Everyone has heard the
expressions "cloud nine" or "walking in the clouds."
Although clouds look like delicious whipped cream or cotton candy, they are
actually masses of condensed water vapor.

Clouds usually form when
cool, moist air rises through the atmosphere. As the earth's surface warms,
water evaporates until the air becomes saturated. As one goes higher in the
atmosphere, pressure decreases while air expands and cools. Water droplets
condense and become visible as clouds.

During storms, dense clouds
form. Water droplets grow heavy and eventually descend as rain. In time, this
moisture will evaporate from the ground and reform in the atmosphere as clouds.
This process is called the water cycle.
 

There are many different
types of clouds. One common type is cumulus, a billowy cloud heaped high on a
flat base. Cirrus clouds are very high clouds that are wispy or streaked. In the
winter, low-level stratus clouds are predominant with fog. Nimbus clouds are
dark sheet-like clouds that bring rain. Cumulonimbus are large anvil shaped
clouds which produce thunder and lightning. Lenticular clouds are smooth,
stationary mid level clouds that develop over mountains.
To find out more about the history of the naming of clouds, click here.

Clouds develop in many
different ways besides the evaporation of water into the atmosphere. When air
passes over a cooler surface, such as the dewy ground at night, the wind may
stir some of the coolness upward, creating fog. Artificial clouds form when damp
air masses of different temperatures mix. Examples include the steam made by
expelled breath in cold weather and the exhaust trails produced by high-flying
aircraft. A rapid local reduction of pressure, along with the expansion and
cooling of the air, leads to the violent rotating motions found in funnel clouds
and tornadoes.

When clouds reach
saturation, they precipitate or produce rain. Under cold conditions, clouds
produce snow but not directly. Condensing water droplets at below freezing
temperatures are said to be supercooled. Occasionally a very small number of
these will freeze. However, ice crystals absorb more water vapor so that they
quickly grow, shed splinters, and multiply. Eventually, the water droplets
completely evaporate and the cloud becomes a pure ice cloud.
 

Many
factors cause clouds to come in many shapes and sizes. As a cumulonimbus hits
the bottom of the stratosphere, a ceiling effect occurs; the top of the cloud
flattens against this invisible surface and stretch into an oval. Persistent
moist winds over mountainous terrain can produce smooth elongated clouds that
stand in one location, known as lenticular clouds.

Source:
http://www.wrh.noaa.gov/spokane/outreach/ttalk/clouds.htm