The word "cirrus" describes high thin wispy clouds. Cirrus clouds are composed of ice crystals and are the highest of the three main cloud types (cirrus, cumulus, and stratus). They are thin, wispy, and often appear feathery or hair-like. Cirrus clouds are formed at high altitudes, typically between 5 and 13 kilometres (3 and 8 miles) above the ground. They are usually composed of ice crystals, which are formed when water vapour freezes at high altitudes. Cirrus clouds are often associated with fair weather, although they can sometimes indicate the approach of a warm front.