Cloud Types for Observers Rev 2014

8/10/2019 Cloud Types for Observers Rev 2014

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Cloud

types

for

observers

Readingthesky


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Cloud

types

for

observers

Readingthesky

Cloud types for observers 1


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Introduction

Clouds

are

continually

changing

and

appear

in

an

infinite

varietyofforms.Itispossible,however,todefinealimitednumberofcharacteristicformsobservedallovertheworldinto

which

clouds

can

be

broadly

grouped.

The

World

MeteorologicalOrganization(WMO)hasdrawnupaclassificationofthesecharacteristicformstoenableanobserver

to

report

the

types

of

cloud

present.

This

publicationillustratesandexplainstheclassifications.

Classificationisbasedon10maingroupsofclouds.Thesearedividedintothreelevelslow,mediumandhigh

according

to

that

part

of

the

atmosphere

in

which

they

are

usually

found.

A

code

figure

designated

CL,CMorCH isused

to

describe

the

clouds

of

each

level.

The

divisions

are

showninthetablebelow.Whenthereismorethanonetypeofcloudofanylevelpresent,anorderofpriorityhasbeen

arranged

by

WMO

to

determine

which

code

figure

shouldbeused.

Inthispublicationaseparatesectionisdevotedtothecloudsofeachlevel.Atthebeginningofeachsectionapictorial

guide

shows

the

priority

of

coding.

Cloud

classification

Thedescriptionsandphotographswhichfollowaregivenin

the

same

order

as

the

code

figures

in

the

pictorial

guide.

Tofindthecorrectcodefigurefromthepictorialguides,startatwhichevercircleisapplicableatthetopofthepageand

follow

the

solid

line

from

description

to

description

as

longasallthecriteriaareapplicable.Ifadescriptionisreachedwhichisnotapplicable,returntothepreviousdescriptionandtakethepeckedlinetoapicturesquare.The

correct

code

figure

will

be

found

in

the

top

right-hand

cornerofthepicturesquare.

Distinguishingfeaturesconnectedwiththe10maingroupsof

clouds

are

listed

at

the

end

of

this

publication.

Observers

mayfindthisausefulguidewhenconsideringwhichcloudsmaybepresent,orwheneliminatingimprobableclouds,

especially

during

darkness.

In

some

meteorological

messages,cloudsareidentifiedaccordingtothe10maingroups.Acodefigure,designatedC,isused.Allreferencesto

C

code

figures

in

this

publication

are

printed

in

red.

IntheUnitedKingdomtheheightofthecloudbaseisreportedinfeet.

Level

Designation

Type

Abbreviation

C

code

High

clouds

(base

usually

20,000

ft

or

above,

over

British

Isles)

CH CirrusCirrocumulus

Cirrostratus

Ci

Cc

Cs

0

1

2

Mediumclouds(baseusuallybetween6,500and20,000ftoverBritishIsles,althoughNsmaylowertoneartheEarth'ssurface)

CM AltocumulusAltostratusNimbostratus

AcAsNs

345

Low

clouds(base

usually

below

6,500

ft

over

British

Isles)

CL

StratocumulusStratus

Cumulus

Cumulonimbus

ScSt

Cu

Cb

67

8

9

Cloud types for observers2


5/45

PictorialguideCL:Sc-St-Cu-Cb

Tofindthecorrectcodefigurebelow,startatwhichever Ifadescriptionisreachedwhichisnotapplicable,returnto

circleisapplicableandthenfollowthesolidlinefrom thepreviousdescriptionandfollowthepeckedline.descriptiontodescription,solongasallcriteriaaremet.

Cb,withclearlyfibrousorstriatedupperpartpresent CL9

Cb

presentNo

Cb

Cb,

without

clearly

fibrous

or

NoCuandScwithbases

at

different

levels

RKPilsbury

CSBroomfield

NoScformedbythe

spreadingoutofCu

Sc,

formed

by

the

spreading

striatedupperpartpresent CL3

outofCu,present CL4

CuofmoderateorgreatCuandScwithbasesat CL2CL8 verticalextentpresentdifferentlevels

St,orraggedStotherthanofbadweather,orboth

CL6CL1

RKPilsbury

SJebson

CSBroomfield

Cuoflittleverticalextent,orraggedCuotherthanofbadweather,orboth NoCuofmoderateor

greatverticalextent.

UseCL =1,5,6or7,

whichever

predominates

RaggedStorraggedCu,ScnotformedbythespreadingCL5outofCu CL7ofbadweather,orboth

Cloud types for observers

CSBroomfield

RKPilsbury

RKPilsbury

RKPilsbury

3


6/45

CL9Cumulonimbuswithanvil

(CloudGroupC9)

RDWhyman

RDWhyman

PHJeffries

Thecharacteristicshapeofthesecloudscanonlybeseenasawholewhenviewedfroma

distance

(top

photograph).

The

tops

of

thesemassivecloudsshowafibrousorstriatedstructurethatfrequentlyresemblesan

anvil

(facing

page,

top

left),

plume,

or

hugemassofhair(facingpage,bottomleft).Theymayoccurasanisolatedcloudor

an

extensive

wall

(facing

page,

3rd

row

right).Squalls,hailand/orthunderoftenaccompanythem.

Underneath

the

base,

which

is

often

very

dark,pannuscloudsCL7frequentlyformand,instorms,thesemaybeonlyafew

StraitofGibraltarhundred

feet

above

the

Earth's

surface.

The Baseofstratocumulus inforegroundabout1,800ft

pannuscloudsmaymergetoformacontinuouslayer.Theremayberaggedcumulus

(bottom

photograph)

or

a

dense

horizontalrollattheshower'sedge.Mammamayform,especiallyontheunderside

of

the

projecting

anvil

(facing

page,2ndrowleft),andmayappearparticularlyprominentwhenthesunislowin

the

sky.

Virga

may

often

be

seen.

Dense

cirrus,

altocumulus,

altostratus,

stratocumulus,cumulusandstratusmayalso

be

present.

Ifthecumulonimbuspassesnearly,ordirectly,overheadthecharacteristictopcanbe

lost

to

view.

An

observer,

seeing

only

theunderside,maythereforeconfuseitwithnimbostratusifawatchhasnotbeenkept

on

the

sky,

but

by

convention,

the

Bracknell

cloudisreportedascumulonimbusifaccompaniedbylightning,thunder,hailorotherprecipitationofashowerynature.

CL=9

is

used

when

it

is

impossible

to

differentiate

between

CL3andCL9.

Cumulonimbus

most

frequently

develop

fromlargecumulusCL2;sometimestheydevelopfromaltocumuluscastellanusCM8,then

the

base

is

unusually

high;

they

may

beembeddedinaltostratusornimbostratus;and/ortheymaydisintegrateinto

dense

cirrus

CH3.

Bracknell

Cloudbase1,0001,200ft



7/45

CL3Cumulonimbus

withoutanvil(CloudGroupC9)

CSBroomfield

KWoodley

RKPilsbury

PJBNye

NewtonBreda

BracknellCloudbaseabout3,000ft

ThecloudsofCL3aregenerallyatan

Crownco

pyright

intermediatestagerepresentingafurtherdevelopment

of

CL2butnotyetreachingthestageofCL9.

Theclear-cutoutlinesandcauliflowertopsof

CL2haveatleastpartiallydisappeared,

but

no

part

of

the

cloud

top

has

acquired

a

fibrousappearanceoranyanvildevelopment.

The

protuberances

tend

to

formawhitishmasswithoutstriations.

Showersorthunderstormsmayoccur.Cumulus,

stratocumulus

or

stratus

may

alsobepresent.

Newclouddomesmaybeproducedwhichmake

the

cumulonimbus

assume,

temporarily, theappearanceoftoweringcumulusCL2,butitshouldstillbecalledcumulonimbus

and

reported

as

CL3.The

occurrence

of

lightning,

thunder

or

hail

sometimesprovidestheonlyindicationofthe

presence

of

a

cumulonimbus.

If,

in

this

case,itisnotpossibletodecidewhetherthecloudisCL3orCL9,thecodingis,byconvention,

CL=9.

Totland

IOW

Cloud

base

1,800

ft

Dishforth



8/45

CL4Stratocumulusfromspreadingoutofcumulus

(CloudGroupC6)

This

type

of

stratocumulus

most

often

formswhentheupperpartofcumulusclouds,thathadbeengainingheightandare

no

longer

able

to

do

so,

begin

to

spreadouthorizontally.Thecumulusgenerallywidentowardsthelevelatwhichthey

spread

out.

Sometimes

the

cumulus

growthisresumed,atleastinsomeplaces,abovethestratocumulus.Raggedmammaoften

appear

on

the

underside

of

the

stratocumulus(seeinsettomiddle

photograph).

The

individual

mamma

elements

are

short-lived

and

do

not

appear

asprominentasthoseshowninthesecondrowphotographonpage36.

Another

form

of

CL4oftenoccursintheeveningwhenthesun'sheatdecreasesand,

in

consequence,

cumulus

clouds

flatten

and

assume

the

appearance

of

patchesofstratocumulus.Thisisdepictedinthebottomgroupofphotographswhichwere

taken

over

a

period

of

about

20

minutes.Cirrusandcirrostratusalsoappearinthesephotographs.

RKPilsbury

jfpGalvin

RK

Pilsbury

RAFCranwell

SGC

ornford

Cirrostratusandcirruscanalsobeseen



9/45

CL8Cumulusandstratocumulusatdifferent

heights(CloudGroupsC6andC8)

CSBroomfield(Crowncopyright)

RKPilsbury

CSBroom

field(Crowncopyright)

Cumulusbase2,000ft.Stratocumulusbase3,000ft

TotlandIOWStratocumulusandcumulus

Cumulusbase2,500ft.Stratocumulusbase6,500ft

ThecodefigureCL=8isusedforcumulusand

stratocumulus,

other

than

the

stratocumulusformedfromthespreadingoutofcumulusCL=4,thathavetheirbasesat

different

heights.

Usuallythecumulusformsbeneathpatchesorasheetofstratocumulusandmayeventhrust

its

way

into

or

through

the

stratocumulus(bottomphotograph).UnlikesomeCL4thecumulusofCL8doesnot

widen

upwards

towards

the

stratocumulus

layer.

A

thinned

or

even

clearedareamaysurroundthecumuluscolumn.

Lessfrequentlythecumulusappearsabovethestratocumulus.

The

captions

to

the

top

and

bottom

photographsshowtheestimatedheightofthecloudbaseatthetimeeachpicturewastaken.



10/45

CL2Cumulusofmoderateorstrongvertical

development(Cloud

Group

C8)

ThesecloudsareadevelopmentofCL1.Their

outline

is

usually

clear

cut,

with

horizontalbasesandcauliflower-shapedtops(topphotograph),althoughinfreshwinds

some

raggedness

may

occur.

Sunlit

partsaremostlybrilliantwhitewhilebasesarerelativelydark.Thecloudsaresometimes

arranged

in

lines,

called

cloud

streets,nearlyparalleltothewinddirection(smallpictures,topright).Theymayalsoform

with

tall

towers

(small

pictures,

top

left)

that

may

be

tilted

by

the

wind.

Whenwelldevelopedthesecloudsmaysometimes

give

showers

and

in

the

tropics

theremaybeabundantrainfall.

SmallcumulusCL1andstratocumulusCL5may

also

be

present,

all

having

their

bases

atthesamelevel.Well-developedcumuluscloudsmaybeaccompaniedbydensecirrus,

CH2orCH3,andaltocumulus,formedfromthespreadingoutofcumulusCM6.

Over

land,

cumulus

clouds

usually

disperse

in

the

late

afternoon

or

early

evening.

Overtheoceans,maximumcumulusactivity

seems

to

occur

in

the

late

hours

of

thenight.

Asthereislittlechangeinthetemperatureofthe

sea

beneath

them,

the

height

of

the

base

ofcumulusintheoceantradewindbeltsisremarkablyuniformataround2,000ft.

ChristchurchBay BracknellCumulusbase2,500ft

RKPilsbury

RKPilsbury

RD

Whyman

RKPilsbury

Crowncopyright

Ge

oAnderson

Cumulusbase3,000ftwithvirga

ChristchurchBayCumulusbase3,000ft

Cumulusmediocris

Cumuluscongestus Largecumulus



11/45

CL1Smallcumulus

(CloudGroupC8)

Cumulus

formation

is

often

preceded

by

hazyspotsoutofwhichthecloudsevolve(topleft).Thecloudsintheirearlystagesof

RKPilsbury

CSBroomfield

WGPendleton

CSBroomfield

CSBroomfield

Base1,200ft Odiham,Hants.Base2,000ft

Fareham,

HantsBase3,000ft

Base2,000ft

West

Meon,

Hants

Penmaen

Cumulusfractus

CSBroomfield formation

are

depicted

in

the

photograph

attopright.Whencompletelyformed,thecloudshaveclear-cuthorizontalbasesandrounded

tops

(centre

photographs).

In

this

stagetheyarecalledfairweathercumulus.

In

the

photograph

at

bottom

left

thecloudshavebeenfrayedbyafairlystrong,turbulentwind.

Over

land,

on

clear

mornings,

cumulus

may

form

as

the

sun

rapidly

heats

the

ground,ormayresultfromthetransformationofstratusCL6.Nearcoasts,cumulusmayformoverthelandbydayinasea-breezeandovertheseaduringthenightinaland-breeze.

Cumulus

in

the

last

stages

of

dissipation

(bottomright)isalsocodedasCL=1.

Ifatleastoneofthecumuluscloudspresent

in

the

sky

shows

moderate

or

strongverticaldevelopment,thecodeCL=2

is

used.



12/45

CL5Stratocumulusnotfromthespreading

ofcumulus(CloudGroupC6)

Stratocumulusoccursinpatchesorlayers,composedofroundedmassesorrolls,atone

or

more

levels.

The

clouds

are

grey

or

whitishandalmostalwayshavedarkparts.Mostoftheregularlyarrangedsmallelements,

when

more

than

30

above

the

horizon,haveanapparentwidthofmorethanthreefingersatarm'slength.

When

in

the

form

of

dark

rolls

(top

photograph)theedgesoftenmerge

together

to

form

a

continuous

layer.

Sometimes

the

elements

lie

in

parallel

bands(middlephotograph).Duetoperspectivethesemayappeartoconverge Oslofjord

Stratocumulusstratiformistowards

the

horizon.

Sometimes

the

cloud

is

not

very

dense

and

gapsmayappearbetweentheelements(bottomphotograph).

In

the

tropics

especially,

stratocumulus

may

occurasalarge,singlerollcloud.Itmayalsooccurintheshapeoflensesoralmonds,

although

this

is

fairly

rare.

One

particular

species,

called

stratocumulus

castellanus,hascumulus-liketurretsrisingfrom

a

common

horizontal

base

(bottom,

inset).Theturretsmaydevelopintolargecumulus,whenthecodingbecomesCL=2C=8,orevencumulonimbus.

StratusCL6C=7maylifttobecomestratocumulus

CL5C=6.StratocumulusoftenformsbeneathnimbostratusCM2

Aldergrove

JFPGalvin

RM

Blackall

Crowncopyright

JFPGalvin

C=5.Stratocumulusstratiformis

10 Cloud types for observers


13/45

CL7Stratusfractusandcumulusfractusofbad

weather(Cloud

Group

C7)

CSBroomfield

Crowncopyright

CSBroom

field

Pannus(dark)600ftbeneathnimbostratus (lightgrey)2,000ft

KingswoodStratusfractus

Cumulusfractus1,500ftbeneathaltostratus8,000ft

These

ragged

shreds

of

low

cloud

always

appearinassociationwithotherclouds.Theyoftenformbeneathloweringaltostratus

or

nimbostratus,

during

precipitationandforashorttimebeforeandafter.Theyalsooccurbeneathcumulonimbus

and

precipitating

cumulus.

Collectivelytheyareknownaspannusorscud.Frequentlythesecloudsbecomeincreasingly

numerous

and

merge

into

a

moreorlesscontinuouslayer,sometimescompletely

obscuring

the

sky

above.

They

appear

dark

or

grey

against

the

lighter

grey

ofthecloudaboveandgenerallymovequicklyacrossthesky,changingshaperapidly.



14/45

CL6Stratus

(CloudGroupC7)

Stratus

most

commonly

occurs

as

a

single,

grey,fairlyuniform,featurelesslayeroflowcloud.Occasionallyitcanbedarkoreventhreatening,

although

at

most

it

can

only

produceweakprecipitation.Thisfeaturemakesitfairlyeasytodistinguishfromnimbostratus,

which

nearly

always

producesrain,snoworicepellets.

However,precipitationfallingfromahighercloud

through

a

dark,

uniform

layer

of

stratusmaycausetheobserversomeconfusion.

Stratus,

when

forming

or

dissipating,

may

appear

as

ragged

shreds

calledstratusfractus.Whenoccurringalonetheseshredsappeargreywhenviewedtowards

the

sun

and

white

when

viewed

awayfromit.Theymayalsoappearbeneathacontinuouslayerofstratus.These

shreds,

unlike

those

of

CL7,arenotaccompaniedbyprecipitation.Fogwilloften

lift

into

a

layer

of

stratus

by

an

increaseinwindorariseintemperature.Stratusissometimescomparativelythinand

the

disc

of

the

sun

or

moon

may

be

seen

with

a

clear

outline

(photograph,

bottomright).

Thetopphotographshowsapatchofstratus

almost

resting

on

the

headland

462ftabovemeansealevel.Inthesecondphotographthetopofan180ftofficeblock

is

lost

to

view

in

low

stratus

on

an

overcastfoggymorning.Patchesbeneathamainlayerareseeninthethirdrowleft.Third

row

right

shows

stratus,

in

a

hilly

region,baselessthan50ftaboveground,thathasdriftedinfromthesea.Alayerofstratus

in

the

process

of

dissipation

is

shown

at

bottom

left.

The

base

of

the

cloudinthisphotographwasestimatedtobe

900

ft.

RKPilsbury

CSBroomfield

FNorton

CSBroomfield

RK

Pilsbury

CS

Broomfield(Crowncopyright)

WestWight

Bracknell

IsleofMan

Bracknell



15/45


16/45

CM2Thickaltostratusornimbostratus

(CloudGroupsC4andC5)

RK

Pilsbury

CSBroomfield

RKPilsbury

Altostratus

CM2isdenserandofadarkergreyorbluish-greythanaltostratusCM1fromwhichitmaydevelop.Thegreaterpart

is

sufficiently

dense

to

completely

maskthesunormoon.RaggedshredsofpannuscloudsCL7mayformataconsiderabledistancebelowthealtostratus.Later,withathickeningofthealtostratusandaloweringofitsbase,thisdistanceisgreatly

reduced.

Pannus

can

be

seen

in

the

threephotographsonthispage.

With

further

thickening

of

altostratus

and

lowering

of

its

base,

the

cloud

may

eventuallybecomethickenoughtomaskBaseofnimbostratusestimatedat6,0008,000ftwith

thesunthroughout.Atthisstageitiscalled pannusat800ftnimbostratus.

The

CM

coderemainsCM=2buttheCcodechangestoC=5,continuously

falling

rain

or

snow

gives

it

a

diffuse

appearance.

Pannus

clouds,

generallymovingfastandchangingshaperapidly,frequentlyoccurbeneathitsbase.These

clouds

appear

dark

or

grey

against

thelighterbackgroundofthecloudabove.Duringheavyprecipitationthepannusmay

disappear.

Ifpannuscloudsmergeintoacontinuouslayerobscuringthecloudabove,thecoding

CM=2shouldbereplacedbya/andthepannuscodedasCL=7C=7.

In

the

tropics,

particularly

during

short

lulls

in

the

rainfall,

nimbostratus

may

break

into

severaldifferentcloudlayerswhichrapidlymergeagain.Thecloudsthenoftenshowa Nimbostratuswithstratusfractus

very

livid

colour

with

variations

in

brightness.

Nimbostratuswithpannus,base800ft,below



17/45

CM1Thinaltostratus

(CloudGroupC4)

RK

Pilsbury

SGCornford

CSBroom

field

Cloudbase10,000ft

Altostratus15,000ftwithstratocumulus,

bases3,000ftand5,000ft,beneath

Altostratusshowingbroadparallelbands

Thin

altostratus

usually

evolves

from

the

gradualthickeningofaveilofcirrostratus.Itnearlyalwaysappearsasalayerofgreathorizontal

extent.

Itisofagreyishorbluishcolour,neverwhite,andthegreaterpartisalwaystranslucent

enough

to

reveal

the

sun

(or

moon)asthroughgroundglass.Theground-glasseffectcanbeseeninthethreephotographs

on

this

page.

Objects

on

the

grounddonotcastshadows,andhalophenomena

are

never

seen.

Pannus

clouds

CL7mayoccur.

Intheirinitialstagesofformationthepannuscloudsaresmallandwellseparatedand

usually

occur

at

a

considerable

distancebelowthealtostratus.Sometimes,especiallyinthetropics,altostratusmayform

from

the

spreading

out

of

the

middle

orupperpartofacumulonimbus.



18/45

CM9Altocumulusofachaoticsky

(Cloud

Group

C3)

Altocumulus

of

a

chaotic

sky

generally

occursatseverallevels.Theskyischaracterisedbyitsheavy,stagnantappearance.

There

are

more

or

less

broken

cloudsheetsofpoorlydefinedcloudsofalltransitionalformsfromratherlow,thickaltocumulus,

to

high,

thin

altostratus.

There

isgenerallyamixtureoflow-levelandhigh-levelcloudsalsopresentinthistypeofsky.

In

the

top

photograph

the

estimated

base

ofthelowestcloudis7,000ft.Thereareconfused

higher

layers

up

to

the

sheet

of

altostratus

at

15,000

ft.

A

prominent

turret

ofaltocumuluscastellanuscanbeseeninthecentreofthephotograph.

In

the

middle

photograph

ragged

altocumulusandaltostratuscanbeseeninseveralill-definedlayers,thelowestappearing

grey

in

the

light

of

the

setting

sun.Thelayersrangeinheightfromabout8,000ftto18,000ft.Virgacanbeseentrailing

beneath

some

of

the

clouds.

The

bottom

picture

shows

poorly

defined

patches

of

altocumulus

and

stratocumulus

beneathextensivelayersofaltostratusandaltocumulus.

CSBroom

field

CSBroomfield

RKPilsbury



19/45

CM8Altocumuluswithtuftsorsproutings

(CloudGroupC3)

RNHughes

DMGBuchanan

RKPilsbury

Altocumulusfloccuswithvirga,base15,000ft

LondonHeathrowAirport

Altocumuluscastellanus,base7,000ft

Great

Gaddeston,

Herts

Altocumuluscastellanus,baseabout15,000ft

Two

species

of

altocumulus

are

coded

underCM8.

Altocumulusfloccusclouds,asdepictedinthe

top

photograph,

occur

as

white

or

grey

scatteredtuftswithroundedandslightlybulgingupperparts.Thesecloudsresemble

very

small

ragged

cumulus

and

areoftenaccompaniedbyfibroustrailsofvirgafromtheirbases.

Altocumulus

castellanus

is

pictured

in

the

other

two

photographs.

This

species

has

sproutingsintheformofsmalltowersorbattlements

and

the

cumiliform

appearance

ismoremarkedthaninaltocumulusfloccus.Thecloudelementshaveacommon

base

and

appear

to

be

arranged

inlines.Thesecharacteristicsareevidentwhenthecloudisseenfromtheside.

Altocumulus

castellanus

may

develop

into

largecumulusCL2orsometimescumulonimbusCL3or9.Altocumulusfloccus

sometimes

results

from

the

dissipationofthebaseofaltocumulus

castellanus,

and

may

itself

dissipate,

leaving

behind

very

white

trails

of

cirrus.

Boththesetypesareassociatedwithdevelopingthunderyconditionsoverawide

area

as

opposed

to

thunderstorms

arisingfromlocallygeneratedcumulonimbusclouds.



20/45

CM7Altocumuluswithaltostratusornimbostratus(Otherthanchaoticsky)

(CloudGroupC3ifaltocumuluspredominates,C4ifaltostratus

predominates,

C5

if

nimbostratus

predominates)

When

altocumulus

occurs

together

with

altostratusornimbostratus,CM iscodedas7(unlessCM=9applies.)

The

clouds

may

occur

as

a

single

or

a

multiplelayer,showingpartlythecharacteristicsofaltocumulus,partlythoseof

altostratus

or

nimbostratus.

This

sky

resultsfromtransformationprocessesby

which

altocumulus

changes

locally

and

acquires

the

appearance

of

altostratus

or

nimbostratus.

AltocumulusandaltostratusatthesameBracknell

level

are

depicted

in

the

top

photograph.

In Cloudbase12,000ft

themiddlephotographtheyoccuratmorethanonelevel.

Code

figure

CM=7isalsousedtoreportaltocumulusintwoormorelayers,orthickaltocumulusinasinglelayer.Thencodefigures

CM=6,5and4takeprecedenceoverCM=7.DescriptionsoftheseothertypesofskyofCM7aregivenonpage22.

IsleofSkyeCloudbasebetween10,000and15,000ft

CS

Broomfield

SGCornford

RKPilsbury



21/45

CM6Altocumulusfromthespreading

outofcumulus(CloudGroupC3)

ShinfieldPark,Reading

Totland,IOW

FN

orton

FNorton

FNorton

ShinfieldPark,Reading ShinfieldPark,Reading

RKPilsbury

RKPilsbury

Cumulusbase3,000ft,altocumulusbaseabout10,500ft,cirrostratusabout25,000ft

On

some

occasions

the

upward

growth

of

cumuluscloudonreachingmediumcloudlevelsisarrested.Thetopofthecumuluscloud

then

spreads

out

to

form

altocumulusCM6.Thetopphotographshows

the

early

stages

of

this

type

of

formation.

The

photograph

beneath

was

takenofthesamecloudsometimelaterwhenthespreadofthealtocumulushadbecome

much

more

extensive.

Cirrostratus

isalsoseenabovethealtocumulusand

cumulus.

Another

example

of

this

type

of

altocumulusdevelopmentisseenatthebottomofthepage(toplineandbottomleft).

Occasionally,afteratemporaryspreadingout,upwardgrowthisresumedinplacesso

that

the

altocumulus

appears

on

the

sideofthecumulus.Thisrenewedupwardgrowthcanbeseeninthephotographatbottom

right.

Altocumulus

CM6canalsooccuronthesideofcumulonimbus.

Because

of

the

way

in

which

it

is

formed,

CM6occursinpatches.Thesearefairlythickatfirstandtheirundersurfacemayappearrippled.Laterthesepatchesthinout

and

break

into

separate

elements.

Altocumulusneverhasthefibrousstructure,silkysheenorwhitenessoftheanvil

of

cumulonimbus.

RK

Pilsbury



22/45

CM5Altocumulusprogressivelyinvadingthesky

(CloudGroupC3)

The

altocumulus

clouds

of

CM5graduallyspreadfromonepartofthehorizon,oftenpassingoverhead,andmayeventuallyreach

the

opposite

horizon.

These

clouds

generallythicken,andusuallyappearthickest,inthedirectionfromwhichtheyfirst

appeared.

The

advancing

edge

may

consistofsmallcloudlets,oftenintheprocessofdissipation,whichmaycoveralarge

expanse

of

the

sky.

The

clouds

often

lieinparallelbandsandmaybeinoneor

more

layers.

The

coding

CM=5

is

no

longer

applicableoncethecloudsstretchfromhorizontohorizon,orwhentheforwardedge

no

longer

progresses.

BracknellBase9,000ft

If,

during

its

progress

across

the

sky,

parts

ofthealtocumuluschangetoaltostratusornimbostratus,thecodingbecomesCM=7insteadofCM=5.

Themiddlephotographwastakenashorttimeafterthetopphotographandtogetherthey

illustrate

the

spread

of

altocumulus

withtime.

Bracknell

RKPilsbury

CSBroomfield

CSBroom

field

Base9,000ft

Totland,IOWAltocumulusstratiformis



23/45

CM4Altocumuluscontinuallychangingshape

lenticularaltocumulus(Cloud

Group

C3)

TotlandIOW

JAW

alton

Aberdeen Cerrig-y-Drudion

RKPilsbury

Aberdeenatsunset SouthernSpain

RKPilsbury

Crowncopyright

PHJeffrie

s

RKPilsbury

The

irregularly

arranged

elements

of

altocumulusofspecificationCM=4arecontinuouslychanginginshape.Theyoften

appear

to

be

dissolving

in

some

placesandforminginothers.Thiscanbeseenbycomparingthetwotopphotographs

which

were

taken

within

five

minutesofeachother.Thesecloudsareusuallythinanddonotprogressivelyinvade

the

sky.

They

often

resemble

a

net

orhoneycomb.

The

altocumulus

of

CM4oftenformsinpatches

in

the

shape

of

almonds

or

lenses

andisthencalledaltocumuluslenticularis.Theseformationsarecausedbywavemotions

in

the

atmosphere

and

are

frequentlyseeninmountainousorhillyareas.Theyareoftencalledwaveclouds.They

may

be

triggered

by

hills

only

a

few

hundredmetreshighandmayextenddownwindforover100km.Thecloudelements

form

at

the

windward

edge

of

the

cloudandarecarriedtothedownwindedgewheretheyevaporate.Thecloudasa

whole

is

usually

stationary

or

slow

moving.

Thesecloudsoftenhaveverysmoothoutlinesandshowdefiniteshading.Atseathey

are

likely

to

be

seen

only

to

landward.

Theymayappearwelldistributedoverthesky(middleleftstratocumulusisalsopresent),

or

as

a

single

element

(middle

right),andcanresembleapileofplateswhentheelementsappearoneontopofthe

other.



24/45


25/45

CM3Semi-transparentaltocumulus

(CloudGroupC3)

SJebson

CSBroomfield

CSBroom

field

Base12,000ft

The

coding

CM=3isusedtoreportaltocumulusatasinglelevel,thegreaterpartofwhichissufficientlytransparenttoreveal

the

position

of

the

sun

or

moon.

Thecloudsdonotprogressivelyinvadethesky,andtheindividualelementschangevery

little.

Theregularlyarrangedelements,aspicturedinthetopandmiddlephotographs,

usually

have

an

apparent

width

of

between

one

and

three

fingers

at

arm'slength,when30ormoreabovethehorizon.

In

the

middle

photograph

an

aircraftcondensationtrailhighabovethealtocumuluscanalsobeseen.

These

thin

altocumulus

clouds

usually

produceacorona.



26/45

CM7Thickaltocumulusinasinglelayer

(CloudGroupC3)

In

addition

to

the

specifications

on

pages

18and22,CM=7isalsousedwhentherearepatches,sheetsoralayerofpredominantly

thick

altocumulus

at

a

singlelevel.Theelementsofthisaltocumulusdonotchangecontinually,nor

does

the

cloud

progressively

invade

the

sky.Mostoftheregularlyarrangedelements,asshowninthemiddleandbottom

photographs,

have

an

apparent

widthofbetweenoneandthreefingersat

arm's

length,

when

30

or

more

above

the

horizon.

Even

if

the

elements

appear

smallerthanthis,thecloudisstillclassifiedaltocumulusifitshowsshading.Inthetop Base8,000ftphotograph

the

cloud

elements

are

irregularinshape.

JFP

Galvin

CSBroomfield

RKPilsbury

Base9,000ft

Altocumulusstratiformis



27/45

PictorialCH:Ci-Cc-Cs

Tofindthecorrectcodefigurebelow,startatwhichever Ifadescriptionisreachedwhichisnotapplicable,returntocircle

is

applicable

and

then

follow

the

solid

line

from

thepreviousdescriptionandfollowthepeckedline.

descriptiontodescription,solongasallcriteriaaremet.

CcaloneormorethananyCiandCscombined CH9

Ci

invading

the

sky CH4

DenseCioriginatingfromCb,present CH3

DenseCi+turretedCi+CiintuftsmorethanotherCi CH2

CSBroom

field

CSBroomfield

CSBroomfield

RKPilsbury

Crowncopyright

NodenseCi

originating

fromCb.

UseCH =1or2,

whichever

predominates

Cinotinvading

the

sky

Csnotcovering

the

wholesky

No

Cs

Cs

present

Cs

invading

thesky

CcaloneormorethananyCiand

Cs

combined

No

Cc

or

Cc

lessthananyCiand

Cscombined

Cs

covering

the

whole

sky CH7

Csnotinvadingthesky CH8

Csexceeding45 CH6

CiinfilamentsorhooksmoreCsnotexceeding45 CH5thanotherCi CH1

RKPilsbu

ry

Crowncopyright

RKPilsbury

RNHughes



28/45

CH9Cirrocumulus

(Cloud

Group

C1)

Real

cirrocumulus

is

uncommon.

Thecloudiscomposedofverysmallelements,mostofwhichhaveanapparentwidth

of

less

than

the

little

finger

held

at

armslength.Theelementsnevershowshading.Theyareoftenarrangedinripplesresembling

those

left

by

the

ebb

tide

in

the

sandontheseashore.Thecloudelementsandclearspacesmayalsobearrangedinamanner

suggesting

a

net

or

a

honeycomb

(topphotograph).Theregularpatternofwaves

and

small

gaps

may

resemble

the

skin

of

a

mackerel,

thus

giving

rise

to

the

popularnamemackerelsky.(Thisnameisalsooccasionallygiventohighaltocumulusclouds.)

In

hilly

regions

the

cloud

may

appearinmoreorlessisolatedpatcheswhicharealmond-shapedandverywhitethroughout.

ThecloudisfrequentlyassociatedwithcirrusorcirrostratusbutcodefigureCH=9shouldonlybeusedwhenthecirrocumuluspredominates.

The

clouds

shown

in

the

photographs

had

estimated

bases

of

20,000

ft

or

above.

RKPilsbury

RKPilsbury

FNorton

Cirrocumulus

stratiformis



29/45

CH7Cirrostratuscoveringthewholesky

(Cloud

Group

C2)

CSBroomfield

RKPilsbury

RKPilsbury

Thecirrostratusinthisexampleisthinandfeatureless

Halonotdetectable.Anyvariationsinthethicknessofthecirrostratus arenoticeablewhenthesunislowinthesky

Code

figure

CH=7isonlyusedwhenaveilofcirrostratuscoverstheentiresky.Althoughitmayberelativelydense,theveilis

sometimes

so

thin

that

it

is

barely

visible.

Itmaybedistinguishedfromaltostratusbyitsthinness,whichallowsshadowstobecast

when

the

sun

is

not

low

in

the

sky,

and

thatitoftendisplayshalophenomena.

Ifthesunisbrightitmaybedifficulttoseea

halo

around

it,

but

by

covering

the

sun

withthehanditisusuallypossibletoseeany

halo

quite

well.

The

distance

between

the

top

of

the

thumb

and

the

little

finger

spreadwideapartatarmslengthisalmostaswideastheradiusofthesmall(22)halo.

(Haloes

are

often

spoken

of

in

weatherloreasforeshadowingstorms,buttheyaretoocommontobereliablesignsofimpending

stormy

weather.)

Itissometimesdifficulttodiscerncirrostratusthroughhaze.Cirrostratusdiffers

from

haze,

in

that

haze

is

opalescent

orhasadirtyyellowishtobrownishcolour.

If

there

are

any

gaps

in

the

veil

of

cirrostratusthroughwhichtheblueofthesky

can

be

distinguished,

the

coding

for

CHshould

be

8.

Cirrus

at

different

levels,

and

cirrocumulus,

mayalsobepresent.



30/45

CH8Cirrostratusneitherprogressivelyinvading

theskynorentirelycoveringit(CloudGroupC2)

The

sky

corresponding

to

CH8ischaracterisedbythepresenceofaveilofcirrostratuswhichisnot(ornolonger)invading

the

sky

progressively

and

which

doesnotcompletelycoverit;theedgeoftheveilmaybeclear-cutorfrayed.

The

code

figure

CH=8isalsousedwhencirrostratusoccursinpatcheswhethertheyincreaseinamountornot.

Cirrus

and

cirrocumulus

may

also

be

present,

but

should

not

predominate

over

thecirrostratus.

RKPilsbury

CSBroomfield

Cirrostratusnotincreasing.Thephotographontherightwastaken90minutesafterthephotographontheleft

Patchofcirrostratus



31/45


32/45

CH4Cirrusprogressivelyinvadingthesky

(CloudGroupC0)

The

cirrus

clouds

of

CH4arethesamespeciesofcirrusasthoseofCH1(page33),butwiththeimportantdifferencethatasCH4theyprogressivelyinvadethesky.Theygenerallybecomedenserasawhole.Theyusuallyseemtofusetogetherinthedirection

of

the

horizon

from

which

they

firstappearedandtheforwardedgemovestowardstheoppositepartofthehorizon.The

clouds

occur

most

frequently

in

the

formofstrandstrailingfromasmallhook

or

tuft,

and

less

frequently

in

the

form

of

straight

or

irregularly

curved

filaments.

Cirrostratusshouldnotbepresent,Totland,IOW

otherwisethecodingwouldbeCH=5or6 Cirrusuncinusasthecasemaybe.

Thecirruscloudsshowninthephotographsonthispageweresteadilyinvading

the

sky

and

had

an

estimated

base

of20,00025,000ft.

Cirrusfibratus

RKPilsbury

RKPilsbury

RKPilsbury

Totland,IOWCirrusuncinus



33/45

CH3Densecirrusfromcumulonimbus

(CloudGroupC0)

RKPilsbury

M

Kidds

JFPGalvin

Wokingham,BerksCirrusspissatus

Reading,Berks

OvertheEnglishChannel

The

code

figure

CH=3isusedonlywhentheobserverisreasonablycertainthatatleastoneofthedensecirruscloudsinthesky

originated

from

the

upper

part

of

a

cumulonimbus.Itmaybepossibletoseethisdevelopmentifawatchcanbekeptonthe

sky

(middle

photograph).

Such

cirrus

cloudsfrequentlyhavehairyorfrayededgesandareoftenintheformofananvil.These

clouds

are

sufficiently

thick

to

veil

thesun,obscureitsoutlineorevenhideit.

In

winter

this

form

of

cirrus

can

occur

well

below

20,000

ft.

Othercirruscloudsmayalsobepresent.



34/45

CH2Densecirrus

(CloudGroupC0)

The

cirrus

of

this

specification

frequently

occursinentangledsheaves(topphotograph),orinpatchesthickenoughtoappear

grey

when

viewed

towards

the

sun

(bottomphotograph).Itdoesnotusuallyincreaseinamount.

Cirrus

of

CH2mayalsooccurinnarrowbandswithsproutingsliketurretsorbattlements(centre)andisthencalledcirrus

castellanus.

Another

species

takes

the

formofcirrusinsmalltufts,thelowerpartoften

being

more

or

less

ragged

(centre

inset).

This

species

is

known

as

cirrus

floccus.

CirrusofcodefigureCH=1mayalsobepresent,

but

should

not

predominate.

Ifanyofthecirrushasoriginatedfromtheupperpartofacumulonimbusthecodingfor

CHshouldbe3.SometimesthecirrusofCH2developsintothickanvilshapeswhichcouldbemistakenfortheCH3ofadecaying

cumulonimbus.

RKPilsbur

y

RDWhyman

WGPendleton

Llandyrnog,

Clwyd

Base20,000ft

Cirruscastellanus25,000ftbeneathothercirrusat30,000ft

FNorton

Cirrusfloccus



35/45

CH1Cirrusinfilamentsorhooksnotprogressively

invadingthesky(CloudGroupC0)

CSBroomfield

RKPilsbury

RDWhym

an

Cloudheightabout30,000ft

The

white,

delicate,

hair-like

cirrus

clouds

of

CH1occurmostofteninnearlystraightorsomewhatcurvedfilaments(topphotograph).

Sometimes

they

are

shaped

likecommastoppedwithahookoratuft(middlephotograph)andinthisformtheyare

popularly

called

mares

tails.

The

elementsmaysometimesbearrangedinamannersuggestingafishskeletonwithaspinal

column

and

filaments

on

either

side

likeribs.Cirrusmayalsooccurinparallel

bands,

sometimes

broad,

which

owing

to

perspective

may

appear

to

converge

towardsthehorizon(bottomphotograph).Theheightofthecirrusinthephotographsopposite

was

at

least

20,000

ft.

ThecirrusofCH1doesnotprogressivelyinvade

the

sky.

This

type

of

cloud

often

occurswithothercirrusclouds,butthehighcloudshouldbecodedasCH=1onlywhen

the

combined

cover

of

all

filaments,

strandsandhooksexceedsthecoverofallothercirrusclouds.



36/45

Specialclouds

NACREOUSCLOUDSresemblepalecirrusorlenticularaltocumulusandshowverymarked

irisation,

the

most

brilliant

colours

occurringwhenthesunisjustbelowthehorizon.Theyaresometimescalledmother-of-pearl

clouds.

They

can

still

be

distinguisheduptoabouttwohoursaftersunsetasthingreycloudsstandingoutagainst

the

starry

sky.

In

moonlight

they

maybevisiblethroughoutthenight.TheyhavebeenobservedmainlyfromNorway,ataltitudes

between

21

and

30

km,

and

Scotland.

They

are

ignored

when

assessing

CH and

C.

NOCTILUCENT

CLOUDS

resemble

thin

cirrus,butareusuallybluishorsilvery,sometimesorangetored,orreddishwhenon

the

horizon.

They

are

extremely

rare,

beingmostcommonlyobservedonclearmidsummernightsbetweenlatitudes55and

65

N.

They

become

visible

at

the

same

timeasthebrighteststarsandappearmorebrilliantaftermidnight.Theiraltitudeisbetween

75

and

90

km.

Particles

collected

byrocketsin1962providedstrong

indications

that

these

clouds

consist

of

icecrystals.

They

are

ignored

when

assessing

CH andC.

CONDENSATIONTRAILS(contrails)forminthe

wake

of

aircraft

when

the

air

is

sufficientlycoldandhumid.Theyareoftenshort-lived,but,especiallywhencirrusandcirrostratus

are

present,

they

may

spread

outandpersistforseveralhours.PersistenttrailsarereportedbyusingtheCH codefigure

most

appropriate;

sometimes

it

is

impossibletodistinguishbetweenoldtrails

and

cloud.

They

may

produce

halo

phenomena

with

exceptionallypurecolours.OvertheUKtheyrarelyformbelow28,000ftinsummerand

20,000

ft

in

winter.

They

may

cast

shadowsonthincloudsbeneaththem.Aseriesofsuchshadowsmaybetheonlyindication

that

there

is

more

than

one

layer

ofcloudpresent.

Lyddington,Oakham

Pershore,Works

CSBroomfield

SJebson

RM

Blackall

PFTomalin

Contrails

Persistentcontrails



37/45

Opticalphenomenaandotherfeatures

JAWalton

RW

Mason

JHallett

JFreem

an

Halo

Sunpillar

HALO

PHENOMENAThesmallhaloof

22radiuscentredonthesunormoonisseenmostfrequently(topphotograph).Awhitehorizontallineatthesameelevationas

the

sun

is

called

the

parhelic

circle

and

thetwobrightspotsonthisarecalledmocksuns(parhelia).Mocksunsappearfurther

from

the

sun

when

it

is

higher

in

thesky.Thebrightspotabovethesunispartofanarcofcontact.Pillarsoflightmayappear

vertically

above

or

below

the

sun

or

moon(second photograph)andaremostfrequentlyseenatsunriseorsunset.These,

coupled

with

a

portion

of

the

parhelic

circle,mayformacross.Alarge,lessbright,haloof46radiusissometimesseen,

its

arcs

of

contact

perhaps

showing

strongcolouration.Rarelyotherarcsmayoccur,butusuallyonlypartofthedisplayisseen.

Halo

phenomena

are

usually

associatedwithCs,sometimesCi.(MocksunsorpillarsaresometimesseeninAc.)

CORONAAbrownishringofsmall

diameteraroundthemoonorsun.Instrongdaylightitmaybeeasiertodetectbyobserving

the

sun's

reflection

in

calm

water.

Outercolouredringswithredoutermostsometimesoccur.Distortedcoronaemaysometimes

occur

when

the

moon

is

not

full.

Most

frequently

associated

with

Ac,

but

sometimes

occurs

with

Cc,

Cs,

As,

Sc,andSt.

IRISATIONColours,predominantlygreenandpink,oftenwithpastelshades,thatsometimes

appear

on

Cc,

Ac

or

Sc.

Thecoloursmayappearasbandsnearlyparalleltothemarginsoftheclouds,orasamosaic

pattern.

Irisation



38/45

Opticalphenomenaandotherfeatures(continued)

VIRGATrailsofprecipitation(fallstreaks)that

do

not

reach

the

earth's

surface,

attachedtotheundersideofacloud.MainlyassociatedwithCc(smalltrails),Ac(pictured),As(maybeclearlyvisible),Ns,Sc(especially

at

very

low

temperatures),

Cu,

andCb.

MAMMADowndraughtscansometimescause

udder-like

protuberances

to

form

on

theundersurfaceofCi,Cc,Ac,As,Sc(irregularandragged),andCb(bulbous,pictured).

The

protuberances

may

appear

prominentwhenthesunislowinthesky.

SMOKECitysmokeandindustrial

pollutioncausesthesuntolookveryredatsunriseandsunsetandtohaveanorangetint

when

high

in

the

sky.

From

a

distance,

suchpollutionmaybeconfusedwithabankofcloudonthehorizon,butpollutiongenerally

appears

light-grey

or

to

have

a

bluish

hue.

RAINBOWAppearsonascreenof

raindropswhentheobserverhashisbacktothesun.Alessbrightsecondarybow,withcolour

sequence

reversed,

sometimes

occurs,withdarkerskybetweenthetwo.Theymaybeborderedbyfainterbows.When

produced

by

the

moon

the

colours

aremuchweakerorareabsent.TherainbowindicatesCborprecipitatinglarge

Cu.

RKPilsbury

PJBNye

W

SPike

Crowncopyright

Primaryandsecondaryrainbows



39/45

Crowncopyright

RKPilsbury

Crownc

opyright

JFPGa

lvin

CREPUSCULARRAYSThesetaketheform

of

pale

blue

or

white

rays

diverging

fromthesunwhenitisbehindCuorCb.Sunbeamspiercingsmallgapsincloudlayers

(sometimes

called

'sun

drawing

water')andshadowscastbycloudsnearthehorizonattwilightarealsocalledcrepuscular

rays.

Jersey.Crepuscularrays

LIGHTNING

Cloud

accompanied

by

lightningisreportedasCb.

SPOUTAnoftenviolentwhirlwind,

revealed

by

the

presence

of

a

funnel

of

cloud

beneath

Cb,

with

a

'bush'

of

matter

raisedfromtheearth'ssurface.Thecloudandbushoftenmeet.Spoutsoccurundernewly-formed

parts

of

Cb,

not

from

where

therainisfalling.WeakspoutsareoccasionallyseenbeneathCu.

Waterspout

VELUMAnaccessorycloudofgreathorizontalextent,closeaboveorattachedto

the

upper

part

of

Cu

or

Cb

which

often

pierceit.PicturedwithCL9C 9.



40/45

Otherclouds

Big

fires

may

produce

dark

clouds

similar

to

large

cumulus.Combustionproductsmaybecarriedbythewindtogreat

distancesandoccasionallycauseablueappearanceofthesunor

moon.

Real

cumulus

may

also

form.

Volcaniceruptionsmaycauselargecumulus-likecloudsthatmayspreadoutatahighaltitudeovervastareas.Theskythenassumes

a

peculiar

tint

which

may

persist

for

several

weeks.

Verylargeexplosionsareusuallyaccompaniedbyacloudofsmokeordustabovewhichvelumisoftenseen.

Industrial

activities

may

also

produce

clouds.

Fire

clouds,

cloudsofsmokeordust,cloudsfromvolcaniceruptions,andveilsofcombustionproductsareignoredwhenconsidering

the

coding

for

CL,

CM,

CH

and

C.

However,

real

cumulus

andcumulonimbuscloudsthatmayresultfromsucheventsare

reportedintheusualway.

Appearanceofclouds

Whenthesunissufficientlyhighabovethehorizon,cloudsin

direct

sunlight

are

white

or

grey

whilst

those

which

receivelightfromtheblueskyarebluish-grey.Someclouds,whicharebrilliantwhiteinreflectedlight,showmarkedcontrasts

in

brilliance

when

illuminated

from

behind.

The

colourofthesunmaychangeasitapproachesthehorizonandcloudsinthevicinitymayshowacorrespondingcolouration.

The

underside

of

a

cloud

may

redden

when

the

sun

is

on

the

horizonasshowninthepictureofstratocumulus(right).

Haze

may

make

distant

clouds

appear

yellow,

orange

or

red.

Dustparticlesintroduceawhitetingetotheblueofthesky;thustheskyisofadeeperbluewhentheairhasitsoriginsinpolar

regions.

Satelliteviewofclouds

ThispicturewastakenfromaTIROS-Nsatelliteduringtheearly

afternoon

of

12

July

1979.

ThespirallingpatternofcloudsindicatesthecentreofadepressionsouthofIceland.Showercloudsofcumulusandcumulonimbus,

organised

into

streets,

follow

the

strong

windsonthesouthernandeasternflanksofthedepression.South-westofthedepressionthecumulusflattensintostratocumulus.

The

wide

band

of

cloud

off

north-west

districtsoftheBritishIslesisassociatedwithabeltofrain,withcloudpresentatalllevelsfromstratocumulus,altocumulus

and

altostratus

up

to

cirrus.

The

cloud

over

northernFranceismostlydensecirrusandthickaltocumuluscastellanusinassociationwithalowpressurearea.

Over

the

United

Kingdom,

land

heating

over

the

Midlands

hascausedshallowcumulustoform.SomecirrusandaltocumulusarepresentoversouthernEnglandassociatedwith

the

cloud

over

France.

Over

Wales,

the

north

of

England

and

the

Southern

Uplands

of

Scotland,

large

cumulus

clouds

haveformedoverthehigherground.

Cumulus

and

cumulonimbus

cloudsproducing

showers

and

thunderstorms

have

formedoverScandinavia.InteractionbetweentheBalticSeaandland,heatinghasproduced

large

cumulus

and

cumulonimbus

alongthesecoasts.

Cumulusfromapowerstation,cirrostratusonhorizon

Univer

sityofDundee

JFPGalvin

RKPilsbury

Stratocumulusatsunset



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Cloudobservationsatnight

The

sky

should

be

watched

towards

dusk

to

obtain

someguidanceonthecloudswhicharelikelytobepresentafter

thedaylighthasgone.

When

the

sun

is

just

below

the

horizon

the

lowest

clouds

lookgrey,cloudsatmediumlevelslookrose-colouredandthoseveryhighappearwhitish.Asnightfallapproaches,medium-level

clouds

turn

grey

while

cirrus

and

aircraft

condensationtrailsturnyellow,thenpinkandfinallygrey.Thiscoloursequenceisreversedatdawn.

After

nightfall

the

sky

should

be

observed

from

a

dark

place,wellawayfromlights.Theobservationshouldnotbemadebeforetheobserver'seyesareadaptedtothedarkness.

Inmoonlight,cloudsarevisiblewhenthemoonismorethanaquarterfull.Allperceptiblecloudsappearblacktogrey,exceptthoseilluminatedbythemoon,whichpresenta

whitish

appearance.

Halo

phenomena

produced

by

the

moonarealwayswhite.Thecoloursofrainbowsproducedbythemoonaremuchweakerthanthoseproducedbythesun

and

sometimes

absent.

Whenthemoonislessthanone-quarterfulltheremaybedifficultyinidentifyingcloudsatlargeangulardistancesfrom

the

moon.

Their

existence

and

approximate

amount

maybededucedfromtheblottingoutofthestars,althoughstarsnearthehorizonmaybeblottedoutbyhaze

alone.

Thedifficultiesare,ofcourse,substantiallyincreasedifthereisnomoonatall.Observationofcirrusisthendifficult,

but

if

thick

and

extensive

it

may

be

noted

by

its

dimmingeffectonstars.Cirrostratuscausesslightdiffusionoflightaroundeachstar,whosebrillianceisatthesametime

dimmed,

but

in

the

absence

of

moonlight

it

is

almost

impossibletodifferentiatebetweencirrusandcirrostratus.

The

brighter

stars

and

planets

are

visible

through

thin

veilsof

cirrus,

cirrocumulus

and

cirrostratus.

Altostratusisgenerallysodensethatthestarsaremasked.Thegradualloweringofasheetofaltostratusmaybeverydifficult

to

detect,

but

as

the

base

is

rarely

quite

uniform,

as

itdescends,smallcontrastscanoftenbediscernedonallbutthedarkestnights.Nimbostratususuallydevelopsfromthickening

altostratus.

If,

on

dark

nights,

doubt

exists

regarding

the

choice

of

designation

altostratus

or

nimbostratusbyconventionthecloudiscallednimbostratusifrainorsnowisreachingthesurface.Nimbostratus

is

usually

associated

with

moderate

or

strong

windsandstratuswithacalmorlightwind,althoughthis

criterion

alone

must

not

be

used

as

a

basis

for

distinction.

Fog

formed

over

the

sea

and

driven

across

the

coast

by

an

onshorewindmayappearinlandasstratuscloud.Itsspreadacrosstheskymaybeveryrapid.

The

intensity

of

the

darkness

is

of

some

assistance

in

decidingwhethertheskyiswhollycoveredornotwithdenselowcloud.Ifthereisanylightatall,variationofcontrast

may

indicate

patches

of

low

cloud

and

medium

or

highcloudabove.Nearbuilt-upareas,cloudsmayoftenberevealedbyilluminationfrombelow,especiallywhensnowis

lying.

Sodium

street

lighting

often

casts

an

orange

glow

onthebaseofthecloud.Alayerofcloudsoilluminatedmayprovideabrightbackgroundagainstwhichlower

fragments

stand

out

in

dark

relief.

Very

low

cloud

may

obscureknownlightsonhillsandtallstructures.Thelightsoflow-flyingaircraft,orwhenhiddenbylowcloudthenoise

of

their

engines,

may

give

a

clue

to

the

cloud

present.

Whereequipmentisavailabletomeasuretheheightofthecloudbase,theknowledgeoftheheightofthebaseisalsohelpful

in

identifying

the

cloud

types

that

may

be

present.



42/45


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44/45

Notes



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Cloud Types for Observers Rev 2014

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