A high, thin cloud made mostly of ice crystals is called cirrus.
Cirrus clouds usually form in the upper troposphere, often above about 6,000 metres, where temperatures are low enough for water to freeze. Their delicate strands or feathery filaments are shaped by strong winds at altitude. The name comes from the Latin word for a lock or curl of hair.
Individual cirrus clouds do not normally produce rain that reaches the ground. Ice particles may fall from them as virga, evaporating before reaching the surface. Despite their fair-weather appearance, increasing cirrus can sometimes signal that a warm front or other large weather system is approaching.
Cirrus is one of the ten principal cloud genera used in the international cloud classification system. It differs from cirrostratus, which forms a more continuous veil, and cirrocumulus, which appears in small, patterned cloudlets.