Cirrus

Photograph of cirrus clouds Click on image for full size version (64K JPG) Courtesy of Lisa Gardiner

Cirrus (weather symbol - Ci) clouds are the most common of the High Cloud (5000-13000m) group. They are composed entirely of ice and consist of long, thin, wispy streamers. They are commonly known as "mare's tails" because of their appearance.

The reason for the long tails is primarily due to the wind. In the upper troposphere, the winds travel at very high speeds from west to east (in the Northern Hemisphere). The winds are so strong at these altitudes that they actually stretch the crystalline structure of the Cirrus cloud, creating a tail.

Cirrus clouds are usually white and predict fair weather.