Cloud types for observers. Reading the sky

 

 

 

 

 

Cloud

types

for

observers

 

 

 

 

 

Cloud

types

for

observers

                                                                                                                                                                                                                                                                                                                         

 

 

     

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

 

      

 

 

 

   

 

 

 

 

 

 

 

   

 

 

 

 

 

Introduction

Cloudsarecontinuallychangingandappearinaninfinite varietyofforms.Itispossible,however,todefinealimited numberofcharacteristicformsobservedallovertheworld intowhichcloudscanbebroadlygrouped.TheWorld MeteorologicalOrganization(WMO)hasdrawnupa classificationofthesecharacteristicformstoenablean observertoreportthetypesofcloudpresent.This publicationillustratesandexplainstheclassifications. Classificationisbasedon10maingroupsofclouds.These aredividedintothreelevels—low,mediumandhigh— accordingtothatpartoftheatmosphereinwhichtheyare usuallyfound.AcodefiguredesignatedCL,CMorCHis

usedtodescribethecloudsofeachlevel.Thedivisionsare showninthetablebelow.Whenthereismorethanone typeofcloudofanylevelpresent,anorderofpriorityhas beenarrangedbyWMOtodeterminewhichcodefigure shouldbeused.

Inthispublicationaseparatesectionisdevotedtothe cloudsofeachlevel.Atthebeginningofeachsectiona pictorialguideshowsthepriorityofcoding.

Cloud

classification

Thedescriptionsandphotographswhichfollowaregiven inthesameorderasthecodefiguresinthepictorialguide. Tofindthecorrectcodefigurefromthepictorialguides, startatwhichevercircleisapplicableatthetopofthepage andfollowthesolidlinefromdescriptiontodescriptionas longasallthecriteriaareapplicable.Ifadescriptionis reachedwhichisnotapplicable,returntotheprevious descriptionandtakethepeckedlinetoapicturesquare. Thecorrectcodefigurewillbefoundinthetopright­hand cornerofthepicturesquare.

Distinguishingfeaturesconnectedwiththe10maingroups ofcloudsarelistedattheendofthispublication.Observers mayfindthisausefulguidewhenconsideringwhich cloudsmaybepresent,orwheneliminatingimprobable clouds,especiallyduringdarkness.Insomemeteorological messages,cloudsareidentifiedaccordingtothe10main groups.Acodefigure,designatedC,isused.Allreferences toCcodefiguresinthispublicationareprintedinred. IntheUnitedKingdomtheheightofthecloudbaseis reportedinfeet.

Level

Designation

Type

Abbreviation

C

code

High

clouds

(base

usually

20,000

ft

or

above,

over

British

Isles)

C

_H

Cirrus

Cirrocumulus

Cirrostratus

Ci

Cc

Cs

0

1

2

Medium

clouds

(base

usually

between

6,500

and

20,000

ft

over

British

Isles,

although

Ns

may

lower

to

near

the

Earth's

surface)

C

_M

Altocumulus

Altostratus

Nimbostratus

Ac

As

Ns

3

4

5

Low

clouds

(base

usually

below

6,500

ft

over

British

Isles)

C

_L

Stratocumulus

Stratus

Cumulus

Cumulonimbus

Sc

St

Cu

Cb

6

7

8

9

 

 

 

                                                                                                                                                                                                                                                                  

Pictorial

guide

C

_L

:

Sc­St­Cu­Cb

Tofindthecorrectcodefigurebelow,startatwhichever Ifadescriptionisreachedwhichisnotapplicable,returnto circleisapplicableandthenfollowthesolidlinefrom thepreviousdescriptionandfollowthepeckedline. descriptiontodescription,solongasallcriteriaaremet.

Cb,withclearlyfibrousor

striatedupperpartpresent

C

L

9

Cb present

No Cb

NoCuandScwithbases atdifferentlevels CS Br oomf ield RK Pilsbur y CS Br oomf ield NoScformedbythe spreadingoutofCu NoCuofmoderateor greatverticalextent.

UseCL=1,5,6or7,

whicheverpredominates

Cb,withoutclearlyfibrousor Sc,formedbythespreading

striatedupperpartpresent

C

L

3

outofCu,present

C

L

4

Cuofmoderateorgreat

CuandScwithbasesat

_C

L

2

C

_L

8

differentlevels verticalextentpresent

Cuoflittleverticalextent,or _{St,orraggedStotherthanof} raggedCuotherthanofbad

_C

_{badweather,orboth}

C

_L

6

L

1

weather,orboth

S

Jebson

RaggedStorraggedCu, Scnotformedbythespreading

_C

L

5

ofbadweather,orboth

C

L

7

outofCu RK Pilsbur y CS Br oomf ield RK Pilsbur y RK Pilsbur y RK Pilsbur y

                                                                                                                                                                                                                                                                                                                                                                                             

 

 

 

 

 

 

                 

C

_L

9

Cumulonimbus

with

anvil

(Cloud

Group

C9)

Thecharacteristicshapeofthesecloudscan onlybeseenasawholewhenviewedfrom adistance(topphotograph).Thetopsof thesemassivecloudsshowafibrousor striatedstructurethatfrequentlyresembles ananvil(facingpage,topleft),plume,or hugemassofhair(facingpage,bottom left).Theymayoccurasanisolatedcloud oranextensivewall(facingpage,3rdrow right).Squalls,hailand/orthunderoften accompanythem.

Underneaththebase,whichisoftenvery dark,pannuscloudsC_L7frequentlyform and,instorms,thesemaybeonlyafew hundredfeetabovetheEarth'ssurface.The pannuscloudsmaymergetoforma continuouslayer.Theremayberagged cumulus(bottomphotograph)oradense horizontalrollattheshower'sedge. Mammamayform,especiallyonthe undersideoftheprojectinganvil(facing page,2ndrowleft),andmayappear particularlyprominentwhenthesunislow inthesky.Virgamayoftenbeseen.Dense cirrus,altocumulus,altostratus,

stratocumulus,cumulusandstratusmay alsobepresent.

Ifthecumulonimbuspassesnearly,or directly,overheadthecharacteristictopcan belosttoview.Anobserver,seeingonly theunderside,maythereforeconfuseit withnimbostratusifawatchhasnotbeen keptonthesky,butbyconvention,the cloudisreportedascumulonimbusif accompaniedbylightning,thunder,hailor otherprecipitationofashowerynature. C_L=9isusedwhenitisimpossibleto differentiatebetweenCL3andCL9.

Cumulonimbusmostfrequentlydevelop fromlargecumulusC_L2;sometimesthey developfromaltocumuluscastellanusCM8,

thenthebaseisunusuallyhigh;theymay beembeddedinaltostratusor

nimbostratus;and/ortheymaydisintegrate intodensecirrusCH3.

StraitofGibraltar

Baseofstratocumulusinforegroundabout1,800ft

Bracknell R D W h y m a n R D W h y m a n P H Je ff ri e s Bracknell Cloudbase1,000–1,200ft

                                                                                                                                                                                                       

 

 

 

 

 

 

                 

C

_L

3

Cumulonimbus

without

anvil

(Cloud

Group

C9)

C S B ro o m fi e ld K W oodle y RK Pilsbur y P JB Ny e NewtonBreda Bracknell Cloudbaseabout3,000ft

ThecloudsofC_L3aregenerallyatan

Cr o wn cop yr ight

intermediatestagerepresentingafurther developmentofC_L2butnotyetreaching thestageofCL9.

Theclear­cutoutlinesandcauliflowertops ofC_L2haveatleastpartiallydisappeared, butnopartofthecloudtophasacquireda fibrousappearanceoranyanvil

development.Theprotuberancestendto formawhitishmasswithoutstriations. Showersorthunderstormsmayoccur. Cumulus,stratocumulusorstratusmay alsobepresent.

Newclouddomesmaybeproducedwhich makethecumulonimbusassume,

temporarily,theappearanceoftowering cumulusCL2,butitshouldstillbecalled

cumulonimbusandreportedasC_L3.The occurrenceoflightning,thunderorhail sometimesprovidestheonlyindicationof thepresenceofacumulonimbus.If,inthis case,itisnotpossibletodecidewhether thecloudisCL3orCL9,thecodingis,by

convention,C_L=9.

TotlandIOWCloudbase1,800ft

                                                                                                                                                                                                     

 

 

 

 

   

 

 

                     

C

_L

4

Stratocumulus

from

spreading

out

of

cumulus

(Cloud

Group

C6)

Thistypeofstratocumulusmostoften formswhentheupperpartofcumulus clouds,thathadbeengainingheightand arenolongerabletodoso,beginto spreadouthorizontally.Thecumulus generallywidentowardsthelevelatwhich theyspreadout.Sometimesthecumulus growthisresumed,atleastinsomeplaces, abovethestratocumulus.Raggedmamma oftenappearontheundersideofthe stratocumulus(seeinsettomiddle photograph).Theindividualmamma elementsareshort­livedanddonotappear asprominentasthoseshowninthesecond rowphotographonpage36.

AnotherformofC_L4oftenoccursinthe eveningwhenthesun'sheatdecreases and,inconsequence,cumulusclouds flattenandassumetheappearanceof patchesofstratocumulus.Thisisdepicted inthebottomgroupofphotographswhich weretakenoveraperiodofabout20 minutes.Cirrusandcirrostratusalsoappear inthesephotographs.

Cirrostratusandcirruscanalsobeseen

RK Pilsbur y jfp Gal vin RK Pilsbur y RAFCranwell SG Cor nf or d

 

 

 

   

 

 

 

 

                                                                                                                                                                                           

C

_L

8

Cumulus

and

stratocumulus

at

different

heights

(Cloud

Groups

C6

and

C8)

CS Br oomf ield (Cr o wn cop yr ight) RK Pilsbur y CS Br oomf ield (Cr o wn cop yr ight)

Cumulusbase2,000ft.Stratocumulusbase3,000ft

TotlandIOW Stratocumulusandcumulus

Cumulusbase2,500ft.Stratocumulusbase6,500ft

ThecodefigureC_L=8isusedforcumulus andstratocumulus,otherthanthe stratocumulusformedfromthespreading outofcumulusCL=4,thathavetheirbases

atdifferentheights.

Usuallythecumulusformsbeneathpatches orasheetofstratocumulusandmayeven thrustitswayintoorthroughthe

stratocumulus(bottomphotograph). UnlikesomeC_L4thecumulusofC_L8does notwidenupwardstowardsthe

stratocumuluslayer.Athinnedoreven clearedareamaysurroundthecumulus column.

Lessfrequentlythecumulusappearsabove thestratocumulus.

Thecaptionstothetopandbottom photographsshowtheestimatedheightof thecloudbaseatthetimeeachpicturewas taken.

                                                                                                                                                                                                                                                             

 

   

 

 

 

 

 

                                                 

C

_L

2

Cumulus

of

moderate

or

strong

vertical

development

(Cloud

Group

C8)

ThesecloudsareadevelopmentofC_L1. Theiroutlineisusuallyclearcut,with horizontalbasesandcauliflower­shaped tops(topphotograph),althoughinfresh windssomeraggednessmayoccur.Sunlit partsaremostlybrilliantwhitewhilebases arerelativelydark.Thecloudsare

sometimesarrangedinlines,calledcloud streets,nearlyparalleltothewinddirection (smallpictures,topright).Theymayalso formwithtalltowers(smallpictures,top left)thatmaybetiltedbythewind. Whenwelldevelopedthesecloudsmay sometimesgiveshowersandinthetropics theremaybeabundantrainfall.

SmallcumulusCL1andstratocumulusCL5

mayalsobepresent,allhavingtheirbases atthesamelevel.Well­developedcumulus cloudsmaybeaccompaniedbydense cirrus,CH2orCH3,andaltocumulus,

formedfromthespreadingoutofcumulus CM6.

Overland,cumuluscloudsusuallydisperse inthelateafternoonorearlyevening. Overtheoceans,maximumcumulus activityseemstooccurinthelatehoursof thenight.

Asthereislittlechangeinthetemperatureof theseabeneaththem,theheightofthebase ofcumulusintheoceantradewindbeltsis remarkablyuniformataround2,000ft.

ChristchurchBay Bracknell Cumulusbase2,500ft RK Pilsbur y RK Pilsbur y RD Wh yman RK Pilsbur y Cr o wn cop yr ight Geo Ander son

Cumulusbase3,000ftwithvirga ChristchurchBay

Cumulusbase3,000ft

Cumulusmediocris

                                                                                                                                                                                                       

 

 

 

 

                                            

C

_L

1

Small

cumulus

(Cloud

Group

C8)

RK Pilsbur y CS Br oomf ield W G P endle ton C S Br oomf ield CS Br oomf ield

Cumulusformationisoftenprecededby hazyspotsoutofwhichthecloudsevolve (topleft).Thecloudsintheirearlystagesof formationaredepictedinthephotograph attopright.Whencompletelyformed,the cloudshaveclear­cuthorizontalbasesand roundedtops(centrephotographs).Inthis

CS

Br

oomf

ield

Base1,200ft Odiham,Hants.Base2,000ft stagetheyarecalled‘fairweather’

cumulus.Inthephotographatbottomleft thecloudshavebeenfrayedbyafairly strong,turbulentwind.

Overland,onclearmornings,cumulus mayformasthesunrapidlyheatsthe ground,ormayresultfromthe

transformationofstratusCL6.Nearcoasts,

cumulusmayformoverthelandbydayin asea­breezeandovertheseaduringthe nightinaland­breeze.

Cumulusinthelaststagesofdissipation (bottomright)isalsocodedasCL=1.

Ifatleastoneofthecumulusclouds presentintheskyshowsmoderateor strongverticaldevelopment,thecodeCL=2

isused.

Fareham,Hants Base3,000ft

Base2,000ft

WestMeon,Hants Penmaen Cumulusfractus

                                                                                                                                                                                                                                                             

 

 

 

 

 

 

 

 

 

           

C

_L

5

Stratocumulus

not

from

the

spreading

of

cumulus

(Cloud

Group

C6)

Stratocumulusoccursinpatchesorlayers, composedofroundedmassesorrolls,at oneormorelevels.Thecloudsaregreyor whitishandalmostalwayshavedarkparts. Mostoftheregularlyarrangedsmall elements,whenmorethan30°abovethe horizon,haveanapparentwidthofmore thanthreefingersatarm'slength. Whenintheformofdarkrolls(top photograph)theedgesoftenmerge togethertoformacontinuouslayer. Sometimestheelementslieinparallel bands(middlephotograph).Dueto perspectivethesemayappeartoconverge towardsthehorizon.

Sometimesthecloudisnotverydenseand gapsmayappearbetweentheelements (bottomphotograph).

Inthetropicsespecially,stratocumulusmay occurasalarge,singlerollcloud.Itmay alsooccurintheshapeoflensesor almonds,althoughthisisfairlyrare.One particularspecies,calledstratocumulus castellanus,hascumulus­liketurretsrising fromacommonhorizontalbase(bottom, inset).Theturretsmaydevelopintolarge cumulus,whenthecodingbecomesCL=2

C=8,orevencumulonimbus. StratusC_L6C=7maylifttobecome stratocumulusCL5C=6.Stratocumulus

oftenformsbeneathnimbostratusC_M2

C=5. Oslofjord Stratocumulusstratiformis Aldergrove Stratocumulusstratiformis Cr o wn cop yr ight JFP Gal vin JFP Gal vin RM Blac kall

                                                                                                                                 

 

 

 

 

 

   

 

 

                                     

C

_L

7

Stratus

fractus

and

cumulus

fractus

of

bad

weather

(Cloud

Group

C7)

CS Br oomf ield Cr o wn cop yr ight CS Br oomf ield

Pannus(dark)600ftbeneath nimbostratus(lightgrey)2,000ft

Kingswood Stratusfractus

Cumulusfractus1,500ftbeneathaltostratus8,000ft

Theseraggedshredsoflowcloudalways appearinassociationwithotherclouds. Theyoftenformbeneathlowering altostratusornimbostratus,during precipitationandforashorttimebefore andafter.Theyalsooccurbeneath cumulonimbusandprecipitatingcumulus. Collectivelytheyareknownaspannusor ‘scud’.Frequentlythesecloudsbecome increasinglynumerousandmergeintoa moreorlesscontinuouslayer,sometimes completelyobscuringtheskyabove.They appeardarkorgreyagainstthelightergrey ofthecloudaboveandgenerallymove quicklyacrossthesky,changingshape rapidly.

                                                                                                                                                                                                                                                                                                                                       

 

 

 

                     

C

_L

6

Stratus

(Cloud

Group

C7)

Stratusmostcommonlyoccursasasingle, grey,fairlyuniform,featurelesslayeroflow cloud.Occasionallyitcanbedarkoreven threatening,althoughatmostitcanonly produceweakprecipitation.Thisfeature makesitfairlyeasytodistinguishfrom nimbostratus,whichnearlyalways producesrain,snoworicepellets.

However,precipitationfallingfromahigher cloudthroughadark,uniformlayerof stratusmaycausetheobserversome confusion.Stratus,whenformingor dissipating,mayappearasraggedshreds calledstratusfractus.Whenoccurringalone theseshredsappeargreywhenviewed towardsthesunandwhitewhenviewed awayfromit.Theymayalsoappear beneathacontinuouslayerofstratus. Theseshreds,unlikethoseofCL7,arenot

accompaniedbyprecipitation.Fogwill oftenliftintoalayerofstratusbyan increaseinwindorariseintemperature. Stratusissometimescomparativelythin andthediscofthesunormoonmaybe seenwithaclearoutline(photograph, bottomright).

Thetopphotographshowsapatchof stratusalmostrestingontheheadland 462ftabovemeansealevel.Inthesecond photographthetopofan180ftoffice blockislosttoviewinlowstratusonan overcastfoggymorning.Patchesbeneatha mainlayerareseeninthethirdrowleft. Thirdrowrightshowsstratus,inahilly region,baselessthan50ftaboveground, thathasdriftedinfromthesea.Alayerof stratusintheprocessofdissipationis shownatbottomleft.Thebaseofthe cloudinthisphotographwasestimatedto be900ft. RK Pilsbur y CS Br oomf ield F Nor ton CS Br oomf ield RK Pilsbur y CS Br oomf ield (Crown copyright) WestWight Bracknell IsleofMan Bracknell

 

 

 

                                                                                                                                                                                                                                                    

Pictorial

guide

C

_M

:

Ac­As­Ns

Tofindthecorrectcodefigurebelow,startatwhichever Ifadescriptionisreachedwhichisnotapplicable,returnto circleisapplicableandthenfollowthesolidlinefrom thepreviousdescriptionandfollowthepeckedline. descriptiontodescription,solongasallcriteriaaremet.

RK

Pilsbur

y

C

_M

7

Acattwoormorelevels

RK

Pilsbur

y

C

_M

5

Acinvadingthesky

RK

Pilsbur

y

C

_M

6

Ac,formedbythespreading outofCuorCb,present SG Cor nf or d

C

_M

2

OpaqueAs,orNs CS Br oomf ield

C

_M

1

Semi­transparentAs

C

_M

7

OpaqueAcpredominant

C

_M

4

Accontinuallychangingin appearanceorintheshapeof almondsorlenses

C

_M

7

AsorNsalsopresent

C

_M

8

TurretedAcorAcintufts present

C

_M

9

Chaoticsky RK Pilsbur y

C

_M

3

Semi­transparent Acpredominant No Ac Ac present Acnotchanging muchnorinthe shapeofalmonds

andlenses Acnotinvadingthe

sky NoAcformedby thespreadingout

ofCuorCb NoAsandnoNs NoturretedAcand

noAcintufts Skynotchaotic

Acatasinglelevel useC_M=3or7, whichever predominates RK Pilsbur y RK Pilsbur y CS Br oomf ield RK Pilsbur y Cr o wn cop yr ight

                                                                                                                                                                                                                                                                                               

 

 

 

 

 

 

 

 

                                         

C

_M

2

Thick

altostratus

or

nimbostratus

(Cloud

Groups

C4

and

C5

)

AltostratusC_M2isdenserandofadarker greyorbluish­greythanaltostratusCM1

fromwhichitmaydevelop.Thegreater partissufficientlydensetocompletely maskthesunormoon.Raggedshredsof pannuscloudsCL7mayformata

considerabledistancebelowthealtostratus. Later,withathickeningofthealtostratus andaloweringofitsbase,thisdistanceis greatlyreduced.Pannuscanbeseeninthe threephotographsonthispage.

Withfurtherthickeningofaltostratusand loweringofitsbase,thecloudmay eventuallybecomethickenoughtomask thesunthroughout.Atthisstageitiscalled nimbostratus.TheC_McoderemainsC_M=2 buttheCcodechangestoC=5,

continuouslyfallingrainorsnowgivesita diffuseappearance.Pannusclouds, generallymovingfastandchangingshape rapidly,frequentlyoccurbeneathitsbase. Thesecloudsappeardarkorgreyagainst thelighterbackgroundofthecloudabove. Duringheavyprecipitationthepannusmay disappear.

Ifpannuscloudsmergeintoacontinuous layerobscuringthecloudabove,the codingCM=2shouldbereplacedbya/and

thepannuscodedasC_L=7C=7.

Inthetropics,particularlyduringshortlulls intherainfall,nimbostratusmaybreakinto severaldifferentcloudlayerswhichrapidly mergeagain.Thecloudsthenoftenshowa verylividcolourwithvariationsin

brightness.

Baseofnimbostratusestimatedat6,000–8,000ftwith pannusat800ft R K P il sb u ry C S B ro o m fi e ld RK Pilsbur y

Nimbostratuswithstratusfractus

                                                                                                                                                                     

 

 

 

 

                                         

C

_M

1

Thin

altostratus

(Cloud

Group

C4)

RK Pilsbur y SG Cor nf or d CS Br oomf ield Cloudbase10,000ft

Altostratus15,000ftwithstratocumulus, bases3,000ftand5,000ft,beneath

Altostratusshowingbroadparallelbands

Thinaltostratususuallyevolvesfromthe gradualthickeningofaveilofcirrostratus. Itnearlyalwaysappearsasalayerofgreat horizontalextent.

Itisofagreyishorbluishcolour,never white,andthegreaterpartisalways translucentenoughtorevealthesun(or moon)asthroughgroundglass.The ground­glasseffectcanbeseeninthethree photographsonthispage.Objectsonthe grounddonotcastshadows,andhalo phenomenaareneverseen.Pannusclouds CL7mayoccur.

Intheirinitialstagesofformationthe pannuscloudsaresmallandwellseparated andusuallyoccurataconsiderable

distancebelowthealtostratus.Sometimes, especiallyinthetropics,altostratusmay formfromthespreadingoutofthemiddle orupperpartofacumulonimbus.

                                                                                                                                                                                                                   

 

     

 

 

 

 

     

C

_M

9

Altocumulus

of

a

chaotic

sky

(Cloud

Group

C3)

Altocumulusofachaoticskygenerally occursatseverallevels.Theskyis characterisedbyitsheavy,stagnant appearance.Therearemoreorlessbroken cloudsheetsofpoorlydefinedcloudsofall transitionalformsfromratherlow,thick altocumulus,tohigh,thinaltostratus.There isgenerallyamixtureoflow­levelandhigh­ levelcloudsalsopresentinthistypeofsky. Inthetopphotographtheestimatedbase ofthelowestcloudis7,000ft.Thereare confusedhigherlayersuptothesheetof altostratusat15,000ft.Aprominentturret ofaltocumuluscastellanuscanbeseenin thecentreofthephotograph.

Inthemiddlephotographragged

altocumulusandaltostratuscanbeseenin severalill­definedlayers,thelowest appearinggreyinthelightofthesetting sun.Thelayersrangeinheightfromabout 8,000ftto18,000ft.Virgacanbeseen trailingbeneathsomeoftheclouds. Thebottompictureshowspoorlydefined patchesofaltocumulusandstratocumulus beneathextensivelayersofaltostratusand altocumulus. CS Br oomf ield CS Br oomf ield RK Pilsbur y

                                                                                                                                                                                                                                                           

 

 

 

 

 

 

 

                                           

C

_M

8

Altocumulus

with

tufts

or

sproutings

(Cloud

Group

C3)

RN Hughes DMG Buc hanan RK Pilsbur y

Altocumulusfloccuswithvirga,base15,000ft

LondonHeathrowAirport Altocumuluscastellanus,base7,000ft

GreatGaddeston,Herts Altocumuluscastellanus,baseabout15,000ft

Twospeciesofaltocumulusarecoded underCM8.

Altocumulusfloccusclouds,asdepictedin thetopphotograph,occuraswhiteorgrey scatteredtuftswithroundedandslightly bulgingupperparts.Theseclouds resembleverysmallraggedcumulusand areoftenaccompaniedbyfibroustrailsof virgafromtheirbases.

Altocumuluscastellanusispicturedinthe othertwophotographs.Thisspecieshas sproutingsintheformofsmalltowersor battlementsandthecumiliformappearance ismoremarkedthaninaltocumulus floccus.Thecloudelementshavea commonbaseandappeartobearranged inlines.Thesecharacteristicsareevident whenthecloudisseenfromtheside. Altocumuluscastellanusmaydevelopinto largecumulusC_L2orsometimes

cumulonimbusCL3or9.Altocumulus

floccussometimesresultsfromthe dissipationofthebaseofaltocumulus castellanus,andmayitselfdissipate,leaving behindverywhitetrailsofcirrus.

Boththesetypesareassociatedwith developingthunderyconditionsovera wideareaasopposedtothunderstorms arisingfromlocallygenerated

                                                                                                                                                                 

 

 

 

 

 

 

 

 

 

 

   

 

 

   

 

   

 

                           

C

_M

7

Altocumulus

with

altostratus

or

nimbostratus

(Other

than

chaotic

sky)

(Cloud

Group

C3

if

altocumulus

predominates,

C4

if

altostratus

predominates,

C5

if

nimbostratus

predominates)

Whenaltocumulusoccurstogetherwith altostratusornimbostratus,C_Miscodedas 7(unlessCM=9applies.)

Thecloudsmayoccurasasingleora multiplelayer,showingpartlythe

characteristicsofaltocumulus,partlythose ofaltostratusornimbostratus.Thissky resultsfromtransformationprocessesby whichaltocumuluschangeslocallyand acquirestheappearanceofaltostratusor nimbostratus.

Altocumulusandaltostratusatthesame levelaredepictedinthetopphotograph.In themiddlephotographtheyoccuratmore thanonelevel.

CodefigureCM=7isalsousedtoreport

altocumulusintwoormorelayers,orthick altocumulusinasinglelayer.Thencode figuresC_M=6,5and4takeprecedenceover CM=7.Descriptionsoftheseothertypesof

skyofC_M7aregivenonpage22.

Bracknell Cloudbase12,000ft CS Br oomf ield SG Cor nf or d RK Pilsbur y IsleofSkye

                                                                                                                                                                                                                                                   

 

 

 

 

 

   

 

 

                                             

C

_M

6

Altocumulus

from

the

spreading

out

of

cumulus

(Cloud

Group

C3)

ShinfieldPark,Reading

Totland,IOW F Nor ton F Nor ton F Nor ton

ShinfieldPark,Reading ShinfieldPark,Reading

RK Pilsbur y RK Pilsbur y

Cumulusbase3,000ft,altocumulusbaseabout 10,500ft,cirrostratusabout25,000ft

Onsomeoccasionstheupwardgrowthof cumuluscloudonreachingmediumcloud levelsisarrested.Thetopofthecumulus cloudthenspreadsouttoform

altocumulusC_M6.Thetopphotograph showstheearlystagesofthistypeof formation.Thephotographbeneathwas takenofthesamecloudsometimelater whenthespreadofthealtocumulushad becomemuchmoreextensive.Cirrostratus isalsoseenabovethealtocumulusand cumulus.

Anotherexampleofthistypeof altocumulusdevelopmentisseenatthe bottomofthepage(toplineandbottom left).

Occasionally,afteratemporaryspreading out,upwardgrowthisresumedinplaces sothatthealtocumulusappearsonthe sideofthecumulus.Thisrenewedupward growthcanbeseeninthephotographat bottomright.AltocumulusCM6canalso

occuronthesideofcumulonimbus. Becauseofthewayinwhichitisformed, CM6occursinpatches.Thesearefairly

thickatfirstandtheirundersurfacemay appearrippled.Laterthesepatchesthin outandbreakintoseparateelements. Altocumulusneverhasthefibrous structure,silkysheenorwhitenessofthe anvilofcumulonimbus.

RK

Pilsbur

                                                                                                                                                                                             

 

 

 

 

 

 

 

         

C

_M

5

Altocumulus

progressively

invading

the

sky

(Cloud

Group

C3)

ThealtocumuluscloudsofC_M5gradually spreadfromonepartofthehorizon,often passingoverhead,andmayeventually reachtheoppositehorizon.Theseclouds generallythicken,andusuallyappear thickest,inthedirectionfromwhichthey firstappeared.Theadvancingedgemay consistofsmallcloudlets,ofteninthe processofdissipation,whichmaycovera largeexpanseofthesky.Thecloudsoften lieinparallelbandsandmaybeinoneor morelayers.ThecodingCM=5isnolonger

applicableoncethecloudsstretchfrom horizontohorizon,orwhentheforward edgenolongerprogresses.

If,duringitsprogressacrossthesky,parts ofthealtocumuluschangetoaltostratusor nimbostratus,thecodingbecomesC_M=7 insteadofCM=5.

Themiddlephotographwastakenashort timeafterthetopphotographandtogether theyillustratethespreadofaltocumulus withtime. Bracknell Base9,000ft Bracknell R K P il sb u ry C S B ro o m fi e ld CS Br oomf ield Base9,000ft Totland,IOW Altocumulusstratiformis

                                                                                                                                                                                                                                                                                         

 

 

 

 

 

 

 

 

                   

C

_M

4

Altocumulus

continually

changing

shape

—

lenticular

altocumulus

(Cloud

Group

C3)

TotlandIOW JA W alt on Aberdeen Cerrig­y­Drudion RK Pilsbur y

Aberdeenatsunset SouthernSpain

RK Pilsbur y Cr o wn cop yr ight PH Jef fr ies RK Pilsbur y

Theirregularlyarrangedelementsof altocumulusofspecificationCM=4are

continuouslychanginginshape.They oftenappeartobedissolvinginsome placesandforminginothers.Thiscanbe seenbycomparingthetwotop

photographswhichweretakenwithinfive minutesofeachother.Thesecloudsare usuallythinanddonotprogressively invadethesky.Theyoftenresembleanet orhoneycomb.

ThealtocumulusofC_M4oftenformsin patchesintheshapeofalmondsorlenses andisthencalledaltocumuluslenticularis. Theseformationsarecausedbywave motionsintheatmosphereandare frequentlyseeninmountainousorhilly areas.Theyareoftencalledwaveclouds. Theymaybetriggeredbyhillsonlyafew hundredmetreshighandmayextend downwindforover100km.Thecloud elementsformatthewindwardedgeofthe cloudandarecarriedtothedownwind edgewheretheyevaporate.Thecloudasa wholeisusuallystationaryorslowmoving. Thesecloudsoftenhaveverysmooth outlinesandshowdefiniteshading.Atsea theyarelikelytobeseenonlytolandward. Theymayappearwelldistributedoverthe sky(middleleft—stratocumulusisalso present),orasasingleelement(middle right),andcanresembleapileofplates whentheelementsappearoneontopof theother.

                                                                                                                                                                                                                   

 

   

 

 

 

 

 

     

C

_M

7

Altocumulus

at

more

than

one

level

(Cloud

Group

C3)

Besidesbeingusedtospecifyaltocumulus togetherwithaltostratusornimbostratus (page18)thecodefigureC_M=7isusedto describepatches,sheets,orlayersof altocumulusattwoormorelevels. Thesepatches,sheets,orlayersmaybeof eithergenerallythinaltocumulus,although thickenoughinplacestomaskthesunor mooncompletely,oraltocumuluswhichis thickthroughout.

Theelementsofthisaltocumulusdonot changecontinually,nordotheclouds progressivelyinvadethesky.

Inthetopphotographthelowergreylayer wasestimatedtobeat8,000ftandthe higherwhitelayerat12,000ft.Inthe middlephotographthelayerswere estimatedtobeat10,000ftand15,000ft. Thebottomphotographwastakenwhen thesunwaslowintheskyandthe

differenceincolouringshowsthetwolayers quitedistinctly.Thebaseofthedark­grey layerwasestimatedtobeat8,000ftand theupperwhitelayerat15,000ft. Note:FurtherspecificationsforC_M=7are givenonpage24. CS Br oomf ield CS Br oomf ield CS Br oomf ield

                                                                                                                                     

 

 

 

 

 

         

C

_M

3

Semi­transparent

altocumulus

(Cloud

Group

C3)

S Jebson CS Br oomf ield CS Br oomf ield Base12,000ft

ThecodingC_M=3isusedtoreport altocumulusatasinglelevel,thegreater partofwhichissufficientlytransparentto revealthepositionofthesunormoon. Thecloudsdonotprogressivelyinvadethe sky,andtheindividualelementschange verylittle.

Theregularlyarrangedelements,as picturedinthetopandmiddle

photographs,usuallyhaveanapparent widthofbetweenoneandthreefingersat arm'slength,when30°ormoreabovethe horizon.Inthemiddlephotographan aircraftcondensationtrailhighabovethe altocumuluscanalsobeseen.

Thesethinaltocumuluscloudsusually produceacorona.

                                                                                                                                         

 

 

     

 

 

 

               

C

_M

7

Thick

altocumulus

in

a

single

layer

(Cloud

Group

C3)

Inadditiontothespecificationsonpages 18and22,CM=7isalsousedwhenthere

arepatches,sheetsoralayerof predominantlythickaltocumulusata singlelevel.Theelementsofthis altocumulusdonotchangecontinually, nordoesthecloudprogressivelyinvadethe sky.Mostoftheregularlyarranged

elements,asshowninthemiddleand bottomphotographs,haveanapparent widthofbetweenoneandthreefingersat arm'slength,when30°ormoreabovethe horizon.Eveniftheelementsappear smallerthanthis,thecloudisstillclassified altocumulusifitshowsshading.Inthetop photographthecloudelementsare irregularinshape. Base8,000ft JFP Gal vin CS Br oomf ield RK Pilsbur y Base9,000ft Altocumulusstratiformis

 

 

                                                                                                                                                                                                                

Pictorial

C

_H

:

Ci­Cc­Cs

Tofindthecorrectcodefigurebelow,startatwhichever Ifadescriptionisreachedwhichisnotapplicable,returnto circleisapplicableandthenfollowthesolidlinefrom thepreviousdescriptionandfollowthepeckedline. descriptiontodescription,solongasallcriteriaaremet.

CcaloneormorethananyCi andCscombined

C

H

9

Ciinvadingthesky

C

H

4

DenseCioriginatingfromCb, present

C

H

3

DenseCi+turretedCi+Ciin

tuftsmorethanotherCi

C

H

2

CS Br oomf ield CS Br oomf ield CS Br oomf ield RK Pilsbur y Cr o wn cop yr ight NodenseCi originating fromCb. UseCH=1or2, whichever predominates Cinot invadingthe sky Csnot coveringthe wholesky NoCs Cspresent Csinvading thesky Ccaloneor morethan anyCiand Cs combined NoCcorCc lessthan anyCiand Cs combined

Cscoveringthewholesky

C

H

7

Csnotinvadingthesky

C

H

8

Csexceeding45°

C

H

6

Ciinfilamentsorhooksmore

Csnotexceeding45°

C

H

5

thanotherCi

C

H

1

RK Pilsbur y Cr o wn cop yr ight RK Pilsbur y RN Hughes

                                                                                                                                                                                                 

 

 

 

       

C

_H

9

Cirrocumulus

(Cloud

Group

C1)

Realcirrocumulusisuncommon. Thecloudiscomposedofverysmall elements,mostofwhichhaveanapparent widthoflessthanthelittlefingerheldat arm’slength.Theelementsnevershow shading.Theyareoftenarrangedinripples resemblingthoseleftbytheebbtideinthe sandontheseashore.Thecloudelements andclearspacesmayalsobearrangedina mannersuggestinganetorahoneycomb (topphotograph).Theregularpatternof ‘waves’andsmallgapsmayresemblethe skinofamackerel,thusgivingrisetothe popularname‘mackerelsky’.(Thisnameis alsooccasionallygiventohighaltocumulus clouds.)Inhillyregionsthecloudmay appearinmoreorlessisolatedpatches whicharealmond­shapedandverywhite throughout.

Thecloudisfrequentlyassociatedwith cirrusorcirrostratusbutcodefigure CH=9shouldonlybeusedwhenthe

cirrocumuluspredominates.

Thecloudsshowninthephotographshad estimatedbasesof20,000ftorabove.

F Nor ton RK Pilsbur y RK Pilsbur y Cirrocumulusstratiformis

                                                                                                                                                                                                                                     

 

 

 

 

 

 

 

 

                                                             

C

_H

7 Cirrostratus covering the whole sky

(Cloud

Group

C2)

CS Br oomf ield RK Pilsbur y RK Pilsbur y

Thecirrostratusinthisexampleisthinandfeatureless

Halonotdetectable.Anyvariationsinthethicknessofthe cirrostratusarenoticeablewhenthesunislowinthesky

CodefigureCH=7isonlyusedwhenaveil

ofcirrostratuscoverstheentiresky.

Althoughitmayberelativelydense,theveil issometimessothinthatitisbarelyvisible. Itmaybedistinguishedfromaltostratusby itsthinness,whichallowsshadowstobe castwhenthesunisnotlowinthesky,and thatitoftendisplayshalophenomena. Ifthesunisbrightitmaybedifficulttosee ahaloaroundit,butbycoveringthesun withthehanditisusuallypossibletosee anyhaloquitewell.Thedistancebetween thetopofthethumbandthelittlefinger spreadwideapartatarm’slengthisalmost aswideastheradiusofthesmall(22°) halo.(Haloesareoftenspokenofin weatherloreasforeshadowingstorms,but theyaretoocommontobereliablesignsof impendingstormyweather.)

Itissometimesdifficulttodiscern cirrostratusthroughhaze.Cirrostratus differsfromhaze,inthathazeisopalescent orhasadirtyyellowishtobrownishcolour. Ifthereareanygapsintheveilof

cirrostratusthroughwhichtheblueofthe skycanbedistinguished,thecodingforCH

shouldbe8.

Cirrusatdifferentlevels,andcirrocumulus, mayalsobepresent.

                                                                                                   

 

 

 

 

 

 

 

 

   

 

 

                                       

C

_H

8

Cirrostratus

neither

progressively

invading

the

sky

nor

entirely

covering

it

(Cloud

Group

C2)

TheskycorrespondingtoC_H8is characterisedbythepresenceofaveilof cirrostratuswhichisnot(ornolonger) invadingtheskyprogressivelyandwhich doesnotcompletelycoverit;theedgeof theveilmaybeclear­cutorfrayed. ThecodefigureCH=8isalsousedwhen

cirrostratusoccursinpatcheswhetherthey increaseinamountornot.

Cirrusandcirrocumulusmayalsobe present,butshouldnotpredominateover thecirrostratus. RK Pilsbur y CS Br oomf ield

Cirrostratusnotincreasing.Thephotographontherightwas taken90minutesafterthephotographontheleft

                                                                                                                                                                                                                                                                                               

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

               

C

_H

5

Cirrostratus

increasing

but

below

45°

elevation

(Cloud

Group

C2)

C

_H

6

Cirrostratus

increasing

and

above

45°elevation

(Cloud

Group

C2)

Bracknell Cirrostratus

Totland,IOW CH5(left)developingintoCH6(right)

45° RK Pilsbur y CS Br oomf ield

Cirrostratusinvadingtheskyprogressively butwithitscontinuouspartstilllessthan 45°abovethehorizonisthemain characteristicofC_H5.Theremaybecirrus, frequentlyseeninbands,filaments,tufts,or resemblingfishskeletons,aheadofthe cirrostratus.Thecirrusmayhavedeveloped fromC_H4.(Whensuchcirruspredominates overthecirrostratusatthesamelevelthe Ccodeis0.)Cirrostratusnearthehorizon maybemistakenforaltostratus,butthe slownesswithwhichitseemstomoveor changeitsappearancecharacterises cirrostratus.Itiswhitishthroughoutand differsfromhazewhichhasadirtyyellowish tobrownishcolour.

IfthecirrostratusofCH5continuesto

invadetheskysothatitiscontinuousto morethan45°abovethehorizon,without coveringthewholesky,thecoding becomesCH=6.Thecirrostratusgenerally

growsdenserasawholeasitprogresses, stilloftenprecededbycirrusasdescribed underC_H5.

Thephotographontherightwastaken30 minutesafterthephotographontheleft.In theearlierpicture,contrailscanbeseen crossingthesky.Inthelaterphotograph,the trailshaveeithermainlydispersedorhave becomeobscuredbythethickening cirrostratus.Smallcumuluscloudscanbe seeninbothphotographs.

Cirrostratusnotcompletelycoveringthesky maybestraight­edgedandclear­cutasin CH5(top).Moreoften,however,itshowsan

                                                                                                                                                               

 

 

 

 

 

 

 

               

C

_H

4

Cirrus

progressively

invading

the

sky

(Cloud

Group

C0)

ThecirruscloudsofC_H4arethesame speciesofcirrusasthoseofCH1(page33),

butwiththeimportantdifferencethatas CH4theyprogressivelyinvadethesky.They

generallybecomedenserasawhole.They usuallyseemtofusetogetherinthe directionofthehorizonfromwhichthey firstappearedandtheforwardedgemoves towardstheoppositepartofthehorizon. Thecloudsoccurmostfrequentlyinthe formofstrandstrailingfromasmallhook ortuft,andlessfrequentlyintheformof straightorirregularlycurvedfilaments. Cirrostratusshouldnotbepresent, otherwisethecodingwouldbeCH=5or6

asthecasemaybe.

Thecirruscloudsshowninthe

photographsonthispageweresteadily invadingtheskyandhadanestimatedbase of20,000–25,000ft. Totland,IOW Cirrusuncinus Cirrusfibratus RK Pilsbur y RK Pilsbur y RK Pilsbur y Totland,IOW Cirrusuncinus

                                                                                                               

 

 

 

 

 

 

                 

C

_H

3

Dense

cirrus

from

cumulonimbus

(Cloud

Group

C0)

RK Pilsbur y M Kidds JFP Gal vin Wokingham,Berks Cirrusspissatus Reading,Berks

OvertheEnglishChannel

ThecodefigureC_H=3isusedonlywhen theobserverisreasonablycertainthatat leastoneofthedensecirruscloudsinthe skyoriginatedfromtheupperpartofa cumulonimbus.Itmaybepossibletosee thisdevelopmentifawatchcanbekepton thesky(middlephotograph).Suchcirrus cloudsfrequentlyhavehairyorfrayed edgesandareoftenintheformofananvil. Thesecloudsaresufficientlythicktoveil thesun,obscureitsoutlineorevenhideit. Inwinterthisformofcirruscanoccurwell below20,000ft.

                                                                                                                                                                       

 

 

 

 

                                 

C

_H

2

Dense

cirrus

(Cloud

Group

C0)

Thecirrusofthisspecificationfrequently occursinentangledsheaves(top

photograph),orinpatchesthickenoughto appeargreywhenviewedtowardsthesun (bottomphotograph).Itdoesnotusually increaseinamount.

CirrusofCH2mayalsooccurinnarrow

bandswithsproutingsliketurretsor battlements(centre)andisthencalled cirruscastellanus.Anotherspeciestakesthe formofcirrusinsmalltufts,thelowerpart oftenbeingmoreorlessragged(centre inset).Thisspeciesisknownascirrus floccus.

CirrusofcodefigureC_H=1mayalsobe present,butshouldnotpredominate. Ifanyofthecirrushasoriginatedfromthe upperpartofacumulonimbusthecoding forCHshouldbe3.Sometimesthecirrusof

C_H2developsintothickanvilshapeswhich couldbemistakenfortheCH3ofa

decayingcumulonimbus. RK Pilsbur y RD Wh yman W G P endle ton Llandyrnog,Clwyd Base20,000ft

Cirruscastellanus25,000ftbeneathother cirrusat30,000ft

F

Nor

ton

                                                                                                                                                                                       

 

   

 

 

 

 

 

 

 

 

             

C

_H

1

Cirrus

in

filaments

or

hooks

not

progressively

invading

the

sky

(Cloud

Group

C0)

CS Br oomf ield RK Pilsbur y RD Wh yman

Cloudheightabout30,000ft

Thewhite,delicate,hair­likecirruscloudsof CH1occurmostofteninnearlystraightor

somewhatcurvedfilaments(top

photograph).Sometimestheyareshaped likecommastoppedwithahookoratuft (middlephotograph)andinthisformthey arepopularlycalled‘mares’tails’.The elementsmaysometimesbearrangedina mannersuggestingafishskeletonwitha spinalcolumnandfilamentsoneitherside likeribs.Cirrusmayalsooccurinparallel bands,sometimesbroad,whichowingto perspectivemayappeartoconverge towardsthehorizon(bottomphotograph). Theheightofthecirrusinthephotographs oppositewasatleast20,000ft.

ThecirrusofCH1doesnotprogressively

invadethesky.Thistypeofcloudoften occurswithothercirrusclouds,butthe highcloudshouldbecodedasC_H=1only whenthecombinedcoverofallfilaments, strandsandhooksexceedsthecoverofall othercirrusclouds.

 

                                                                                                                                                                                                                                                                                                                                                                                             

Special

clouds

NACREOUSCLOUDSresemblepalecirrus orlenticularaltocumulusandshowvery markedirisation,themostbrilliantcolours occurringwhenthesunisjustbelowthe horizon.Theyaresometimescalled ‘mother­of­pearl’clouds.Theycanstillbe distinguisheduptoabouttwohoursafter sunsetasthingreycloudsstandingout againstthestarrysky.Inmoonlightthey maybevisiblethroughoutthenight.They havebeenobservedmainlyfromNorway,at altitudesbetween21and30km,and Scotland.Theyareignoredwhenassessing CHandC.

NOCTILUCENTCLOUDSresemblethin cirrus,butareusuallybluishorsilvery, sometimesorangetored,orreddishwhen onthehorizon.Theyareextremelyrare, beingmostcommonlyobservedonclear midsummernightsbetweenlatitudes55° and65°N.Theybecomevisibleatthesame timeasthebrighteststarsandappearmore brilliantaftermidnight.Theiraltitudeis between75and90km.Particlescollected byrocketsin1962providedstrong indicationsthatthesecloudsconsistofice crystals.Theyareignoredwhenassessing CHandC.

CONDENSATIONTRAILS(contrails)formin thewakeofaircraftwhentheairis

sufficientlycoldandhumid.Theyareoften short­lived,but,especiallywhencirrusand cirrostratusarepresent,theymayspread outandpersistforseveralhours.Persistent trailsarereportedbyusingtheCHcode

figuremostappropriate;sometimesitis impossibletodistinguishbetweenoldtrails andcloud.

Theymayproducehalophenomenawith exceptionallypurecolours.OvertheUK theyrarelyformbelow28,000ftinsummer and20,000ftinwinter.Theymaycast shadowsonthincloudsbeneaththem. Aseriesofsuchshadowsmaybetheonly indicationthatthereismorethanonelayer ofcloudpresent. Lyddington,Oakham Pershore,Works CS Br oomf ield S Jebson RM Blac kall PF T omalin Contrails Persistentcontrails

                                                                                                                                                                                                                                                                                  

 

 

 

 

                                         

Optical

phenomena

and

other

features

JA W alt on R W Mason J Halle tt J Fr eeman Halo Sunpillar

HALOPHENOMENA—Thesmallhaloof 22°radiuscentredonthesunormoonis seenmostfrequently(topphotograph).A whitehorizontallineatthesameelevation asthesuniscalledtheparheliccircleand thetwobrightspotsonthisarecalled ‘mocksuns’(parhelia).Mocksunsappear furtherfromthesunwhenitishigherin thesky.Thebrightspotabovethesunis partofanarcofcontact.Pillarsoflightmay appearverticallyaboveorbelowthesunor moon(second photograph)andaremost frequentlyseenatsunriseorsunset.These, coupledwithaportionoftheparhelic circle,mayformacross.Alarge,less bright,haloof46°radiusissometimes seen,itsarcsofcontactperhapsshowing strongcolouration.Rarelyotherarcsmay occur,butusuallyonlypartofthedisplayis seen.Halophenomenaareusually

associatedwithCs,sometimesCi.(Mock sunsorpillarsaresometimesseeninAc.)

CORONA—Abrownishringofsmall diameteraroundthemoonorsun.Instrong daylightitmaybeeasiertodetectby observingthesun'sreflectionincalmwater. Outercolouredringswithredoutermost sometimesoccur.Distortedcoronaemay sometimesoccurwhenthemoonisnotfull. MostfrequentlyassociatedwithAc,

butsometimesoccurswithCc,Cs,As,Sc, andSt.

IRISATION—Colours,predominantlygreen andpink,oftenwithpastelshades,that sometimesappearonCc,AcorSc. Thecoloursmayappearasbandsnearly paralleltothemarginsoftheclouds,orasa mosaicpattern.

                                                                                                                                                                                                                                                         

 

 

 

 

 

Optical

phenomena

and

other

features

(continued)

VIRGA—Trailsofprecipitation(fallstreaks) thatdonotreachtheearth'ssurface, attachedtotheundersideofacloud.Mainly associatedwithCc(smalltrails),Ac

(pictured),As(maybeclearlyvisible),Ns,Sc (especiallyatverylowtemperatures),Cu, andCb.

MAMMA—Downdraughtscansometimes causeudder­likeprotuberancestoformon theundersurfaceofCi,Cc,Ac,As,Sc (irregularandragged),andCb(bulbous, pictured).Theprotuberancesmayappear prominentwhenthesunislowinthesky.

SMOKE—Citysmokeandindustrial pollutioncausesthesuntolookveryredat sunriseandsunsetandtohaveanorange tintwhenhighinthesky.Fromadistance, suchpollutionmaybeconfusedwitha bankofcloudonthehorizon,butpollution generallyappearslight­greyortohavea bluishhue.

RAINBOW—Appearsonascreenof raindropswhentheobserverhashisbackto thesun.Alessbrightsecondarybow,with coloursequencereversed,sometimes occurs,withdarkerskybetweenthetwo. Theymaybeborderedbyfainterbows. Whenproducedbythemoonthecolours aremuchweakerorareabsent.The rainbowindicatesCborprecipitatinglarge Cu. RK Pilsbur y P JB Ny e W S Pik e Cr o wn cop yr ight

                                                                                                                                                                                        Cr o wn cop yr ight RK Pilsbur y Cr o wn cop yr ight JFP Gal vin

CREPUSCULARRAYS—Thesetakethe formofpaleblueorwhiteraysdiverging fromthesunwhenitisbehindCuorCb. Sunbeamspiercingsmallgapsincloud layers(sometimescalled'sundrawing water')andshadowscastbycloudsnear thehorizonattwilightarealsocalled crepuscularrays.

Jersey.Crepuscularrays

LIGHTNING—Cloudaccompaniedby lightningisreportedasCb.

SPOUT—Anoftenviolentwhirlwind, revealedbythepresenceofafunnelof cloudbeneathCb,witha'bush'ofmatter raisedfromtheearth'ssurface.Thecloud andbushoftenmeet.Spoutsoccurunder newly­formedpartsofCb,notfromwhere therainisfalling.Weakspoutsare

occasionallyseenbeneathCu.

Waterspout

VELUM—Anaccessorycloudofgreat horizontalextent,closeaboveorattached totheupperpartofCuorCbwhichoften pierceit.PicturedwithCL9C 9.

 

                                                                                                                                                   

 

 

 

                                                                                                 

 

 

 

                                                                                                                                                                                                   

Other

clouds

Bigfiresmayproducedarkcloudssimilartolargecumulus. Combustionproductsmaybecarriedbythewindtogreat distancesandoccasionallycauseablueappearanceofthesun ormoon.Realcumulusmayalsoform.

Volcaniceruptionsmaycauselargecumulus­likecloudsthat mayspreadoutatahighaltitudeovervastareas.Theskythen assumesapeculiartintwhichmaypersistforseveralweeks. Verylargeexplosionsareusuallyaccompaniedbyacloudof smokeordustabovewhichvelumisoftenseen.

Industrialactivitiesmayalsoproduceclouds.Fireclouds, cloudsofsmokeordust,cloudsfromvolcaniceruptions,and veilsofcombustionproductsareignoredwhenconsidering thecodingforC_L,C_M,C_HandC.However,realcumulusand cumulonimbuscloudsthatmayresultfromsucheventsare reportedintheusualway.

Appearance

of

clouds

Whenthesunissufficientlyhighabovethehorizon,clouds indirectsunlightarewhiteorgreywhilstthosewhich receivelightfromtheblueskyarebluish­grey.Someclouds, whicharebrilliantwhiteinreflectedlight,showmarked contrastsinbrilliancewhenilluminatedfrombehind.The colourofthesunmaychangeasitapproachesthehorizon andcloudsinthevicinitymayshowacorresponding colouration.

Theundersideofacloudmayreddenwhenthesunisonthe horizonasshowninthepictureofstratocumulus(right). Hazemaymakedistantcloudsappearyellow,orangeorred. Dustparticlesintroduceawhitetingetotheblueofthesky; thustheskyisofadeeperbluewhentheairhasitsoriginsin polarregions.

Satellite

view

of

clouds

ThispicturewastakenfromaTIROS­Nsatelliteduringthe earlyafternoonof12July1979.

Thespirallingpatternofcloudsindicatesthecentreofa depressionsouthofIceland.Showercloudsofcumulusand cumulonimbus,organisedintostreets,followthestrong windsonthesouthernandeasternflanksofthedepression. South­westofthedepressionthecumulusflattensinto stratocumulus.Thewidebandofcloudoffnorth­west districtsoftheBritishIslesisassociatedwithabeltofrain, withcloudpresentatalllevelsfromstratocumulus, altocumulusandaltostratusuptocirrus.Thecloudover northernFranceismostlydensecirrusandthickaltocumulus castellanusinassociationwithalowpressurearea.

OvertheUnitedKingdom,landheatingovertheMidlands hascausedshallowcumulustoform.Somecirrusand altocumulusarepresentoversouthernEnglandassociated withthecloudoverFrance.OverWales,thenorthofEngland andtheSouthernUplandsofScotland,largecumulusclouds haveformedoverthehigherground.

Cumulusandcumulonimbusclouds producingshowersandthunderstormshave formedoverScandinavia.Interaction betweentheBalticSeaandland,heatinghas producedlargecumulusandcumulonimbus alongthesecoasts.

Cumulusfromapowerstation, cirrostratusonhorizon U niv er sity of Dundee JFP Gal vin RK Pilsbur y Stratocumulusatsunset