8/10/2019 Cloud Types for Observers Rev 2014
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Cloud
types
for
observers
Readingthesky
8/10/2019 Cloud Types for Observers Rev 2014
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8/10/2019 Cloud Types for Observers Rev 2014
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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
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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
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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
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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
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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
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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.
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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
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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.
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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
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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.
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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
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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
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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.
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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
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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.
Cloud types for observers 17
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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
18 Cloud types for observers
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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
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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
20 Cloud types for observers
8/10/2019 Cloud Types for Observers Rev 2014
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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.
Cloud types for observers 21
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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.
Cloud types for observers 23
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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
24 Cloud types for observers
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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
Cloud types for observers 25
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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
26 Cloud types for observers
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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.
Cloud types for observers 27
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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
28 Cloud types for observers
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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
30 Cloud types for observers
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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.
Cloud types for observers 31
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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
32 Cloud types for observers
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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.
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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
34 Cloud types for observers
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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
Cloud types for observers 35
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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
36 Cloud types for observers
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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.
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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.
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Notes
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