Cirrus clouds generally occur in fair weather, but they can also hearld the leading edge of a far-away storm or may mark the edge of a storm passing to the north.
Cirrus clouds are transparent wispy curls or filaments of cloud floating high in the sky. They commonly point in the direction of the air movement at their elevation.
Cirrus clouds are formed by the freezing of rising water vapor into tiny ice crystals. Cirrus clouds are so high in the upper regions of the sky that the temperature there is always below freezing and water vapors that rises that high will freeze and remain frozen.
Anytime you see a cirrus cloud, you can know that it is 20,000 feet (6,000 m) in the sky or higher. That is the altitude at which water vapor turns into a solid.
All clouds between 20,000 and 50,000 feet high bare the prefix “cirro”–cirro in Latin means curl—as in curly lock of hair or ringlet.
Cirrus clouds are so curly, wispy, and feathery that they are often called “mares’ tails.” Children sometime mistake cirrus clouds for the goose down from a broken pillow.
Cirrus clouds are so insubstantial that they do not dim the brightness of the sun or moon like other clouds. As well, they cast no shadows. From earth cirrus clouds appear transparent white or light gray.
Cirrus clouds sometimes appear straight, other times they look like floating commas, and other times they may become tangled. A cirrus cloud that is drawn out in a flat, featureless weave of filaments is called a crirrostratus cloud.
When cirrus clouds appear in clusters of tiny balls or tufts they are called cirrocumulus clouds. A collection of cirrocumulus clouds are sometimes referred to as a “mackerel sky” or “buttermilk sky.”
The ice crystals in cirrus clouds do not fall to the ground like rain. When they fall, they are carried on the wind and fall much more slowly. Their fall is often seen as streaks in the sky—called fall streaks. These falling ice crystals evaporate long before reaching the ground. The change in wind with height determines how quickly ice crystals fall and, in turn, determines the shapes and sizes of fall streaks.
Contrails that appear behind high flying jets are manmade cirrus clouds. They are formed when water vapor from engine exhaust condenses into ice crystals.