Cloud families by height, not to scale. Low clouds below 6,500 feet: stratus, a flat grey layer near the ground; stratocumulus, a lumpy layer; and nimbostratus, a thick dark layer with rain (nimbo means rain). Middle clouds from 6,500 to 20,000 feet, prefix alto: altostratus, a grey sheet, and altocumulus, rounded masses. High clouds above 20,000 feet, prefix cirro, made of ice crystals: cirrus wisps, cirrocumulus ripples and a thin cirrostratus veil with a halo. Then a small cumulus grows upward into a towering cumulus and a cumulonimbus with an anvil top, lightning and heavy rain, the most dangerous cloud.
