Cirrus clouds are at a high enough altitude that the water in them is either frozen or supercooled. The tiny ice crystals are tossed about by the upper-level winds, creating their wispy appearance. Long, thin cirrus clouds are sometimes called mares' tails. Cirrus are never very thick. A sheet-like layer of cirrus is called cirrostratus, and numerous tiny poofball cirrus are called cirrocumulus. Cirrus clouds can be attached to other types of clouds - they are at the leading edge of the anvil of a thunderstorm, and sometimes they will be found at the very beginnings of a cold front.