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Photograph by Anna Pankowska, My Shot
Clouds form when humid air cools enough for water vapor to condense into droplets or ice crystals. Clouds formations (stratus, stratocumulus—seen here—nimbostratus, fog, altocumulus, altostratus, cirrus, cirrostratus, cirrocumulus, cumulus, and cumulonimbus) are classified into four basic categories, depending largely on the height of their bases above the ground: low-level, mid-level, high-level, and convective clouds. -
Photograph by Rishi Agrawal, My Shot
Low-level clouds lie below 6,500 feet (2,000 meters). They're often dense, dark, and rainy (or snowy) though they can also be cottony white clumps interspersed with blue sky. Meteorologists refer to them as stratus clouds. Fog (pictured above) is considered a low-level cloud. -
Photograph by Darlisa Black, Your Shot
Mid-level clouds form between 6,500 feet (2,000 meters) and 19,500 feet (6,000 meters). They are referred to as "alto" clouds and bear such names as altostratus, which are flat, or altocumulus (pictured), which are puffy. -
Photograph by JR Smith, My Shot
Altostratus mammatus clouds (seen here) are mid-level clouds typically associated with strong thunderstorms (cumulonimbus clouds) and very turbulent conditions. Mammatus appear at levels below 10,000 feet and they hang below the base of cumulonimbus. When there are mammatus clouds present, meteorologists worry about tornados forming. -
Photograph by Jenn Slater, My Shot
High-level clouds, called cirrus clouds, can reach heights above 20,000 feet (6,000 meters) and are typically thin. Cirrus clouds (mare's tails like these) can indicate a change in weather. They do not produce rain and are usually made up of ice. Other high-level clouds are cirrocumulus and cirrostratus. -
Photograph by Jonathan Groner, Your Shot
Cumulus clouds are fair-weather clouds. When they get big enough to produce thunderstorms, they are called cumulonimbus. These convective clouds form when hot air blows high into the atmosphere. The clouds appear to be boiling. -
Photograph by Tom Dietrich, My Shot
Cumulonimbus, along with cumulus, are the most dramatic types of cloud. Rather than spreading out in bands at a fairly narrow range of elevations, like other clouds, they can span 6,500 to 20,000 feet (2,000 to 6,000 meters)—sometimes well above the level of jetliner flights.
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