Six-In-One Weather Station
(630-0868)            About Clouds/Forecasting        Faxback Doc. # 18071

CLOUDS AND VISIBILITY

A cloud is simply a big group of water molecules or ice particles in the
form of water vapor that stays clumped together and floats in the air
above the earth.  Clouds most often form as the result of air rising from
the earth in one of these ways:

         Convection - the heating of the ground creates bubbles of warm air
                      that rise into the atmosphere.

Topographic Lifting - the lifting of air up the side of a mountain.  The
                      windward side of a mountain (the side that the wind
                      is blowing against) tends to have more clouds than
                      the leeward side (the side facing away from the
                      wind).

        Convergence - the meeting of air from several different directions
                      at the earth's surface near low pressure systems.  A
                      hurricane (over water) is an example of a low
                      pressure center that causes air to converge at the
                      earth's surface, resulting in severe thunderstorms
                      over land.

    Frontal Lifting - the lifting of air along a frontal boundary (the
                      front of a storm, for example).  Cold fronts and warm
                      fronts force air to rise.  The most dramatic cloud
                      formations result from a cold front, because the
                      slope of the front is steeper than a warm front.

CLASSIFYING CLOUDS

When you look up at clouds in the sky, some clouds seem close enough to
touch, and some seem to be very far away.  Some days, the clouds even seem
to form in neat, clean shapes!

Clouds can float from a few feet above the earth (fog) all the way up to
40,000 feet, and higher.  There are 10 different classes of clouds.  As
the chart on page 19 in the manual illustrates, you can tell clouds apart 
based on how high they are in the sky (altitude) and their shape or how
they look.

CLOUD CHART

Cirrus - 20,000 to 40,000 feet

Cirrus clouds are made of very fine ice crystals, which is why they have a
somewhat soft, fuzzy appearance.

Cirrostratus - 20,000 to 40,000

Cirrostratus clouds are the classical harbinger (sign) of bad weather.
They form slowly, like a screen, until they finally cover the whole sky.

Cirrocumulus - 20,000 to 40,000 feet 

Cirrocumulus clouds usually appear in large patches and are arranged in
attractive looking formations.  They most often form ahead of warm fronts,
when a high-level airstream is blowing from the south.  Cirrocumulus
clouds are a sign of increasing humidity in the upper air.

Altostratus - 8,000 to 20,000 feet

From its looks, an altostratus cloud might be called the most "boring"
type.  Altostratus clouds most often appear gray in color, but can
sometimes appear blue-gray.  A growing, thick layer of altostratus clouds
usually means it will rain soon, because they usually form when a storm
front is approaching.

Altocumulus - 8,000 to 20,000 feet

Altocumulus clouds are the most common type of cloud and appear as big
fleecy shapes, and are often confused with cirrocumulus clouds.  Jet vapor
trails, which look like long, straight clouds, form only at cirrocumulus
level, and help to tell the difference between cirrocumulus and 
altocumulus clouds.

Stratocumulus - below 8,000 feet

Stratocumulus clouds are the most widespread type of cloud.  Seen at sea
under almost all weather conditions, they usually appear as a dense,
low-hanging cover and usually dissolve quickly.  Stratocumulus clouds often
darken in the evening, but they are not a sign of bad weather.

Nimbostratus - below 8,000 feet

Nimbostratus is another classic bad-weather cloud.  Prolonged rainfall
usually comes with this cloud, which typically forms and appears with a
warm front.  Nimbostratus is a layer cloud, extending from just above the
earth's surface to the upper boundary of the troposphere (about 7-10 miles
above the earth).

Cumulus - 8,000 to 45,000 feet (base to top)

There are many different types of cumulus clouds - small, medium, or large
size.  Cumulus clouds usually formed at or around the spot in the sky
where you see them, and almost always disappear with an hour.  Their
formation means upward movement of the air.

Stratus - below 8,000 feet

Stratus clouds are the lowest-lying type of cloud, and generally appear in
the form of fog or mist.  When the weather report refers to a high, 
fog-type cloud, it is referring to a stratus cloud.  Stratus clouds have no
structure (shape) and often form over the sea and along the coast.

Cumulonimbus - 10,000 to 60,000 feet (base to top)

Cumulonimbus clouds are thunderstorms.  They form when a cumulus cloud
shoots upwards so strongly that its upper region freezes.  Cumulonimbus
clouds usually develop a delicate, fuzzy structure, but they also can form
caps or collars that resemble cirrus clouds.

When a cumulonimbus cloud's ragged-looking upper region spreads out like
an anvil, it has moved against a barrier layer preventing any further
upward movement.

FORECASTING THE WEATHER

Every day, meteorologists on TV and the radio give a weather "forecast."
What they are actually doing is predicting what the weather will be for
the next 24 hours, or even the next few days, based on the current weather
conditions.  Meteorologists use a variety of different tools, such as
weather maps and models, to explain their forecast.


(PH 11/22/95)