Cirrus clouds are composed almost exclusively of ice crystals. These crystals are generally very small, which, together with their sparseness, accounts for the transparency of most Cirrus clouds.
Dense Cirrus patches or Cirrus in tufts may contain ice crystals large enough to acquire an appreciable terminal velocity. Trails of considerable vertical extent fall from the base of Cirrus.
Infrequently, the ice crystals in the trails melt into small water droplets; the trails are then greyish, in contrast with their usual white appearance. A rainbow may form.
The trails curve irregularly or slant as a result of wind shear and of the variation in size of the constituent particles; consequently, Cirrus filaments near the horizon do not appear parallel to it.
Halo phenomena may occur; circular haloes almost never show a complete ring, due to the narrowness of the Cirrus clouds.