This June 20, 2006 photo taken from a downtown Cedar Rapids, Iowa office building shows what may become the first new cloud type to be recognized by scientists since 1951. Meteorologists vertically divide clouds into three levels: high, mid-level and low. Within each of these level (or etages) are 9 categories.
Credit: AP Photo/Jane Wiggins
Cirrus, Cirrocumulus and Cirrostratus are high-level, typically thin clouds at elevations ranging from 10,000 to 60,000 feet (varying due to season or region -- polar, temperate or tropical). Cirrus clouds usually form in nearly straight or curved strands, filaments or hooks.
Dense cirrus occur in patches or entangled sheaves.
Cirrostratus progressively invade the sky, becoming denser, extending more than 45
Halos form when light from the sun or moon is refracted by ice crystals associated with thin, high-level cirrostratus clouds.
Credit: Carlye Calvin/UCAR
Altocumulus, Altostratus and Nimbostratus are mid-level clouds, composed of water droplets or ice crystals, found at 6,500-25,000 feet. These altocumulus clouds are semi-transparent.
Altocumulus often form in patches called Altocumulus Lenticularis (wave clouds). These formations in the shape of almonds or lenses are seen in mountainous or hilly areas.
Semi-transparent altocumulus can spread from one horizon to the other, sometimes in more than one layer.
Altocumulus with tufts or sproutings.
Altostratus are dense middle-level clouds that cover the sky, found at altitudes of 6,500-26,000 feet. Usually gray or blue-gray in color, these appear red due to the setting sun.
Credit: Gary Barnes, UCAR
Cumulus, Stratocumulus, Stratus and Cumulonimbus are low clouds (surface-6,500 ft.) composed of water droplets. Cumulus clouds have clear-cut horizontal bases and flattened or slightly rounded tops. These are called "fair weather cumulus."
Cumulus clouds may form tall towers tilted by the wind. Showers can occur but no lightning and thunder.
Cumulonimbus features an anvil or plume shape, with a darkened base, and occur as an isolated cloud or in an extensive wall and squalls. They are often accompanied by hail and/or thunder. A variety of other clouds (dense cirrus, altocumulus, altostratus, stratocumulus, cumulus and stratus) may also be present.
Mammatus, which results from the sinking of moist air into dry air, are associated with thunderstorms, but do not necessarily indicate sever weather.
This low, horizontal wedge-shaped formation, called a shelf cloud, is associated with a thunderstorm gust front or cold front.