Cirrus (Ci) are delicate, wispy clouds composed entirely of ice crystals, found in the upper troposphere at altitudes of 6–12 km. Their name comes from the Latin for "curl of hair", perfectly describing their filamentous, hook-shaped streaks across the sky. Cirrus are among the most common clouds worldwide and are often the first visible sign of an approaching warm front.
The ice crystals that form cirrus are typically hexagonal columns or plates that fall slowly through the atmosphere. As they descend through layers of differing wind speed, they create the characteristic trailing streaks called fall streaks (or virga). When these streaks are hooked, the cloud is called cirrus uncinus — one of the most reliable natural indicators that a warm front and its associated rain are 24–48 hours away. Dense patches of cirrus (cirrus spissatus) can reduce solar radiation noticeably.
Cirrus play an important role in Earth's radiation budget. Being thin and high, they allow most incoming solar radiation to pass through but trap outgoing infrared radiation from the surface — a net warming effect. This contrasts with low clouds like stratus, which have a net cooling effect. Aircraft contrails that persist and spread can create extensive artificial cirrus, contributing to warming. Solar halos — rings of light around the sun — are formed by refraction through cirrus ice crystals.