Cirrus Clouds

The most common variation of high-level clouds, cirrus are thin, whispy clouds composed of ice crystals. These ice crystals come from the freezing of supercooled water droplets, and exist at heights where the temperatures are typically below -38 degrees Celcius. Cirrus generally occur in fair weather and move from west to east across the sky, indicating the direction of the prevailing winds at their elevation.

Cirrus can form from almost any cloud that has undergone glaciation and be observed in a variety of shapes and sizes. Possibilities range from the "finger-like" appearance of cirrus Fall Streaks, commonly seen during pleasant weather conditions, to the uniform texture of more Extensive Cirrus clouds, which can be the first sign of an approaching warm front.

For More on Extensive Cirrus Clouds

An example of cirrus streaks:

"Fall Streaks" form when snowflakes and ice crystals fall from high cirrus clouds, while the change in wind with height, and how quickly these ice crystals actually fall, regulate the shapes and sizes fall streaks can attain. Since ice crystals fall much more slowly than do rain drops - about 1 meter per second compared with about 8 meters per second for large raindrops - these streamers tend to be stretched out horizontally as well as vertically.