Definition of cirrus clouds

1) High clouds that are formed entirely from ice crystals. They appear delicate and wispy and can reach a height of 35,000 feet (10,700 meters). Other types of cirrus clouds include cirrostratus and cirrocumulus. Cirrostratus is a thin sheet of cloud that often causes a halo to appear around the sun or moon. Cirrocumulus look like many small tufts of cotton; however, these clouds rarely form.

source: [Journal of Applied Meteorology. v 31, page 370, 1991.] [Journal of the Atmospheric Sciences. v 46, page 2293, 1989.]