How Do Clouds Form?
A complete description follows this diagram:
sun heats the ground.
warm ground in turn heats the air above. (In the
above diagram, the warm ground and air are shaded
warm air at the ground is lighter and less dense
than the surrounding air above, so it begins to
rising air takes on the shape of a round "parcel."
things happen as the parcel rises up through the
higher up into the atmosphere you go, the
lower the pressure. As the parcel of air
rises, it experiences this drop in
pressure. As the pressure on the air
parcel drops, the parcel expands and
increases in size. (In the diagram I drew
the rising parcel getting bigger as it
air expands due to a drop in pressure, it
also gets cooler. So, as the parcel rises
and expands, it also cools. (In the
diagram, the green and blue colors
indicate the parcel getting cooler.) The
parcel cools at a rate of approximately
10 degrees C per 1000 meters of rise (or
5.5 degrees F per 1000 feet of rise.)
This is called the dry adiabatic lapse
air is able to hold larger amounts of water
vapor (water in gas form) than cool
air. The cooling parcel is no longer able
to hold all of the water vapor it was
able to hold when it was warm. The excess
water vapor begins to condense out
of the air parcel in the form of liquid
cloud droplets. When water vapor begins
to condense into liquid droplets (moisture),
this means the air parcel is saturated
and has reached its dewpoint
temperature. The higher the parcel
rises, the more it expands, the cooler it
gets, and the greater the amount of
moisture that condenses out of it.
rising, expanding, cooling air parcel finally
rises so high that the surrounding atmospheric
air is the same temperature as the cooled parcel.
This is called equilibrium temperature.
When equilibrium is reached, the parcel loses
momentum and stops rising.
the time the air parcel has reached equilibrium
temperature, enough moisture has condensed out of
it to form a visible cloud!
heights and shapes vary according to the
atmospheric conditions. On a hot day, clouds will
tend to form higher, because the rising air
parcels must rise higher in order to cool enough
to reach dewpoint. When air parcels continue to
rise after dewpoint has been reached and
condensation has occurred, the cloud becomes very
tall. This is how cumulus congestus and
cumulonimbus clouds form. If air parcels only
rise a short distance over a large geographical
area after reaching dewpoint, layer clouds (such
as stratus) will form.
Cloudy Skies © 1998,