This is the main type of high cloud, occurring in the upper troposphere, mainly in the tropics and the mid-latitudes. The water content of cirrus clouds is small and the layers of long, fibrous filaments, streaks, plumes or tufts (Figure 8.4) consist entirely of ice crystals because of the low temperatures at the top of the troposphere. For instance, temperatures at 12 km in midlatitudes are typically about -55°C, which is well below the -40°C to which water can sometimes be supercooled. Growth of the crystals makes them settle out as fall streaks, long curved wisps of cloud. Large differences of wind speed or direction in adjacent air (i.e. strong wind shear) cause hooked cirrus, or 'mares' tails'.
Artificial cirrus is produced by the condensation trails (i.e. contrails) of high-flying aircraft, as ice crystals form from the exhaust water-vapour.
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