Clouds are visible accumulations of water droplets or solid ice crystals that float in the Earth's troposphere (the lowest part of the Earth's atmosphere), moving with the wind. From space, clouds are visible as a white veil surrounding the planet.
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:
Prefixes and Suffixes Used to Describe Clouds:
Clouds are defined by both the way they look and how high they are in the atmosphere. For example, cirro (meaning "wisp of hair") is a prefix given to high-altitude clouds (above 20,000 feet). Alto (meaning "high") is a prefix given to mid-altitude clouds (between 6,000 and 20,000 feet). There is no prefix for low-altitude clouds. When clouds are by the ground we call them fog.
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.
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