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The most common way clouds form is through lifting. In this process, a "parcel" of air is forced upward by convective uplift, orographic lifting, frontal wedging or convergence. As the parcel of air rises, it encounters lower pressures, expands, cools adiabatically, and eventually reaches saturation at the lifting condensation level.
Clouds are composed of minute liquid water droplets and/or ice crystals depending on the cloud's temperature. When the outside air temperature is between 0 degrees C and -20 degrees C, clouds are largely composed of supercooled water droplets, and usually contain some ice particles. Ice crystals usually predominate at temperatures lower than -20 degrees C. However, supercooled water droplets can exist at temperatures as low as -40 degrees C. Cloud particles have an average radius of 10 to 100 microns and are clustered together closely enough to make them visible. (AFH 11-203, Vol 1, 1997).
Clouds are classified using a combination of form and height. In general, form refers to characteristics such as size, shape, color and physical composition. Height refers to the altitude above the Earth's surface.
There are three forms
clouds may take: cirrus, cumulus and stratus.
Cirrus are
the highest clouds, thin, white and wispy. They are composed primarily
of ice crystals.
Cumulus are
globular, "cauliflower-like" in appearance, and tend to have flat bases.
Stratus form
in sheets or layers and cover much of the sky.
Height is also divided into three catagories: low, middle and high. There are variations in these altitudes due to seasonal and latitudinal variations. Colder seasons (winter) and geographic locations (North & South Poles) have lower heights for each catagory, and warmer seasons (summer) and geographic locations (Equator) have higher heights.
Low clouds are
found below 2000 meters or 6,500 feet*.
Middle clouds
are found between 2000-6000 meters or 6,500-20,000 feet.
High clouds
are found above 6000 meters or 16,000-45,000+ feet.
*Feet values are taken from AFH 11-203, Vol 1, 1997 and are based on mid-latitude locations above ground level (AGL). Clouds below 50 feet are considered fog. Metric values are taken from Lutgens & Tarbuck, 1998.
Clouds are broken down into four families: high, middle, low, and clouds of vertical development.
HIGH
Composed of ice crystals
and are not considered
precipitation makers.
There are three members in
this family.
Cirrus.
These are thin, white, feathery clouds found
in patches or narrow
bands. As the ice crystals fall into
slower winds below,
the clouds may take on a curved
appearance, often
referred to as "mares tails". These
"mares tails" indicate
upper level wind direction and speed.
Cirrus may indicatate
that bad weather is approaching.
cirrus
Cirrostratus.
These are thin, white clouds that appear as
a sheet or veil.
The Sun or Moon will appear dimmer
through these clouds,
and may produce a halo. Classic
cirrostratus usually
signal the approach of a warm front by
12 to 24 hours. (AFH
11-203, Vol 1, 1997)
cirrostratus
Cirrocumulus.
These clouds are thin, closely-spaced,
white globular "cotton
balls" that appear similar in size to
the sun. They
may be found in small patches or larger
layers, sometimes
arranged in a regular pattern giving it a
distinctive look called
"mackerel sky", due to its similarity
to fish scales.
Their appearance indicates upper level
atmospheric instablility.
Cirrocumulus
MIDDLE
May be composed of
ice crystals, water droplets, or
super-cooled water
droplets. There are two members
in this family.
Altostratus.
These are fairly uniform grey sheets covering
much or all of the
sky. The Sun may be visible through
the upper layers,
but diminishes as the layer thickens.
These clouds signal
the arrival of a warm front, and light
precipitation is possible.
altostratus
Altocumulus.
These are white or grey in color with
rounded or rolled
shapes, commonly found in large
patches across the
sky. They are composed of water
or ice as they may
be found at varied altitudes. Alto-
cumulus clouds are
associated with different fronts and
often indicate a weather
change within the next few hours.
altocumulus
LOW
May be composed of
ice crystals or super-cooled water
droplets during the
colder months, but are generally
water droplets during
the warmer months. There are
three members in this
family.
Stratus.
These form a low, uniform, sheet-like layer.
They often cover much
of the sky, and can produce
light precipitation.
Status can form from fog that is
slowly lifting off
the ground or being blown onshore
from fog formed over
water bodies.
stratus
Stratocumulus.
These appear as large, greyish, globular
masses or rolls.
They often result from a layer of stable
air lifted and mixed
by wind blowing over rough terrain.
Stratocumulus also
form from the breaking up of a stratus
layer and from the
spreading out of cumulus clouds.
stratocumulus
Nimbostratus.
These are grey to dark grey layered
clouds. They
generally cover a large area of the sky
and are associated
with long, light, continuous precipitation
over a widespread
area. Although their bases are in the
low region, and are
found in stable atmoshpheric
environments, they
may extend vertically upwards
to great heights.
nimbostratus
CLOUDS
OF VERTICAL DEVELOPMENT
These clouds are not
restricted to the height boundaries
as their bases are
in the low layer, but their tops may
extend through the
middle or high layers. They are
related by their instability.
There are two members in
this family.
Cumulus (fair weather).
These are the puffy,
white "cotton ball"
clouds we associate with typical
clouds. Cumulus
are characterized by their flat
bases, rounded tops,
and "cauliflower-like"
appearance.
They generally form on clear days
when unequal surface
heating causes parcels of air to
rise above the condensation
level. Little precipitation
is associated with
these clouds.
cumulus (fair weather)
Once they enlarge past
the low level range, they
are referred to as
towering
cumulus, or cumulus
congestus,
and may even form into the great
cumulonimbus.
cumulus congestus (towering cumulus)
Cumulonimbus.
These are large, dark, dense,
billowy clouds of
great vertical height. Many form
the characteristic
"anvil head" at the very top. The
lower layers are comprised
of water, and the upper
layers of ice.
These clouds are synonmyous with
thunderstorms, and
may be accompanied by strong
winds, heavy rain,
lightning, hail and tornadoes.
Precipitation is strong,
usually of short duration,
and intermitant over
the geographic area.
cumulonimbus
CLOUD
VARIETIES
In addition to the
names given to the 10 basic cloud
types, adjectives
may also be used to describe variations
of a particular cloud
type. (Lutgens & Tarbuck, 1998)
Unicus. Describes
something hook-shaped like certain cirrus clouds.
Fractus.
When stratus or cumulus clouds appear broken into smaller pieces.
Mammatus.
Rounded protrusions on the bottom of a cloud.
Lenticular.
Lens-shaped clouds, often associated with mountains.
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