Cirrus clouds are made of ice crystals and look like long, thin, wispy white streamers high in the sky. They are commonly known as “mare’s tails” because they are shaped like the tail of a horse. Cirrus clouds are often seen during fair weather. But if they build up larger over time and are followed by cirrostratus clouds, there may be a warm front on the way.
Cirrus clouds also form in the atmospheres of other planets, including Mars, Jupiter, Saturn, Uranus, and Neptune, and have been seen even on Titan, one of Saturn’s larger moons. Some of these extraterrestrial cirrus clouds are composed of ammonia or ices of methane, much as with terrestrial water ice. (The term cirrus also applies to certain interstellar clouds, composed of sub-micrometer-sized grains of dust.
Cirrus comes in four distinct species; Cirrus castellanus, fibratus, spissatus, and uncinus; which are each divided into four varieties: intortus, vertebratus, radiatus, and duplicatus. Cirrus castellanus is a species that has cumuliform tops caused by high-altitude convection rising up from the main cloud body. Cirrus fibratus looks striated and is the most common cirrus species. Cirrus uncinus clouds are hooked and are the form that is usually called mare’s tails. Of the varieties, Cirrus intortus has an extremely contorted shape, and cirrus radiatus has large, radial bands of cirrus clouds that stretch across the sky. Kelvin–Helmholtz waves are a form of cirrus intortus that has been twisted into loops by vertical wind shear.
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