Cirrus clouds are at a high enough altitude that the water in them is either frozen or supercooled. The tiny ice crystals are tossed about by the upper-level winds, creating their wispy appearance. Long, thin cirrus clouds are sometimes called mares' tails. Cirrus are never very thick. A sheet-like layer of cirrus is called cirrostratus, and numerous tiny poofball cirrus are called cirrocumulus. Cirrus clouds can be attached to other types of clouds - they are at the leading edge of the anvil of a thunderstorm, and sometimes they will be found at the very beginnings of a cold front.

Cirrus clouds are beautiful things, forming all kinds of fantastical wispy shapes. They also cause quite an array of gorgeous optical phenomena: Sun dogs, which are bright coloured patches just over 22º away from the sun; circumzenith arcs, beautiful dry-weather rainbows high up in the sky; and halos at 22 and 46 degrees away from the sun, which are particularly associated with cirrostratus. Sun dogs and circumzenith arcs, on the other hand, are mainly found in thicker cirrus formations consisting of horizontally-aligned plate-like ice crystals.

Cir"rus (?), n.; pl. Cirri (#). [L., lock, curl, ringlet.] [Also written cirrhus.]

1. Bot.

A tendril or clasper.

2. Zool. (a)

A soft tactile appendage of the mantle of many Mollusca, and of the parapodia of Annelida. Those near the head of annelids are Tentacular cirri; those of the last segment are caudal cirri.


The jointed, leglike organs of Cirripedia. See Annelida, and Polychaeta.

In some of the inferior animals the cirri aid in locomotion; in others they are used in feeding; in the Annelida they are mostly organs of touch. Some cirri are branchial in function.

3. Zool.

The external male organ of trematodes and some other worms, and of certain Mollusca.

4. Meteor.

See under Cloud.


© Webster 1913.

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