Cirrus cloud
Formation

Cirrus clouds are usually formed as warm, dry air rises, causing water vapor to undergo deposition onto rocky or metallic dust particles at high altitudes. The average cirrus cloud altitude increases as latitude decreases, but the altitude is always capped by the tropopause. These conditions commonly occur at the leading edge of a warm front. Because absolute humidity is low at such high altitudes, this genus tends to be fairly transparent. Cirrus clouds can also form inside fallstreak holes (also called "cavum").

At latitudes of 65° N or S, close to polar regions, cirrus clouds form, on average, only 7,000 m (23,000 ft) above sea level. In temperate regions, at roughly 45° N or S, their average altitude increases to 9,500 m (31,200 ft) above sea level. In tropical regions, at roughly 5° N or S, cirrus clouds form 13,500 m (44,300 ft) above sea level on average. Across the globe, cirrus clouds can form anywhere from 4,000 to 20,000 m (13,000 to 66,000 ft) above sea level. Cirrus clouds form with a vast range of thicknesses. They can be as little as 100 m (330 ft) from top to bottom to as thick as 8,000 m (26,000 ft). Cirrus cloud thickness is usually somewhere between those two extremes, with an average thickness of 1,500 m (4,900 ft).

The jet stream, a high-level wind band, can stretch cirrus clouds long enough to cross continents. Jet streaks, bands of faster-moving air in the jet stream, can create arcs of cirrus cloud hundreds of kilometers long.

Cirrus cloud formation may be effected by organic aerosols (particles produced by plants) acting as additional nucleation points for ice crystal formation. However, research suggests that cirrus clouds more commonly form on rocky or metallic particles rather than on organic ones.
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