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Textbook, chapter 4, p. 100-106
Cloud classification
• Originally by Luke Howard (1850’s)
• Currently used nomeclature based on Abercromy & Hildebrandsson (1887)
• Linnean system (genus, species)
• morphological only, not genealogical
Latin Root
cumulus
stratus
cirrus
nimbus
Translation
heap
layer
curl of hair
rain
Example
fair weather cumulus
altostratus
cirrus
cumulonimbus
Cloud subclassification
Further classification identifies clouds by height of cloud base.
3 levels
Examples:
the prefix "cirr-" (as in cirrus clouds) refers to high levels,
the prefix "alto-" (as in altostratus) refers to middle levels.
Relative heights vary zonally
20,000
18,000
H
M
L
H
M
L
H
M
16,000
meters agl
14,000
tropopause
12,000
10,000
8,000
6,000
4,000
2,000
0
Tropics
Midlatitudes
Poles
L
Cloud types and relative altitudes
High clouds
composed of ice
crystals
fibrous or filamentous
shape
ice crystal concentration
generally very small
Cirrus uncinus (mares’ tails)
Height (km): 7-10 km+
Falling light snow [fallstreaks], not reaching the ground.
distorted by upper-tropospheric wind shear.
Cirrus uncinus
Cirrus floccus
Cirrus fibratus vertibratus
Cirrus uncinus
Cirrus
Cirrus uncinus
Cirrus spissatus
Cirrus spissatus with virga
Cirrus
Cirrostratus
continuous
sun shines thru, no precipitation
process: widespread ascent aloft,
or old thunderstorm anvil
halo
Cirrocumulus
Cs broken into waves
sun shine thru, no precipitation
process: widespread ascent combined with
convective overturning in a thin layer.
Middle clouds
* occur at altitudes where temperatures range between 0 and -25ºC (32 and -13ºF).
* composed of supercooled water droplets, or are in mixed-phase (supercooled
water droplets and ice crystals).
Altostratus clouds occur as uniformly gray
or white layers that totally or partially
cover the sky. They are usually so thick
that the sun is only dimly visible, as if
viewed through frosted glass.
Altostratus
Height (km): base=2-6 km, can be thick, liquid or ice, no precipitation
reaching the ground. Process: widespread stable ascent, often preceding a
surface warm front.
Altostratus has a
uniform and diffuse
coverage
Iridescent Altostratus (difraction)
Altocumulus
base=2-6 km, usually thin, usually liquid, no precipitation.
Process: widespread ascent combined with convective overturning aloft
(left) or with wave activity (right)
clear regions  descending air
cloudy regions  ascending air
Altocumulus
Sharp cloud boundaries
indicate the presence of
water droplets rather
than ice crystals.
Altocumulus castellanus
Altocumulus stratiformis
Altocumulus lenticularis
Low-Level Clouds
* usually at temperatures above -5ºC (23ºF)
* composed mostly of water droplets.
geostationary
satellite
precipitation
Stratus
Base>0.1 km, top<3 km,
liquid (or ice), may have drizzle falling.
Process: stable ascent, mixing
Fog occurs when stratus meets the ground; when it lifts, it may
break up into stratocumulus.
Nimbostratus
steady light precipitation
Stratocumulus
Shallow, usually liquid, no precipitation
Process: forced or spontaneous overturning in a shallow layer
Marine stratocumulus
cumulus
Buoyantly rising air parcels (thermals) become saturated
Cu cloud base … LCL
Shallow or deep
Cumulonimbus
Cu mediocris
Cu humulis
LCL
Fair-Weather Cumulus
Widely separated heap clouds of small
vertical development. With flat bottoms
and rounded tops, they resemble a flock
of sheep grazing in a pasture.
Cu humilis or mediocris
Cumulus congestus
Larger and deeper  cauliflower appearances to
the tops. Base is flattish, representing the LCL.
Can produce showers.
Cloud top=5-7 km, above the
freezing level but usually still
liquid.
Cumulonimbus
Look at this animation
12 June 2004, Hastings NE. Photo by Doug Raflik
Cumulonimbus: the most active member of the cumulus family
Convective family over the South China Sea
Cumulonimbus
Pop quizzes: cloud identification
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A: cirrus
B: cirrostratus
C: cirrocumulus
D: altostratus
E: altocumulus
F: stratus
G: stratocumulus
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H: cumulus humilis
I: cumulus congestus
J: cumulonimbus
J: nimbostratus
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
a mid-level
cloud in the
lee of
mountain
ranges
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
Hint: it is overcast, but not raining
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
Hint: this cloud layer is low
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
Hint: this cloud layer is high
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
this is a halo
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cirrus
cirrostratus
cirrocumulus
altostratus
altocumulus
stratus
stratocumulus
cumulus humilis
cumulus congestus
cumulonimbus
nimbostratus
Hint:
it is raining
A few other cloud visual clues
1. anvil development
process: buoyant ascent
lightning usual
(lightning is rare over the oceans)
much precip evaporates if LCL is high
view this animation
Cumulonimbus: Severe weather
lightning, thunder, heavy
rains, hail, strong winds,
and tornadoes …
Model simulations:
Typical thunderstorm
Severe thunderstorm
Cumulonimbus: Shelf clouds
We are just ahead of a severe thunderstorm. Dewpoints are in the mid-70s.
Winds gust to over 40 mph with the passage of this shelf cloud.
Evaporatively cooled air is pushed out of the precipitation area by the
downdraft, warm air slides up and over the gust front forming the concaveshaped shelf cloud.
Cumulonimbus: Mammatus clouds
pockets of negatively-buoyant air, filled with snow
suspended from the anvil base
Mesoscale Convective Complexes
Radar animation
Orographic and wave clouds
Orographic clouds are produced by the flow of air interacting with
mountainous terrain. They often indicate areas of clear air turbulence.
lee wave
mountain wave
Altocumulus
lenticularis
Lenticular clouds
Altocumulus lenticularis are the
"mountain-wave clouds." As
strong horizontal winds encounter
a mountain range, gravity waves
propagate upwards, and leeward.
Mountain waves
are stationary.
Ac lenticularis elsewhere
RH stratification may yield a ‘stack of pancakes’
Plymouth, NH, 3 Dec ‘96
Which one is liquid
which one ice ?
Unusual AC lenticularis
Mountain-wave turbulence
Föhn wall cloud, Rockies
turbulent rotor cloud in Owens Valley, CA, downwind of the Sierras.
Mountain-wave turbulence :
Banner clouds
Banner cloud or lenticular cloud ?
(same mountain)
Kelvin-Helmholtz waves and KH billows
KH waves form when strong wind shear overturns a stable layer (i.e., an
inversion).
They are usually invisible!
(CAT)
Kevin-Helmholtz instability
Breaking billows – an aviation hazard
 Kelvin-Helmholtz
waves on
Jupiter.
Colors
indicate the
clouds'
altitudes:
blue is lowest
through red
as highest.
Cap clouds
Pileus, Latin for "skullcap,"
is a smooth cloud that forms
when a stable, humid layer aloft
forced to rise by a penetrating
Thunderstorm cell.
Pileus cloud
attached to
the top of a
cumuliform
cloud.
 cap cloud on a Cb
“Morning glory” roll clouds
These occur when a deep stable layer overruns a shallower stable layer.
Gravity waves ahead of the deep layer result in roll clouds, and may
cause severe turbulence.
 ~1000 km
Air rises and condenses at the leading edge, and evaporates behind
These cigar-shaped clouds also occur in the US, mainly in spring
note the stably stratified layer below, and the more turbulent one above