How Clouds Form: Clouds form when water vapor (water that has evaporated from the surface of the Earth) condenses (turns into liquid water or solid ice) onto microscopic dust particles (or other tiny particles) floating in the air. This condensation (cloud formation) happens when warm and cold air meet, when warm air rises up the side of a mountain and cools as it rises, and when warm air flows over a colder area, like a cool body of water. This occurs because cool air can hold less water vapor than warm air, and excess water condenses into either liquid or ice. For more on the water cycle, click here.

Types of Clouds:

Nimbo (meaning "rain") as a prefix, or nimbus added as a suffix, in a cloud name indicates that the cloud can produce precipitation (rain, snow, or other forms of falling water). Cumulo (meaning "heap") refers to piled-up clouds. Strato (meaning "layer") refers to flat, wide, layered clouds.

Type of Cloud (Genus)	Abbreviation	Appearance	Composition	Altitude (height)
Cumulo-nimbus =Thunderheads	Sb	Can cause lightning, thunder, hail, strong rains, strong winds, and tornadoes		Near ground up to 75,000 feet (Vertical clouds)
Cirro-stratus	Cs	Thin, wispy, appears in sheets. Located above thunderheads		Above 18,000 feet (High-altitude clouds)
Cirrus	Ci	Thin, wispy, filamentous, or curly	Mostly composed of ice crystals	Above 18,000 feet (High-altitude clouds)
Cirro-cumulus	Cc	Small, puffy, patchy and/or with a wavelike appearance		Above 18,000 feet (High-altitude clouds)
Alto-cumulus	Ac	Medium-sized puffy, patchy, scattered clouds - often in linear bands		6,500 - 20,000 feet (Middle-altitude clouds)
Alto-stratus	As	Thin, uniform		6,500 - 20,000 feet (Middle-Alttude clouds)
Strato-cumulus	Sc	Broad and flat on the bottom, puffy on top,		Below 6,500 feet (Low-altitude clouds)
Cumulus	Cu	Puffy and piled up.		Below 6,500 feet (Vertical clouds)
Stratus	St	Uniform, flat, thick to thin layered clouds will ill-defined edges	Mostly composed of liquid droplets	Below 6,500 feet (Low-altitude clouds)
Nimbo-stratus	Ns	Uniform, dark, flat, low, featureless clouds that produce precipitation	Mostly composed of liquid droplets	Below 6,500 feet (Low-altitude clouds)
Fog	Very low stratus clouds		Mostly composed of liquid droplets	In contact with the gound (Ground-hugging clouds)

Other types of clouds:
Mammatus clouds are dark clouds shaped like sagging pouches. These clouds often appear after a tornado.

Orographic clouds are clouds that are formed as moist air rises over mountains or other major geographic features. The air floats up the side of the mountain and cools quickly, condensing and turning into a cloud.

A pileus cloud is a smooth cloud that is found over or on the top of a major geographic feature, like a mountain.

A contrail (short for CONdensation TRAIL) is a cloud-like vapor trail that forms behind some aircraft when flying in cold, clear, humid air. The contrail forms from the water vapor contained in the jet's engine exhaust.