Cirrus

Thin, wispy, high-altitude clouds composed entirely of ice crystals, typically found above 20,000 feet MSL. Cirrus clouds are often the first visible sign of an approaching warm front or large-scale weather system, appearing 24 to 48 hours ahead of surface frontal passage. While cirrus clouds themselves do not produce significant precipitation or turbulence at lower altitudes, their presence in increasing amounts with lowering bases indicates deteriorating weather is likely on the way.