are the 10 cloud names that I am hoping you will
learn. My recommendation is that you
concentrate on the "key words" shown above that
tell you something about a cloud's altitude and
appearance. Then you'll not only be able
to name the cloud, but will be able to describe
it in words.
Drawing a chart
like this on a blank sheet of paper is a
good way to review cloud identification
and classification. There are 10 boxes in
this chart, one for each of the 10 main cloud
types. Eventually, you should be able to
put a cloud name, a sketch, and a short
written description in each square.
Start by writing down
the key words for altitude along the vertical side
of the chart and key words for appearance across the
top. You'll find a completed version of the
chart at the end of today's notes.
classified according to the altitude at which
they form and the appearance of the
cloud. We'll discuss altitude
Clouds are grouped into one
of three altitude categories: high, middle level,
and low. It is very hard to just look
up in the sky and determine a cloud's
altitude. You will need to look for other
clues to distinguish
between high and middle altitude clouds.
We'll learn about some of the clues when we look
at cloud pictures later in the class.
identifies a high altitude cloud. There are
three types of clouds found in the high altitude
in a cloud name means the cloud is found at
middle altitude. The arrow connecting
altostratus and nimbostratus indicates that they are
basically the same kind of cloud. When an
altostratus cloud begins to produce rain or snow its
name is changed to nimbostratus. A
nimbostratus cloud may become somewhat thicker and
lower than an altostratus cloud. Sometimes it
might sneak into the low altitude category.
There is no key word for low altitude clouds.
Low altitude clouds have bases that form 2 km or
less above the ground. The summit of Mt. Lemmon in the Santa Catalina mountains
north of Tucson is about 2 km above the
valley floor. Low
altitude clouds will have bases that form at or
below the summit of Mt. Lemmon.
High clouds are found up near the top of
the troposphere. Moving from left to right
across the top of the picture are cirrocumulus,
cirrostratus, and cirrus. It's cold up at
high altitude and violet in this picture means
the cloud is composed entirely of ice crystals.
Green at the bottom of the figure indicates
clouds that form in warmer air and are composed
of just water droplets, As you move left
to right at the bottom of the picture are
cumulus, stratus, and stratocumulus. Note
how the base of these low clouds is at or below
the summit of the mountain in the sketch.
Blue, at middle levels indicates a mixture of
ice crystals and supercooled water droplets
(water that has cooled to below freezing but
hasn't frozen). The figure shows
altocumulus, altostratus, and nimbostratus
producing rain and snow.
The tall cloud at right is a thunderstorm, also
named cumulonimbus. The cloud starts in
the low altitude category and grows all the way
up to the top of the troposphere, the top of the
high altitude category.
A key word for cloud appearance is the
second part of most cloud names.
We'll start with cumuliform. In
some ways cumulus clouds resemble a head of
cauliflower. C for cumuliform and C for
cauliflower. Cumulus clouds might also
have a lumpy, patchy or puffy appearance and
might form in waves or ripples. You'll
find cumulo or cumulus in a cloud that has
texture like this
Clouds can have a patchy of
puffy (or lumpy, wavy, splotchy or ripply)
appearance. These are cumuliform clouds and
will have cumulo or cumulus
in their name. In an unstable atmosphere
cumuliform clouds will grow vertically.
Strong thunderstorms can produce dangerous severe
- as in rock strata, stratosphere
Stratiform clouds grow
horizontally and form layers. They form when
the atmosphere is stable. You'll find strato or stratus
in the cloud name.
at the Grand Canyon
A side view of a layer cloud.
How much sunlight is able to shine through the
cloud depends on how thick the cloud is.
A person on the ground may or may not cast a
A view from the ground
look up at the sun through a middle level
layer cloud. The sun is visible but
There's a 5th key word that I
have been forgetting to mention.
or nimbus, means precipitation (it is also the
name of a local
brewing company). Only two of the 10
cloud types are able to produce (significant
amounts of) precipitation. It's not as easy
as you might think to make precipitation.
We'll start to look at precipitation producing
processes in the next class.
tend to produce fairly light precipitation over a
large area. Cumulonimbus clouds produce heavy
showers over localized areas. Thunderstorm
clouds can also produce hail, lightning, and
tornadoes. Hail would never fall from a Ns cloud.
While you are still learning the cloud names you
might put the correct key words together in the
wrong order (stratonimbus
instead of nimbostratus, for example). You
won't be penalized for those kinds of errors in this
class because you are putting together the right two
we looked at photographs of most of the 10 cloud
types. You'll find the written
descriptions of the cloud types in the images
below on pps 97-98 in
the ClassNotes. You won't find the pictures,
you should draw those in yourself.
HIGH ALTITUDE CLOUDS
High altitude clouds are thin because the air at
high altitudes is very cold and cold air can't
contain much moisture, the raw material needed to
make clouds (the saturation mixing ratio for
cold air is very small). These clouds are
also often blown around by fast high altitude
winds. Filamentary means "stringy" or
"streaky". If you imagine trying to paint a
Ci cloud you might dip
a small pointed brush in white paint brush it
quickly and lightly across a blue colored
canvas. Here are some pretty good
photographs of cirrus clouds (they are all from a
article on Cirrus Clouds)
A cirrostratus cloud is a thin uniform white
layer cloud (not purple as shown in the figure)
covering part or all of the sky. They're so
thin you can sometimes see blue sky through the
cloud layer. Haloes are a pretty sure
indication that a cirrostratus cloud is
overhead. If you were painting Cs clouds you
could dip a broad brush in watered down white paint
and then paint back and forth across the canvas.
detour to briefly discuss haloes and sundogs.
Haloes are produced when white light
(sunlight or moonlight) enters a 6 sided ice
crystal. The light is refracted
(bent). The amount of bending depends on the
color (wavelength) of the light
(dispersion). The white light is split into
colors just as light passing through a glass
prism. Crystals like this (called columns)
tend to be randomly oriented in the air.
That is why a halo forms a complete ring around
the sun or moon. You don't usually see all
the colors, usually just a hint of red or orange
on the inner edge of the halo.
is a flatter crystal and is called a plate.
These crystals tend to all be horizontally
oriented and produce sundogs which are only a
couple of small sections of a complete halo.
A sketch of a sundog is shown below.
Sundogs are pretty common.
Keep an eye out for them whenever you see high thin
clouds in the sky at sunrise or sunset.
very bright halo is shown at upper left with the
sun partially blocked by a building (the cloud
is very thin and most of the sunlight is able to
shine straight through). A halo like this
would draw a crowd. Note the sky inside
the halo is darker than the sky outside the
halo. The halo at upper right is more
typical of what you might see in Tucson.
Thin cirrus clouds may appear thicker at sunrise
or sunset because the sun is shining through the
cloud at a steeper angle. Very bright
sundogs (also known as parhelia) are shown in
the photograph at bottom left. The sun in
the photograph at right is behind the
person. You can see both a halo and a
sundog (the the left of the sun) in this
photograph. Sources of
these photographs: upper
If you spend enough time outdoors
looking up at the sky you will eventually see all 10
cloud types. Cirrus and cirrostratus clouds
are fairly common. Cirrocumulus clouds are a
little more unusual. The same is true with animals, some
are more commonly seen in the desert around Tucson
(and even in town) than others.
To paint a Cc cloud you could dip a
sponge in white paint and press it gently
against the canvas. You would leave a
patchy, splotchy appearing cloud (sometimes you
might see small ripples). It is the patchy
(or wavy) appearance that makes it a cumuliform
The table below compares
cirrostratus (the cloud on the left without
texture) with a good example of a cirrocumulus
cloud (the "splotchy" appearing cloud on the
right). Both photographs are from the
Wikipedia article mentioned earlier.
Altocumulus clouds are pretty
common. Note since it is
hard to accurately judge altitude, you must rely
on cloud element size (thumbnail size in the case
of Ac) to determine whether a cloud belongs in the
high or middle altitude category. The
cloud elements in Ac clouds appear larger than in Cc
because the cloud is closer to the ground. A
couple of photographs are shown below (source: Ron
Holle for WW2010 Department
of Atmospheric Sciences, the University of
Illinois at Urband-Champaign
There's a much larger collection in this gallery
of images. The fact that there are so many
examples is an indication of how common this
particular type of cloud is.
Altostratus clouds are
thick enough that you probably won't see a
shadow if you look down at your feet.
The sun may or may not be visible through
the cloud. Three examples are shown
below (the first is from a
the middle and right photograph are from
Canada web page)
When (if) an altostratus
cloud begins to produce precipitation, its
name is changed to nimbostratus.
were there and could see if it was raining or
snowing you might call this an altostratus or
even a stratus cloud. The smaller darker
cloud fragments that are below the main layer
cloud are "scud" (stratus fractus) clouds (source
of this image).
LOW ALTITUDE CLOUDS
This cloud name is a
little unusual because the two key words for cloud
appearance have been combined, but that's a good
description of this cloud type - a "lumpy layer
cloud". Remember there isn't a key word for
low altitude clouds.
Because they are closer
to the ground, the separate patches of Sc are
bigger, about fist size (sources of these images:left
The patches of Ac, remember, were about thumb
nail size.. If the cloud fragments in
the photo at right are clearly separate from
each other (and you would need to be
underneath the clouds so that you could look
to make this determination) these clouds would
probably be "fair weather" cumulus. If
the patches of cloud are touching each other
(clearly the case in the left photo) then
stratocumulus would be the correct
didn't show any photos of stratus clouds in
class. Other than being closer to the
ground they really aren't much different from
altostratus or nimbostratus.
clouds come with different degrees of vertical
development. The fair weather cumulus clouds
don't grow much vertically at all. A cumulus
congestus cloud is an
intermediate stage between fair weather cumulus
and a thunderstorm.
A photograph of "fair
weather" cumulus on the left (source)
and cumulus congestus or towering cumulus on the
FIT INTO ALL 3 ALTITUDE CATEGORIES
are lots of distinctive features on cumulonimbus
clouds including the flat anvil top and the
clouds sometimes found on the underside of the
dense downdraft winds hit the ground below a
thunderstorm and spread out horizontally
underneath the cloud. The leading edge of
these winds produces a gust front (in Arizona
dust front might be a little more
descriptive). Winds at the ground below a
thunderstorm can exceed 100 MPH, stronger than
The top of a
thunderstorm (violet in the sketch) is cold enough
that it will be composed of just ice
crystals. The bottom (green) is composed of
water droplets. In the middle of the cloud
(blue) both water droplets and ice crystals exist
together at temperatures below freezing (the water
droplets have a hard time freezing). Water
and ice can also be found together in nimbostratus
clouds. We will see that this mixed phase
region of the cloud is important for precipitation
formation. It is also where the electricity
that produces lightning is generated.
top left photo shows a thunderstorm viewed from
space (source: NASA
Earth Observatory). The flat anvil top
is the dominant feature. The remaining three
photographs are from the UCAR
Digital Image Library. The bottom left
photograph shows heavy by localized rain falling
from a thunderstorm. At bottom right is a
photograph of mammatus clouds found on the
underside of the flat anvil cloud.
air spilling out of the base of a thunderstorm is
just beginning to move outward from the bottom
center of the storm in the picture at left. In
the picture at right the cold air has moved
further outward and has begun to get in the way of
the updraft. The updraft is forced to rise
earlier and a little ways away from the center of
the thunderstorm. Note how this rising air
has formed an extra lip of cloud. This is
called a shelf cloud.
Here's a photograph of the dust
stirred up by the thunderstorm downdraft winds
(blowing into Ahwatukee, Pheonix on Aug. 22,
2003). The thunderstorm would be off the left
somewhere and the dust front would be moving toward
the right. Dust storms like this are often
called "haboobs" (source
of this image)
. We'll learn more
about the hazards associated with strong downdraft
winds later in the semester when we cover
Shelf clouds can sometimes be quite
impressive (the picture above is from a
Wikipedia article on arcus clouds
The main part of the thunderstorm would be to the
left. Cold air is moving from left to right in
this picture. The shelf cloud forms along the
advancing edge of the gust front
the completed cloud chart and here's a link to a cloud
chart on a National Weather Service
webpage with actual photographs.
See if you can fill in the cloud names using just
the abbreviations and pictures of the clouds as clues.