Clouds
MICHAEL DAVIS
PHD CANDIDATE
THE OHIO STATE UNIVERSITY
Altocumulus
(Ac)
Midlevel
2400-6100 m (650020000 ft)
Appear in either masses
or rolls and are similar to
cirrocumulus but the
masses are usually larger
and darker
Normally signal
convection and will take
on a wavy sheet-like
appearance
Occur before a cold front
and may signal
thunderstorm
development later in the
day
Altostratus
(As)
Midlevel
2400-6100 m (650020000 ft)
Appears as a gray sheet and
is typically lighter than
nimbostratus but darker
than cirrostratus
The sun may be seen
through the cloud and
appear as a ‘bright spot’
Formed by the elevated
condensation of an air
mass usually a frontal
system and can be very
expansive
May produce very light
rainfall
Arcus
A low level cloud
usually on the front
edge of a thunderstorm
with the outflow
boundary
2 types of arcus clouds:
Shelf cloud – low wedge
shaped cloud that is
attached to the
thunderstorm
Roll cloud – detached,
tube shaped cloud that
appears to roll along a
horizontal axis
Cirrocumulus
(Cc)
High level
+6000 m (+20000 ft)
Convective
environment and
composed primarily of
ice and supercooled
water droplets.
Generally short lived as
they may evaporate or
precipitate as virga in
either rain or snow
Patterns may look like
fish scales so sometimes
called “mackerel sky”
Cirrostratus
(Cs)
High level
+6000 m (+20000 ft)
Thin and mostly
composed of ice crystals
May produce an ice
halo as seen in photo
Possible indication of
upper level moisture
Form in front of warm
fronts and rain may be
expected in the next 1224 hours
Cirrus (Ci)
High level
+7000 m (+23000 ft)
Thin and wisplike,
sometimes called mare’s
tails
Composed of ice crystals
Can indicate wind shear
based on the streaks
Cover approximately 30%
of the Earth and warm
the planet via IR
absorption
Usually indicate a change
in the weather in the near
future or blow off from a
thunderstorm
Contrails
Artificial clouds created
by passing aircraft
exhaust
As hot exhaust is
expelled from the
engine, the water vapor
freezes quickly in the
surrounding cold air
and forms ice in trails
Time duration is highly
variable
May act to warm the
planet by trapping
outgoing longwave
radiation
Cumulonimbus
(Cb)
Vertical
2000-16000 m (650060000 ft)
Tall cloud that signifies
the presence of an
intense thunderstorm
Can form alone, in
clusters, or along a cold
front
Form from convection
and strong updrafts
which elevate the cloud
top to the tropopause
Jet stream shears the top
creating an anvil like
appearance
Cumulus (Cu)
Low to mid level
Below 2000 m (Below
6500 ft)
Appears as heaps and
may form alone, in lines,
or in clusters
Often precede other types
of clouds as they
encompass a large variety
of cloud types
Forms when warm air
rises and condenses
May appear flat bottomed
if at the lifted
condensation level or
form ‘cloud streets’ in a
windy environment
Cumulus
Congestus
(TCu)
Vertical
Up to 6000 m (up to
20000 ft)
Form in unstable
conditions usually
undergoing convection
May appear like
broccoli
Usually signal the
progression from
cumulus to
cumulonimbus
Fog
Ground level
Low level cloud that
reaches the ground
resulting in a decrease
in visibility
Forms when saturation
occurs and little wind
Many different types of
fog
Grand Banks,
Newfoundland is
recognized as the
foggiest place in the
world averaging 200
days of fog annually
KelvinHelmholtz
High level
Cirrus like
Forms when there is a
stark difference in
velocities between two
layers of fluid
(remember, the
atmosphere is a fluid!)
This causes motion
between two stratified
layers creating a
shallow layer of
instability
Lenticular
Mid level
6000-12000 m (2000040000 ft)
Lens shaped cloud
typically forming over
areas of orography
Forms when stable moist
air flows up the
orography, creating
standing waves on the
leeward side. If the
temperature reaches the
dew point and condenses,
a lenticular cloud forms
They may be mistaken for
UFOs
Mammatus
Low level
Forms on the underside
of a violent
thunderstorm and
through descending air
Sometimes called a
breast cloud
May appear as smooth,
ragged, or lumpy lobes
averaging 1-3 km wide
and lasting 10-15
minutes
Nimbostratus
(Ns)
Low level
Below 3000 (Below
10000 ft)
Appears uniform and
dark
Will be precipitating as
‘nimbo’ is Latin for
precipitation. If not,
then it is stratus
Precipitation is
moderate to heavy and
may last for several
days
Pileus
Sometimes called a
scarf cloud or cap cloud
Horizontal cloud that
appears on top of
cumulus or
cumulonimbus
Formed by strong
updrafts uplifting moist
low levels causing the
air to cool to the dew
point as it ascends
Indicates strong
updrafts as they tend to
be associated with
thunderstorms
Stratocumulus
(Sc)
Low level
Below 2400 m (8000 ft)
Appear rounded, dark
masses that usually form
in lines or clusters
Weak updrafts and dry,
stable air above keep
clouds low
Don’t usually precipitate
but can produce light rain
or snow. However, they
are seen at either the
front or end of
thunderstorms
Stratus (St)
Low level
Below 2000 m (Below
6000 ft)
Horizontal uniform
gray layer of clouds that
looks like a bed sheet
Usually obscure the sky
thus blocking out the
sun
Essentially above the
ground fog