These clouds have bases between 16,500 and 45,000 feet in the mid latitudes. At this level they are composed of primarily of ice crystals. Some clouds at this level are cirrus, cirrocumulus, and cirrostratus

see Clouds, High.

Mean height of more than 20,000ft. Usually composed of ice crystals. They are: (a) Cirro-cumulus (mackerel sky). Layer cloud of small white flakes or of small globular masses. Often resembles the ripples in the sand on the seashore. (b) Cirro-stratus. A thin whitish veil which does not blur the outlines of the sun or moon but gives rise to haloes. (c) Cirrus (Mare's tails). Detached clouds of delicate and fibrous appearance.