HIGH clouds above 10,000 feet:

• (Ci) Cirrus - Thin and wispy, usually transparent white in color but can display magnificent colors when the sun is low on the horizon. Cirrus generally occur in fair weather and point in the direction of air movement at the cloud’s level.  
• (Cs) Cirrostratus - Thin and sheet-like. Sometimes the only sign of their presence is a halo around the sun or moon. Halos result from the refraction of light by the cloud's ice crystals.  
• (Cc) Cirrocumulus - Tiny, white and puffy heap.  

MIDDLE clouds 6,500 to 20,000 feet:

• (Ac) Altocumulus - Grayish white and puffy heap. Altocumulus clouds usually form by convection in an unstable layer aloft. This may result from the gradual lifting of air in advance of a cold front. Altocumulus clouds on a warm and humid summer morning commonly signal afternoon thunderstorms. 
• (As) Altostratus - grayish white and sheet-like.  
• (Ns) Nimbostratus - Dark gray and sheet-like with rain or snow falling.  

LOW clouds below 6,500 feet:

• (St) Stratus - Low, grayish white, sheet-like. 
• (Sc) Stratocumulus - Low, grayish white, sheet-like but with a bumpy bottom.  

VERTICAL DEVELOPMENT clouds below 6,500 feet:

• (Cu) Cumulus - Small, white and puffy heap with distinctive flat bases. These fair weather cumulus are harmless. These clouds are formed by thermals, that rise upward from the earth's surface. As they rise, the water vapor within cools and condenses forming cloud droplets. If conditions are right, however, they may develop into cumulonimbus clouds and produce thunderstorms with heavy precipitation.  
• (Cb) Cumulonimbus - Giant cumulus clouds with black bottoms. These are full of precipitation and are the thunderstorm clouds with tops that may extend more than 50,000 feet. The lower levels of these clouds are generally made of water droplets while the upper levels of the cloud may be made of ice. These dangerous clouds may produce tornados and hail as part of powerful thunderstorms known as supercells. They can exist as individual towers or form a line of towers called a squall line.

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