandPMsecondarysatellitesrespectively;NOAA17servesastheAMbackup,
NOAA18
serves
as
the
PM
primary,
and
NOAA
19
is
currently
undergoing
operational
verification.
NOAAsatellitesThesixthsatelliteiscalledMETOPA,wasdevelopedbyaconsortiumofEuropean
companies,andispartofanewEuropeanundertakingtoprovideweatherdataservicesusedtomonitor
climateandimproveweatherforecasts. METOPAservesastheAMprimarysatellite.
4.5.2.2 DefenseMeteorologicalSatelliteProgram(DMSP)
Sincethemid1960's,whentheDepartmentofDefense(DoD)initiatedtheDefenseMeteorological
SatelliteProgram(DMSP),lowearthorbitingsatellitesprovidedthemilitarywithimportant
environmentalinformation. TheDMSPsatellites"see"suchenvironmentalfeaturesasclouds,bodiesof
water,snow,fire,andpollutioninthevisualandinfraredspectra. Scanningradiometersrecord
informationwhichcanhelpdeterminecloudtypeandheight,landandsurfacewatertemperatures,
watercurrents,
ocean
surface
features,
ice,
and
snow.
Communicated
to
ground
based
terminals,
the
dataisprocessed,interpretedbymeteorologists,andultimatelyusedinplanningandconductingU.S.
militaryoperationsworldwide.
Therearecurrently6DMSPsatellitesinorbit. F12isusedtoprovidetacticaldata,F13,14,and15are
secondarysatellites,whileF16and17serveasprimarysatellites. EachDMSPsatellitehasa101minute,
sunsynchronous,
near
polar
orbit
at
an
altitude
of
830
km
above
the
surface
of
the
earth.
The
visible
andinfraredsensorscollectimagesacrossa3000kmswath,providingglobalcoveragetwiceperday.
Thecombinationofday/nightanddawn/dusksatellitesallowsmonitoringofglobalinformationevery6
hours.Themicrowaveimager(MI)andsounders(T1,T2)coveronehalfthewidthofthevisibleand
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4.5.2.3 TheNationalPolarorbitingOperationalEnvironmentalSatelliteSystem(NPOESS)
NPOESSisthenextgenerationoflowearthorbitingenvironmentalsatellites. TheNPOESSwillcirclethe
Earthapproximatelyonceevery100minutes. Duringtheserotations,theNPOESSwillprovideglobal
coverage,monitorenvironmentalconditions,andcollect,disseminateandprocessdataaboutthe
Earthsweather,atmosphere,oceans,land,andnearspaceenvironment.
NPOESSwillhave5majorsensorsonboard. TheMIS(MicrowaveImager/Sounder,willperformkey
measurementsfortheNPOESSsystemtoincludesoilmoistureandseasurfacewindsbycollectingglobal
microwaveradiometryandsoundingdata. ATMS,AdvancedTechnologyMicrowaveSounder,will
operateinconjunctionwiththeCrosstrackInfraredSounder(CrIS)toprofileatmospherictemperature
andmoisture. CrIS,inconjunctionwiththeATMS,willcollectatmosphericdatatopermitthe
calculationoftemperatureandmoistureprofilesathightemporalresolution. OMPS,OzoneMapping
andProfilerSuite,willmonitorozonefromspace. Andfinally,VIIRS,theVisible/Infrared
Imager/RadiometerSuite
will
collect
visible
and
infrared
imagery
and
radiometric
data.
NPOESSisbeingdevelopedunderanhistoricagreementamongcivil,scientificandmilitarycommunities
andwilleventuallyreplacebothPOESandDMSP.
4.5.2.4 TropicalRainfallMeasuringMission(TRMM)
TheTropical
Rainfall
Measuring
Mission
(TRMM)
is
ajoint
mission
between
NASA
and
the
National
SpaceDevelopmentAgency(NASDA)ofJapan. TRMMisaresearchsatellitedesignedtohelpour
understandingofthewatercycleintheatmosphere. Bycoveringthetropicalandsemitropicalregions
oftheEarth,TRMMprovidesmuchneededdataonrainfallandtheheatreleaseassociatedwithrainfall.
Thishelpsunderstandtheinteractionsbetweenwatervapor,cloudsandprecipitation,whicharecentral
to
regulating
the
earths
climate.
The
TRMM
satellite
carries
five
instruments;
the
first
space
borne
PrecipitationRadar(PR),aVisibleandInfraredScanner(VIRS),aLightningImagingSensor(LIS),aCloud
andEarthRadiantEnergySystem(CERES),andtheTRMMMicrowaveImager(TMI). Thesensorsallow
ustomeasurethesurfacerainrate,atmosphericliquidwater,aswellasdissecttropicalcyclonesat
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4.6 CLOUDANDNONCLOUDFEATUREIDENTIFICATION
4.6.1 SatelliteImageryInterpretation
Wheninterpretingsatelliteimagery,itisimportanttoensureyoulookatthewholeimagetodetermine
whatthefeatureisandhowitfitsintothesynopticsituation. Donotgettunnelvisionandjustlookat
onefeature. Ensuredifferenttypesofimagery(VIS,IR,WV,and/ormicrowave)areusedtogetherto
takeadvantageofeachoftheiruniqueproperties. Usingthedifferenttypesofimagerytogetherwill
usuallyeliminateseveralobstaclestoaccurateinterpretationandhelpnarrowidentificationdownto
onespecificfeature. Visualimagerywilldefinesmallscalefeaturessuchasterrain,cloudshadows,
texture,andsmalllowclouds. Infraredimagerymakesrelativecloudheightanalysispossible,andthus,
specificcloudtypes. Toassistwithsatelliteinterpretationatnightwhennovisualimageryisavailable,
comparethelastvisibleandinfraredimagesoftheday. Identifyeachfeatureevidentonvisibleimagery
anddeterminehowthosefeaturesarerepresentedininfraredimagery. Onceyouhaveidentifiedeach
featurein
infrared
imagery,
using
the
visible
imagery
as
atool,
follow
those
features
using
infrared
imagerythroughoutthenight,keepinginmindhowtheyappearedonthelastvisibleimageoftheday.
Alwaysuseanatlasorlocalterrainmapwheninterpretingtheimagery. Thisensuresthatyoudonot
mistaketerrainfeatures,suchassnowonmountaintops,forclouds.
4.6.1.1 CloudType
Thethreedesignatedcloudtypesarecumuliform,stratiform,andcirriform. Understandinghoweachof
theseappearinimagerycanhelppredictthetypeofprecipitation,ifany,aparticularregionmaybeor
willbereceiving. Cumuliformcloudsproduceshoweryprecipitation,whilestratiformcloudsproduce
intermittenttocontinuousprecipitation. Cirriformcloudsformintheupperlevelsoftheatmosphere
andarecomposedmostlyoficecrystals. Anyprecipitationfallingfromthemtypicallyevaporatesbefore
hittingthesurfaceoftheEarth.
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4.6.1.2 CloudElement
Acloudelementisthesmallestdistinguishable
unitinacloudmassthatcanbedisplayedina
satelliteimageandisdeterminedbythespatial
resolutionofthesensor). SeeFigure412.
4.6.1.3 CloudFingers
Thesearenarrowcloudbands,lessthanonedegree
latitudeinwidth,whichdevelopasaresultoflowlevel
convergence. Thesearefoundinthewarmsector
aheadofacoldfront. SeeFigure413.
4.6.1.4 CloudStreets
Cloudstreetsarecomposedofaseriesofaligned
individualcloudelementsthatarenot
interconnected. Cumuliformcloudswillorganizeintolines
paralleltothelowlevelwinddirectionasseeninFigure4
14,
may
be
curved
or
form
in
straight
lines,
and
are
usually
evenlyspaced. Cloudstreetsusuallyformwhenthelow
levelsoftheatmosphereareunstable,butdescendingair
(subsidence)formsacapandlimitsverticalextentofcloud
development.
4.6.1.5
CloudLines
Acloudlineismuchlikecloudstreetsexcepttheelementsareconnectedandhaveageneralwidthof
lessthan1degreelatitude. Cloudlinesaremostpredominantovertropicaloceansbutareobservedat
all latitudes This cloud formation primarily develops off the east coasts of continents where a
Individual cloud elements
Figure412. Cloudelements.(Source:NOAA)
Arrows representstreamlinesindicating winddirection.
Figure413. Cloudfingers.
(Source:NOAA)
Arrow indicateswind direction.Figure414. Cloudstreets.
(Source:NOAA)
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4.6.1.6 CloudShield
Thesearelargeextensive,commashapedcloudareasmostcommonlyassociatedwithlargescale
synopticsystems. Cloudshieldscanexistseparatelyfromlargescalesystems,butshouldbewatched
fordevelopment.
4.6.2 CloudIdentification
ThelowcloudetageextendsfromtheEarthssurfaceupto6,500feet. Cloudsthatdevelopwithinthis
rangecan
have
asignificant
impact
on
flying
operations.
The
middle
cloud
etage
extends
from
6,500
feettoanywherebetween18,000feetand22,000feet,dependingonlatitude. Thesecloudsare
sometimesindicatorsofstormsystemsmovingintothearea. Thehighcloudetageextendsanywhere
from18,000feetto22,000feetandabove,dependingonlatitude. Becausetheairattheseelevationsis
quitecoldand"dry,"highcloudsarecomposedalmostexclusivelyoficecrystalsandareusuallyrather
thin.
4.6.2.1 FogandStratus(ST)
Fogandstratuslayersappearsmooth,fairlyuniforminareaandslightlygrayormilkyinimagery. In
visibleimagery. OnVISimagery,fog/stratusappearswhitetolightgrayinauniformsheetwithlittle
textureasseeninFigure415overeasternKentucky,WestVirginia,andwesternNewYork. Alsoin
Figure415,noticeoninfraredimagery,fogisdifficult,ifnotimpossibletointerpretduetothelow
contrastintemperaturebetweentheearthssurfaceandthewarmtemperaturesofthefog. Continuing
inFigure415,watervaporimageryisapoortooltouseforfogandstratussincethistypeofimagery
detectsmoistureonlyinthemidandupperlevelsoftheatmosphere. Watervaporimagerydisplaysa
darkregionthroughtheareaofstationsreportingfog. Ifterrainfeaturespenetratethecloudtop,fog
andstratus
will
have
sharp
boundaries
at
these
points.
In
mountainous
or
hilly
terrain,
fog
and
stratus
insmallvalleysoftenhaveabranchingorveinlikeappearancecalledadendriticpatternasseenin
Figure416
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4.6.2.2 Stratocumulus(SC)
Stratocumulusformsduetoashallowlayerofinstabilityinthelowlevelswithastableatmospherealoft,
asshowninFigure417. Onvisibleimagery,stratocumulusappearsascontinuouscloudsheets
composedofparallelrollsorcellularelements. Itwillappearlightgraytowhitewithatexturedlook,as
seeninFigure418overnorthernKansas,Nebraska,andIowa. Oninfraredimagery,itappearsdarkgray,
indicatingwarmtemperatures,asseeninFigure419. Thecellularortexturedappearanceevidentin
visibleimageryisnotobservedoninfraredimageryduetothelowerresolutionoftheIRsensorand
contamination.
WV SFC
IR VIS
WV SFC
IR VIS
Figure415. VISimageoffog/stratus
(Source:NOAA)
Figure416. DendriticPattern
(Source:NOAA)
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concentratedareasofcumulusareusuallydiscernibleandtheywillappearasdarkgraysincethecloud
temperatureisaveragedwiththewarmertemperatureoftheunderlyingsurfacearoundtheclouds.
Morevertically
developed
cumulus
clouds
within
the
field
will
assist
greatly
in
identification
on
infrared.
4.6.2.4 Cumulonimbus(CB)
Cumulonimbusappearbrightwhiteonvisibleimagery,duetotheirhighalbedo,witharoundor
elongatedanvilplumeasseeninFigure422overeastcentralAfrica. Theyhaveasharpupstreamcloud
edge
and
a
thin,
diffuse
anvil,
which
spreads
out
downstream.
Oninfraredimagerycumulonimbusappearbrightwhiteeveninunenhancedimagery,asseeninFigure
423.
With
enhancement,
step
contouring
will
help
identify
cells.
A
tight
gray
shade
or
color
enhanced
gradientisoftenpresentontheupstreamedgeoftheanvilcirrusandwillloosenrapidlydownstream.
Thistightgradientindicatesstrongupdraftsresistingthe
upperlevelflow. Individualupdraftcellsknownas
overshootingtopsareoftenvisibleasabulgeabovethe
otherwisesmooth
anvil
top.
Overshooting
tops
normallyindicatesevereweatherbelowthe
cumulonimbus. Theymayalsocastshadowsonlower
clouddecksifthesunangleislowasseeninFigure424
Figure422. VISimageofCB. Figure423. IRimageofCB.
(Source: NOAA)
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4.6.2.5 StratocumulusLines
Stratocumuluselementsforminareasoflowlevel
instability,whichiscausedbyairseatemperature
differences. Thelinesarecreatedbythestrongvertical
windshearandaninversion,whichcapsthevertical
developmentofthecloud. Stratocumulusappearon
visibleimageryaslightgraytowhitedependingonthe
amountof
contamination.
The
smallest
cells
are
normallyontheupstreamsideoftheline.Theupstream
edgeofthecloudsmayconformtothecoastlineifthe
airisdestabilizingbecauseitismovingoutover
warmerwater. Acloudfreeareausuallyexists
betweenthe
coast
and
where
the
first
elements
form.
Thedistancebetweenthecoastandthefirstelement
isafunctionofhowmuchmoistureexistsintheair
massbeforeitmovesoutoverthewaterandwind
speed,asseeninFigure426. Oninfraredimagery,
theyappearmediumtodarkgrayandtheindividual
linesmayormaynotbeidentifiabledependingonthe
sensorresolution.
4.6.2.6 ClosedcellStratocumulus
Closedcellstratocumulusaretypically
foundin
large
sheets
associated
with
anticyclonicflowinthestableareaofthe
subtropicalhighoveroceanicregions.
Cloudelementsaretypicallyanywhere
Figure426.
Stratocumulus
lines
off
EastCoastofU.S.(Source:NOAA)
Figure425.
WV
image
of
CB.
(Source:NOAA)
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identifyit.Thecloudsareoftenfoundinthesoutheast,southern,andsouthwesternperipheriesof
surfacehighpressurecenters.
Theyappearonvisibleimagery,asseeninFigure428overthePacificOceanoffthecoastofCalifornia,
aswhiteinthecenterandmediumgraytowhiteontheedges. Duetosensorresolution,closedcell
stratocumuluswillusuallyappearsimilartostratuswithaveryuniform,mediumtodarkgrayshadeon
infraredimagery,asseeninFigure429. NoticethecloudformalongtheCaliforniacoastline,thisisnot
stratocumulus,butfog/stratusbutoninfraredimagery,thedifferenceisdifficulttodiscern. The
stratocumulusformsfurtheroutovertheocean,awayfromthecoldCaliforniaCurrent,overareas
wherethewaterismuchwarmerandconducivetothedevelopmentofstratocumulus.
4.6.2.7 OpencellCumulus
Opencellcumuluscloudswhich
formoverwaterbehindmid
latitudecyclonesandarecausedby
theresultinginstabilityofcoldair
advectionoverwarmerwater.
Thereistypicallyanywherefroma
20kmto100kmgapbetween
cloudelements,asseeninFigure4
Figure428. VISimageofclosedcellSC. Figure429. IRimageofclosedcellSC.
(Imagerysource: NOAA)
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Onvisibleimagery,opencellcumulusappearasopenandcircularringletsofcumuluswithclearcenters,
asviewedinFigure431overtheNorthPacificOcean. Instronglowlevelwinds,theseringletswill
becomedistorted
into
lines
and
individual
elements
may
become
difficult
to
detect
due
to
sensor
resolution. Oninfraredimagery,theyappearmediumtodarkgrayduetosensorresolutionand
contaminationasviewedinFigure432. Eventhoughopencellcumulusarenotcappedandgrowto
greaterverticalextentthanclosedcellstratocumulus,opencellcumulusmayappearwarmerdueto
contamination.
Figure431. VISimageofopencellcumulus. Figure432. IRimageofopencellcumulus.
(VisualandInfraredImagerySource: NOAA)
4.6.2.8 EnhancedCumulus
Thesetell
tale
clouds
are
found
in
an
area
of
open
cell
cumulusandwillappearmoreverticallydeveloped
thantheremainderoftheopencellcumulusfield,as
depictedinFigure433. Invisibleimagery,theyappear
similartotoweringcumulusorsmallcumulonimbus
cloudswhile
the
elements
comprising
the
feature
will
formintoacomashapeindicatingavorticitymaximum
inthemidlevelsoftheatmosphere. Oninfrared
imagery,enhancedcumulusappearssimilartotoweringFigure433. Enhancedcumulus.(Source:PDC)
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stratocumulusfieldorsometimesdevelopinginthemiddleofthestratocumulusfieldonthesouthern
sideofthehigh,locatedwellwithinthewarmairmass. Onvisibleimagery,theyappearlightgraydue
tocontamination,
often
with
afish
bone
or
chicken
wire
appearance.
Actiniform
are
slightly
warmer
on
infraredimagerysowillappearmediumtodarkgray;slightlygrayerthanopencellcumulus,and
individualelementsarenotidentifiable. Iftoomuchcontaminationoccurs,theareawillappearasa
darkspotorholeinthestratocumulusfield.
Figure434. Closeupofactiniformclouds Figure435. ActiniformcloudswestofSCfield.
(ImagerySource: NOAA)
4.6.2.10 RopeClouds
Aropecloudiscomposedofcumulusor
toweringcumuluscloudswhichareorganized
intoalineatthetrailingedgeofanoceaniccold
front,anddepictstheexactlocationofthat
surfacecoldfront.Onvisibleimagery,itappears
asaverynarrowlineofcumuliformcloudsandis
bestidentified
by
observing
the
rope
like
configuration,asseeninFigure436justoffthe
westcoastofAfrica. Oninfraredimagery,the
rope cloud formation is somewhat difficult to
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4.6.2.11 ArcClouds
Acurvedlineofcumuluscloudsformedduetoa
thunderstormdowndraftofcoldair,asdepictedin
Figures37and38. Asthecoldairdowndraftofa
cumulonimbusspreads
out
in
all
directions,
it
acts
asapseudocoldfrontanddisplacestherelatively
warmerairatthesurfaceformingcloudsatthe
liftingcondensationlevel(LCL). Onvisibleimagery
anarccloudsappearssimilartoaropecloud
exceptthat
the
parent
thunderstorm
is
normally
still
in
the
vicinity
on
the
concave
side
of
the
arc
cloud.
Oninfraredimagerytheymaybeundetectabledependingontheirverticalextentandsensorresolution.
4.6.2.12 AltostratusandNimbostratus(AS/NS)
Altostratusandnimbostratusareextensivesheetsof
stratiformcloudinessfoundinthemidlayers. These
cloudsarefoundintheeasternportionofsurfacecyclones,
aheadofwarmfronts,andontheleadingedgeoffrontal
systems. Thesecloudsareoftenmaskedbycirrostratus
shieldsinactiveregionsofcommacloudsystems,as
depictedbytheredarrowsinFigure439. Onvisible
imagery,they
will
appear
as
bright
white
and
uniform,
normallycoveringextensiveareas. Shadowscastonorby
theAS/NSfromhighcloudswillhelpdistinguishthemfrom
CSorST. Oninfraredimagery,theyappearinuniformgrayFigure439. AS/NScoveredbycloudshield.
Figure438. Sideviewofarccloud.
Figure437. Topdownviewofarccloud.
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4.6.2.13 Altocumulus(AC)
Oftenappearonvisibleimageryasabrightwhitecloudsheetwithatexturedappearance. Oninfrared
imagery,altocumulusmayappearsimilartoaltostratusornimbostratusduetosensorresolution.
4.6.2.14 AltocumulusStandingLenticular(ACSL)
Alsoknownasmountainwaveclouds,theyformwhenstrong
midlevelwindsflownearlyperpendiculartoamountain
rangeinastableatmosphere,asdepictedinFigure440.
Thesecloudsareanindicatorofmoderatetoextreme
turbulence,dependingontheirproximitytothemountain
range. Onvisibleimagery,theyarelightgraytowhitewitha
washboard(banded)
appearance,asviewedinFigure441. Oninfraredimagery,
theyrange
from
medium
gray
to
white.
Individual
elements
areoftentoosmalltobedetectedbytheinfraredsensor.
4.6.2.15Cirrus(CI)
Thincirrusisverydifficulttodetectonvisibleimagerydueto
contaminationfrombelow,asdepictedinFigure442bythe
redarrow. Densecirruslookslikepatches,streaks,orbands,
andhasamilkyappearance. Oninfraredimagery,thincirrusisnormallycontaminated,butiscold
enoughtobediscernibleoverthewarmbackground,asseeninthesameareaonFigure443overthe
AtlanticOcean. Densecirrusappearscold. Onwatervaporimagery,itrangesfromlightgraytobright
white,asviewedinFigure444overthesamearea,dependingoncloudthickness.
4.6.2.16 Cirrostratus(CS)
Cirrostratusisashieldofcontinuous,variablydensecloudscoveringanextensivearea. Onvisible
imagery,itwillappearbrightwhite,asseeninFigure445associatedwiththelowpressurecenterover
Figure440. FormationofACSL.
(Source:UniversityofWisconsin)
Figure441.
Altocumulus
standing
lenticular.(Source:NOAA)
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Figure441. VISimageofcirrus Figure442. IRimageofcirrus.
Figure443. WVimageofcirrus. Figure444. VISimageofCS.
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Inenhancedinfraredimagery,withaspecificallydesignedtemperaturecorrelatedgrayscale,the
assistantforecasterandforecastercandeterminecloudheightsbycomparinggrayscaleshadesto
upperlevelcharts. Inwatervaporimagery,itwillshowupasbrightwhite,asviewedinFigure447.
OutsideofCBs,cirrostratusarenormallythecoldestcloudtops.
4.6.2.17 Cirrocumulus(CC)
Duetospacingandsizeofcirrocumulus,thiscloudtypemaynotreadilyshowuponvisibleimagerydue
toextensive
contamination.
Infrared
imagery
may
serve
as
the
better
tool
to
identify
cirrocumulus
due
totheirverycoldtemperatures.
4.6.2.18 CirrusStreaks
Cirrusstreaksaresmallisolatedpatchesofcirrusgenerallyoccurringawayfromotherclouds. Theyare
elongatedbytheupperwindflow,developwherethereisinsufficientmoistureforanentirecirrus
shieldtoform,andareassociatedwithjetstreammaximums. Onvisibleimagery,asshowninFigure4
48overSouthDakota,theyareverydifficulttodiscernbecausetheyarehighlycontaminatedfrom
below. Oninfraredimageryacirrusstreakwillappearmediumgraytowhitebutmaystillsuffera
degreeofcontaminationasevidencedinFigure449. Inwatervaporimagery,cirrusstreakswillappear
lightgraytowhiteandyoumaybeabletodeterminecurvatureassociatedwiththecirrusstreak
associatewithavorticitymaximum. CirrusstreaksinawatervaporimagecanbeinterpretedinFigure
450.
Figure449CirrusStreak
IR
7 JUL 09 1945Z
Figure450CirrusStreak
WV
7 JUL 09 1945Z
Figure448CirrusStreak
VIS
7 JUL 09 1945Z
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4.6.2.19 TransverseBands
Thiscloudformationconsistsofirregularlyspaced,
parallelbandsofthincirrusfilamentsandstrands
orientedperpendiculartothewindflow,asshownin
Figure451overNorthernMexico. Windspeedsin
transversebandsareusuallygreaterthan80knots.
Transversebandsaremostoftenassociatedwiththe
subtropicaljet.
4.6.2.20 LeeoftheMountainCirrus
Thiscloudformationisamultilayeredcirruscloud
shieldthatoccursontheleesideofamountainrange. Asharp,stationary,upstreamcloudedgealong
theridgelineindicatesthepresenceofstandingmountainwaveclouds. Ittendstoformlateatnight
whenanocturnalinversiondevelopsoverthe
mountainsanddissipatesduringtheafternoonwhen
theinversionisbrokenbydaytimeheating. The
occurrenceofleeofthemountaincirrusseems
highlydependentuponthepresenceofahighlevel
moisturesourceinastrongwindzone. Onvisible
imagery,itappearsasbrightwhiteandasthickas
cirrostratus,withasharpedgealongthemountain
ridgeline,upwind,becomingmorediffusedownwind
wherecontaminationcanbecomesignificant. On
infraredimagery,
it
appears
bright
white,
as
shown
inFigure452,eastoftheRockyMountainsinColorado. Onwatervaporimagery,theyappearbright
whiteextendingdownstreamfromthemountains. Adarkband,whichextendsupstreamand
downstreamfromthemountainsonthenorthernedgeofthiscloudformation,indicatesthepositionof
Figure51. TransverseBandingOverMexico.
(Source:NOAA)
Figure452. LeeofthemountainCI.
(Source:NOAA)
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4.6.2.21
BillowClouds
Thesecloudsareregularlyspacedcloudsthatadvectwiththewindandusuallyonlylastforafew
minutesatatime. Theyarecausedbyverticalwindshearduetostrongerwindsaloftandcanoccurin
themidtoupperlevelsoftheatmosphere,asdepictedinFigure453. Generally,billowcloudslookvery
similartoACSLclouds,althoughspacingtendstobelessbetweenthebillowcloudsthanwiththeACSL
clouds.
Onvisible
imagery,
they
appear
as
light
gray
to
white
with
awashboard
appearance
and
will
showslightmovement. Contaminationisoftenaproblemduetothesmallspacingbetweenthe
elements. Oninfraredimagery,theirappearancerangesfrommediumgraytowhite. Individual
elementsareoftentoosmalltobedetectedbytheinfraredsensor.
4.6.2.22 AnvilCirrus
Thiscloudformationisexhaustfromthe
topofathunderstormthatformsasharp
upwindedgewithafuzzy,diffuse
downstreamedge. Blowofffrom
numerouscellsmayformanextensive
cirrostratuscanopy.
On
visible
imagery,
theanvilwillappearasbrightwhiteonthe
upstreamedgeandgraduallydarken
downstream as it thins, as seen on Figure
Figure453. Formationofbillowclouds.(Source:UniversityofWisconsin)
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indeterminingcloudheightbyagaincomparinggrayshadestotheimagetemperaturescale,then
determiningatwhatleveloftheatmospherethattemperatureexists.
4.6.3 NONCLOUDFEATURES
Inadditiontotheseveraldifferentcloudfeatures,noncloudfeaturesarealsopredominantand
importanttoaforecaster. Insomeways,noncloudfeaturescanbeconfusedwithcloudfeatures,soit
isimportanttounderstandwhatthetelltalesignsareseparatingthetwo.
4.6.3.1
Snow
Snowcoveredgroundappearsonvisibleimagerybetterthan
infraredimagerybecauseofthebrightnesscontrastbetween
thereflectivesnowfieldandthesurroundingbareground.
Snowhasadendritic(veinlike)patterninmountainareas,as
viewedin
Figure
455
over
the
Sierra
Nevada
Mountains.
Snowfieldsoverflat
regionscanbe
identifiedbythe
snowfreeriversand
lakesand
also
tend
to
be
long,
narrow,
and
smooth
with
sharp
edges,asseeninFigure456overIllinois. Snowwillalso
exhibitamottled(blotchy)appearanceinforestedareas,as
seeninFigure457overcentralMississippi. Itwillbebright
whiteinthe
plainsanddecreasebrightnesswithincreasingvegetation
densityandheight. Oninfraredimagery,freshsnowwill
oftenappearcolderthansurroundingsnowfreeareas,
especiallyaroundsunrise,butotherwiseblendsinwith
Figure456. SnowfieldsoverIllinois.
Source: NOAA
Figure455. Dendriticsnowpattern.
(Source: NOAA)
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4.6.3.2 Ice
Iceisusuallyonlydiscernibleonsatelliteimageryin
largelakes,bays,andseas. Offshorewindswillmove
andbreakupiceadjacenttotheshore. Waterand
new,thintransparenticewillappearasdarkbands
alongtheshore. Onvisibleandinfraredimagery,ice
willhavethesamegrayshadeassnowandisdifficult
todistinguish
from
snow
cover.
There
may
be
dark
fracturesorcracksintheicetohelpidentifyit,as
seeninFigure458overLakeErie. Knowledgeofthe
locationofbodiesofwatercomparedtolandisvery
importantwhenidentifyingice.
4.6.3.3 Sand/Dust
Sandanddust,alsoreferredtoaslithometeors,are
suspendedsurfaceparticlescarriedaloftbystrong
synopticscalesurfacewinds,oftenforlongdistances,with
themostcommonoccurrencesindesertregions. The
upstreamedgeisusuallynotwelldefinedandmaybe
difficulttodistinguishonvisibleimagery,appearingasa
diffuseareaofamediumtolightgray,asseeninFigure4
59overIraq. Itisverydifficulttodetectoninfrared
imagerysince
the
sand
or
dust
are
usually
near
the
sametemperatureastheland,butifcanbeseen,it
willbeadarktomediumgrayshade.
Figure458. IceoverLakeErie.
(Source: NOAA)
Figure459. Blowingdust
overtheTexasPanhandle.(Source: NOAA)
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4.6.3.4 Haze
Hazeisverydifficulttodetect,especiallyoninfraredimagery. Visibleimagerywilldepictamilky
appearance,similartothincirrus,butitistypicallymorewidespread,asshowninFigure460overthe
UnitedStateseasternseaboard. Infraredimagerywillfavortheunderlyingsurfacetemperatures. Haze
willbeevidentandpersistunderstagnantconditions(subsidencefromahigh). Sunangleisalsoakey
factorininterpretinghaze. Hazeismuchmoreidentifiablewithalowsunangle,comparedtoatime
duringahighsunangle.
4.6.3.5 Smoke/Ash
Smokefromfiresandindustryusuallyhasasharp
boundaryatthesourceoftheplume,asseeninFigure
461overMexico. Smokeandashfromvolcanoesmay
bediscernibledependingonthelevelofvolcanic
activity,buttheashreachesambientairtemperature
veryrapidlysomaybecomelessidentifiableovertime.
Ifavolcanicplumereacheshighaltitudes,theashcloud
willappearcoldoninfraredimagery. Ashcloudsthat
reachtheupperlevelscanbeadvectedlongdistances
downstreamby
upper
level
wind
flow.
The
upstream
edgeisnormallythick,whiledownstream,the
ashcloudismorediffuseandthin. Ifthick,it
willlookthesameasthickcirrusonvisibleand
infraredimagery,asseeninFigure462over
NewZealand.
Figure461. Smokeplumes.
(Source: NOAA)
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4.6.3.6 WaterTemperature
Differencesinwatertemperaturearereadily
recognizedingoodinfraredimagery,aslongas
thereisnosignificantlowlevelcloudcoverage,as
seeninFigure463. TheNIRbandisusuallybestfor
interpretingwatertemperature. Water
temperatureisnotrecognizableonvisibleimagery
becauseillumination
is
the
key
means
of
deriving
theproduct,nottemperature. Watertemperature
isalsonotrecognizableonwatervaporimagery. Visibleimagerymustbeused,ifavailable,toverify
thereisnosignificantcloudcoverageinthelowerlevels.
4.6.4 SYNOPTICSCALECLOUDORGANIZATIONS
Commacloudsareassociatedwithsynopticscale,lowpressuresystemswithinthemidlatitude
westerlies. Thereisonebasiccloud
systemthatstartsthecommacloudand
threebasiccloudsystemswhichmakeup
thesynopticscalecommaclouditself.
Certainparts
of
acomma
cloud
have
their
ownspecialnamesandcanbeusedto
readilyidentifydifferentfeatures,as
depictedinFigure464. Thesurgeregion
iswheredry,subsidingairflowsintothe
commacloud.
It
is
also
known
as
the
dry
slot. Thecommaheadisthenorthwest
portionofthecloudsystem. Itis
composedofthedeformationzonecloud
Figure
4
63.
Gulf
Stream
water
temperatures.
(Source: NOAA)
Figure464. Partsofthesynoptic
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4.6.4.1 BaroclinicLeafs
Abaroclinicleafisamidandupperlevelcloudpatternassociatedwithasystemwhichisjustbeginning
todevelop,asshowninFigure465overtheNorthPacificOcean. Itnormallyhasashallow"S"shapeon
thesharpupstreamedgeofthecloudsystem. A
uniquecharacteristicisthe"V"notchinthetailof
theleaf(thisiswherethepolarfrontjetisentering
theleaf). Baroclinicleavesaresmallerthanthe
moredeveloped
synoptic
scale
comma
clouds
and
maynotbeevidentonthesurfaceanalysis. They
varymoreinshapethantheothercloudsystems,so
identifyinglargeregionsoforganizedorunorganized
midandupperlevelcloudinesscanpossiblyidentify
thepresenceofabaroclinicleaf,suchasthearea
overMontanainthesameimage,Figure465.
4.6.4.2 BaroclinicZoneCloudSystem
Thebarocliniczonecloudsystemisalarge,
extensiveareaofmultilayeredcloudswhichare
associatedwithabarocliniczone(frontalzone). A
largecirrostratusshieldassociatedwithcoldand
warmfrontsusuallytopsthemultilayeredclouds.
Thecloudshieldformsinanareawherethe
temperaturefield
is
out
of
phase
with
the
pressure/windfield(describedasbaroclinicity,or,
simplyput,whenthermaladvectionisoccurring.)
Precipitation falling from this cloud shield is usually
Figure465. Baroclinicleafs.(Source: NOAA)
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4.6.4.3 VorticityCommaCloudSystem
Thevorticitycommacloudsystemiscomposedofanareaoflowormidlevelconvectiveclouds
organizedintoacommashapeandiscausedbytheupwardverticalmotionresultingfromthe
divergenceaheadofavorticitymaximum,asshowninFigure467. Iftheatmosphereisunstable
enoughtosupportstrongconvection,thecommacanalsobecomposedofthecirrusanvilsof
cumulonimbuscloudsandprecipitationisconvectivewithrainshowersorsnowshowers.
Figure467. Vorticitycommacloudsystem.(Source: NOAA)
4.6.4.4
DeformationZone
Cloud
System
Thedeformationzonecloudsystemisanelongatedareaofmultilayeredcloudsthatarebeing
stretched"and"sheared"byanupperleveldeformationzone,asdepictedinFigure467. Themulti
layeredcloudsareusuallytoppedbyacirrostratusshield. Thecloudmasselongatesalongtheaxisof
dilatationandshrinksalongtheaxisofcontraction. Theupwardverticalmotioncausingthecloudsis
usuallydue
to
divergence
aloft
associated
with
the
ascent
of
the
cold
conveyor
belt
and
the
divergent
quadrantofajetmaximum. Thedeformationoftheupperlevelwindfieldthen"rearranges"thecloud
massintotheclassicdeformationpattern. Precipitationisnormallystratiformandheaviestinthe
southern portion of the deformation zone cloud system.
Vorticity Comma Cloud System
Deformation Zone Cloud System
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zonecloudsystemislowerandthinnerthanthebarocliniczonecloudsystem. TypeAsystemsare
formedprimarilyfromMeridionalcyclogenesis.
Figure468. TypeAOccludedSystem Figure469. TypeBOccludedSystem
(Source: NOAA)
4.6.4.6
TypeB
Occluded
Systems
TypeBsynopticscalecommacloudsystemsshowthebaroclinicanddeformationzonecloudsystems
merged. Whilethewindcontinuestoflowacrossthebarocliniczonecloudsystemanddeformation
zonecloudsystemssuchasinTypeAsystem,thewindspeeddropstolessthanjetstreamcriteria.
Hencewesaythejetstopsandreformsonthenorthernsideofthebarocliniczonecloudsystem.
Thedeformation
zone
cloud
system
is
approximately
the
same
height
(temperature)
as
baroclinic
zone
cloudsystem,asviewedinFigure469. TypeBsystemsareformedprimarilyfromsplitflow
cyclogenesis.
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4.7 ANALYSISOFMETEOROLOGICALFEATURES
LearningObjective
Identifysynopticscalemeteorologicalfeaturesidentifiableonsatelliteimagery.Lowlevelfeaturesareeasiesttoidentifyonvisiblesatelliteimagerysincethecontrastbetweenlow
cloudsandtheEarth'ssurfaceisthegreatest. Still,theidentificationoflowlevelfeaturesismore
difficultthantheidentificationofupperlevelfeatures. Sincevisibleimageryisonlyavailablehalfofthe
time,you
will
need
to
be
able
to
estimate
the
position
of
these
features
using
infrared
data.
As
an
assistantforecaster,youwillbelocatingfrontsandpressuresystemsusingsatelliteimagerywhichdoes
nothavetheresolutionofvisibleimagery,soexactpositionofweatherelementsisoftenmuchmore
difficulttodetermine. Thebestapproachistocombineconventionaldata,whereavailable,with
satelliteimageryindata
sparseregions.
4.7.1 JETSTREAMS
Thefirstruleforplacingthe
jetstreamaxisonsatellite
imageryistoplaceitabout1
oflatitude
poleward
of
the
sharpnorthernedgeofthe
barocliniczonecloudshield,
asshowninFigure470.
Thesecondrulefor
placementis
where
the
upper
level
clouds
are
advanced
downstream
the
furthest,
as
viewed
in
Figure
471withtheredarrowwherethejetaxisandcloudbandintersects. Thisismostoftenseenwith
occludedsystemswherethejetaxisentersthesurgeregion. Thesouthernportionofthesurgeregions
typically displays a U shape where the jet stream starts to intersect the cloud band and will display a
Figure470. Placingthejetinrelationtothebarocliniccloudshield.
(Source: NOAA)
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Figure471. Secondruleofjetplacement.
Thethirdruleforplacementofthejetstreamiswhennohighcloudsarepresent,theaxisofthejet
streamwill
normally
be
located
13
of
latitude
on
the
cold
side
of
the
boundary
between
the
open
cell
cumulusandclosedcellstratocumuluscloudfields,asshowninFigure472. Eventhoughthejetstream
isanupperlevelfeature,wecanstillusethelowlevelcloudsintheabsenceofcirrusbasedonthe
temperaturesthoseparticularcloudsdevelopin. Opencellcumulusisassociatedwithcoldunstableair,
whileclosedcellstratocumulusisassociatedwithmorestable,warmair. Sincethejetstreamexists
where
there
is
a
significant
thermal
discontinuity,
we
can
place
the
jet
stream
in
its
approximate
positionusingtheboundarybetweenthesetwodifferentlowlevelclouds.
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Thepolarfrontjetaxisflowsthroughsynopticscalecommacloudsystemsinoneoftwoconfigurations,
TypeAorTypeB. InaTypeAsystem,thejetiscontinuousacrossthesystemandcrossesthecloud
massat
or
just
north
of
the
surface
front
triple
point.
There
will
be
adistinct
separation
between
the
deformationzonecloudsystemandthebarocliniczonecloudsystem. InaTypeBsystem,thejetis
discontinuousthroughthecloudsystem. Thereisnoseparationbetweenthedeformationzonecloud
systemandthebarocliniczonecloudsystem. Latentheatreleasehascausedthejettofanoutandwrap
intothelow.
Watervapor
imagery
is
avaluable
tool
when
analyzingthejetstream. Often,watervapor
imageryistheonlyimagerywhichcanbeusedto
accuratelyplacethejetaxisincloudfreeregions.
Thejetstreamisassociatedwiththeboundary
betweendarker(drier)stratosphericairand
lighter(moister)troposphericare,asseenover
thecentralUnitedStatesinFigure473. To
analyzethejetstreamusingwatervapor
imagery,locatethetightestmoisturegradient
(wheretheimageturnsmostrapidlyfromwhitetodark)andplacethejetaxisinthedarkbandclosest
tothe
area
of
moisture
as
seen
in
Figure
474.
Figure
475
provides
explanation
on
this
technique.
The
PolarFrontJet(PFJ)liesbetweenthemidlatitudeFerrelCellandthePolarCell. ImaginethePolarFront
Jet(PFJ)inFigure475ispropagatingawayfromyouandintothepage. Thecoldandmoistupper
troposphereliestotherightofthePFJaxis,whilethewarmer,dryairofthestratosphereliestotheleft
ofthePFJaxis. AsthePFJpropagates,itisalsorotatinginacounterclockwisefashion(asyouare
viewingit.)
To
the
right
of
the
PFJ,
motion
is
upward
lifting
any
existing
moisture
and
enabling
cloud
formation. TotheleftofthePFJ,motionisdownwardandsubsidenceisprevalenteliminatingany
possibilityofcloudformation. Adarkbandthatbecomesbetterdefined(darkening/widening)indicates
astrengtheningoftheassociatedjetstream.
Fi ure473. WVima eof etstream.
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4.7.2 UPPERLEVELBAROCLINICLOWS
Upperlevelbarocliniclowsthatsupportbarocliniclowsatthesurfacewillexhibitalargeverticaltiltand
areusuallyfoundwiththeupperleveldeformationzone. Atthisstageinthedevelopmentofthe
system,anextensiveshieldofcoldcloudtopswillbeassociatedwiththesystem. Theupperlowis
locatedatthecuspoftheupperleveldeformationzonecloudsystem,asdepictedinFigure476. Water
vaporimageryshowsadarkslotspiralingaroundthelowandadarkregionnormallyonthewestor
northwestside. Thisisassociatedwiththeaxisofdilatationinthedeformationzone.
Figure474. WVimageofjetstream.
Tro os here
Stratos here
Figure475. PolarFrontJetin
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4.7.3 UPPERLEVELLOWSASSOCIATEDWITHDECAYINGWAVES
Upperlevellowsassociatedwithdecayingwavesoftenshowcoldcloudtopsthatarefragmentedand
disorganized,indicating
the
system
is
weakening.
The
upper
low
is
located
in
the
dry
region,
as
depicted
inFigure477. Watervaporimagerywillindicateadarkbandthatwrapsalmostcompletelyaroundthe
low.
Figure477. Upperlevellowassociatedwithadecayingwave.
(Source: NOAA)
4.7.4 CUTOFFLOWS
Cutoff
lows
are
deep
pools
of
cold
air
located
equatorward
of
the
main
polar
front
jet.
There
are
three
typicalcloudformationsassociatedwithcutofflows,asdepictedinFigure478.
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Inthebarocliniczonecirrus,thejetstream
normallyflowsfromthesouthwesttonortheast
justeast
of
the
low
center.
The
deformation
zone
cirrusisabandofcirrus,whichstretchesout,inan
eastwestornortheastsouthwestdirection,north
oftheupperlow,asviewedinFigure479. This
bandofcirrusdevelopsduethedeformationzone
createdbytheconvergencebetweenthelow
circulationandtheprevailingflow. Core
convectionappearsasconvectivecloudslocated
directlyundertheupperlowwithinthecoldupperlevelcore. Watervaporimageryindicatesadark
bandspiralingaroundthelow. Weakcutofflowsmaybebarelydiscernableoninfraredimagery,but
standoutclearlyonwatervaporimagery.
4.7.5 SURFACELOWS
Duringtheinitialstagesofdevelopmentof
amidlatitudecyclone,thefrontalclouds
associatedwithaslowmovingcoldfrontor
stationaryfront
will
begin
to
widen
and
haveaslightSshapeonthe+nsideofthe
cloudband,asviewedinFigure480. The
surfacelowwillbelocatedhalfwayintothe
cloudpatternfromtheinflectionpointin
thefrontalband. Thispatternisquite
commonwithstablewavesoryoung,
unstablewavesalongthefrontalboundary.
Duringtheintensificationstageofthesurfacelowpressurecenter,thesynopticscalecommacloudwill
Figure480. Initialstageofdevelopmentofa
baroclinicsurfacelow. (Source:NOAA)
Figure479. CutofflowovertheMidwestU.S.
(Source:NOAA)
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Figure481. Intensificationstageofabaroclinicsurfacelow.(Source:NOAA)
Duringthematurestage,thesurfacelowisstartingtobecomemoreverticallystackedwiththeupper
low. Asthesurfacelowoccludes,itmigratesontothecoldsideofthejet,asseeninFigure482. The
positionofthesurfacelowwillbeontheupstreamedgeofthevorticitycommacloud,beneaththe
upperleveldryslot,justeastofthedeformationzonecloudsystem. Inmanycases,thedeformation
zonecirrusandthedryslothavewrappedaroundthesurfacelow.
H th l l d f l i thi t l ti l i t t th t f
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However,theupperlevelandsurfacelowsare,inthisstage,nearlyverticalinstructuresothecenterof
thecyclonicswirlinthelowcloudswillidentifythegenerallocationofthesurfacelowpressurecenter.
asseen
in
Figure
483.
Figure483. Decayingstageofasurfacelow.(Source:NOAA)
4.7.6 FRONTS
Frontsarenormallylocatedwithinacommacloudstructureoranorganizedmultilayeredcloudband. In
mostcaseswithsatelliteimagery,youcanonlyplacethegeneralpositionofthefront. Togetan
accuratefrontalposition,youwillneedtosupplementsatelliteanalysiswithconventionalsynopticdata.
Frontsarenormallyeasiertoidentifyoverwaterthanlandbecausemoremoistureisavailableforcloud
formationalongthefrontalboundary.
4.7.6.1 ColdFronts
Typically,acoldfrontislocatedunderthemultilayered,barocliniczonecirrusofthecommacloud
structurebeginningattherearportionofthecommacloudstructureclosesttothelow,movingtoward
the middle of the cloud formation about midway through the comma then toward the forward part of
An active cold front is slow moving between 5 and 15 knots and will have a more stratified cirrus cloud
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Anactivecoldfrontisslowmoving,between5and15knots,andwillhaveamorestratifiedcirruscloud
shieldwithuniformcloudtoptemperatureswithanextensivebarocliniczonecloudshield. Thecold
frontin
the
case
of
an
active
cold
front,
will
be
located
near
the
east
side
of
the
comma
tail
with
the
majorityofthecloudinessbehindthefront,asdepictedinFigure484. Thepolarfrontjetflowsparallel
tothefrontanddoesnotpushthefrontalong,hencethereasonforitsslowmovementandrelatively
widecloudshield. Temperatureswilldroprapidlyacrossthefrontanddewpointswilldropgradually
duetotheassociatedmoisturebeinglocatedatandbehindthefront. Theslopeofthefrontistypically
shallow.
Figure484. Active,slowmovingcoldfront.(Source:NOAA)
Clouds,ifpresent,alongafastmoving,inactivecoldfrontareconvectiveinnature. Strong
perpendicularwindflow,relativetothefront,pusheslowlevelconvergenceaheadofthefrontoften
formingsqualllinesaheadofthesurfacefront. Afastmovingcoldfrontwillnothavealargebaroclinic
zonecloud
shield.
The
cold
front
will
be
located
along
the
backside
of
the
cloudiness,
often
in
the
clear
airasseeninFigure485inthevicinityoftheNorthandSouthCarolinacoastline. Thepolarfrontjetwill
bemoreperpendiculartothefront,pushingthefrontalongatafasterpace. Temperatureswilldrop
d ll h f d d ll d dl h d l d
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Figure485. Inactive,fastmovingcoldfront.
4.7.6.2 WarmFronts
Warmfrontsaremoredifficulttoposition
becausecloudinessrangesfromscattered
tomultilayeredclouds. Thesurfacewarm
frontislocatedwithintheVnotchor
wedgeonthewarmsideofthebaroclinic
zonecloudshield,asdepictedinFigure4
86. Alowcloudbandnormallyextends
eastwardalongthefront. Thiscloudiness
isformedbywarmmoistairascendingthe
warmfrontalslope. Imbeddedconvection
mayform
ahead
of
the
warm
front
due
to
theliftingofunstableairalongthewarm
frontalsurface.
baroclinic zone cloud system as depicted in Figure 487 On water vapor imagery there is a gray shade
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barocliniczonecloudsystem,asdepictedinFigure4 87. Onwatervaporimagery,thereisagrayshade
differencebetweenthebarocliniczonecirrusandthelowervorticitycommacloud.
Figure487. TypeAoccludedfront. Figure488. TypeBoccludedfront.
(Source:NOAA)
InType
B
systems,
triple
point
placement
is
not
so
straightforward.
Extrapolation
of
the
warm
and
cold
frontalpositionsisrequiredtofindthetriplepoint. Oncethecoldandwarmfrontshavebeenlocated,
simplyextendthefrontalsurfacesuntiltheyintersectonthe+nsideofthebarocliniczonecirrus,as
showninFigure488.
4.7.6.4 StationaryFronts
Thestationaryfrontisnormallylocatedalongthesouthernedgeofthecloudbandthattypicallyextends
moreeasttowestasseeninFigure489. Overwatertheremaybearopecloud,whichindicatesthe
exactlocationofthefront. Cloudsarenormallycumuliformonthewarmsideofthefront,whilethe
typeofcloudinessonthecoldsideofthefrontdependsonthestabilityoftheoverlyingwarmairand
thesteepnessofthefront. Iftheoverlyingwarmairisstable,stratiformcloudinesswilldevelop. Ifthe
overlyingwarm
air
is
unstable,
stratiform
cloudiness
with
embedded
cumuliform
activity
will
develop.
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Figure489. Placementofastationaryfront.(Source:NOAA)
4.7.7 UPPERLEVELHIGHS
Upperlevelhighsaredifficulttopositionsincetheyproducesubsidence,downwardverticalmotion,and
inhibitverticalclouddevelopment. Watervaporimageryisparticularlyusefulforidentifyingclosed
upperlevelhighs. Withinthelongwaveridge,watervaporimageryindicatesaboundarybetweenthe
moistairtothewestanddrierairtotheeast. Thisboundaryisnormallyragged.
Figure490. IRimageofU/Lhigh Figure491. WVimageofU/Lhigh
(Source: NOAA)
4.7.8 SURFACEHIGHS
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Highsoverlandcanbelocatedbynotingthe
effectof
the
wind
flow
on
low
clouds.
In
late
falltoearlyspring,stratocumuluslines
developontheleewardsideoflakesandoff
coastlines. Fogandstratusnormallydevelop
withupslopeflowassociatedwithwestside
of
a
high,
as
viewed
in
Figure
4
92
over
OklahomaandArkansas. Surfacehighscan
alsobeidentifiedbyalackofmidandupper
levelcloudinessasseenovertheAppalachian
Mountainsinthesameimage.
Over
water,
the
high
center
is
located
using
a
combinationofcloudpatternsandthelowpressure
centerlocation. Inthesouthernhalfofahigh,
closedcellstratocumulusisnormallypresentwitha
clearzonetothewest. Westoftheclearzoneis
anotherfrontalsystemwithlowlevelcloudiness.
Thehighcenterisnormallylocatedintheeastern
portionoftheclearzone,asseeninFigure493. The
ridgeaxisislocatedwherethecloudsshow
thestrongestanticyclonicturninginthe
cumuluslines.
Whentwofrontalsystemscomeincloseproximity,asharpsurfaceridgeisfoundbetweenthem,as
depictedinFigure494. Onthenorthernsideofthesurfacehigh,windsshiftfromasouthwesterly
directiontoanorthwesterlydirectioneastofthesurfaceridge. Thesurfaceaxisispositionedalonga
Figure492. Upslopeflowshowingahightotheeast.
(Source:NOAA)
Figure493. Highpressurecenterlocatedoverthecoast
ofCalifornia.(Source:NOAA)
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Figure494. Surfaceridgebetweentwoclosefrontalsystems.
(Source:University
of
Wisconsin)
4.7.9 TROPICALWEATHERSYSTEMS
Tropicalcyc