Clouds can be classified according to their usual altitudes. The altitudes listed here apply to the temperate zone. In polar regions, these clouds may be found in lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.
High clouds generally have bases above 18,000 feet (5,486 meters). The main types of high clouds are cirrus, cirrocumulus, and cirrostratus.
CIRRUS (Ci)
Except for cumulonimbus clouds that build and occasionally reach high up into the atmosphere, cirrus clouds are the highest clouds in the sky. White and wispy in appearance, their name is Latin for ╥hair curl.╙ Since they resemble flowing hair, they are often referred to as "mares' tails." Cirrus clouds are thin and commonly found at altitudes of 20,000 to 30,000 feet in the atmosphere. Because the air is very cold at these levels, cirrus clouds are composed of ice crystals.
CIRROSTRATUS (Cs)
The word stratus means ╥sheetlike╙ in Latin. Sometimes cirrus clouds descend to around 20,000 feet in the troposphere. They begin to form closer together, spreading out in a thick, sheetlike appearance. Meteorologists call these clouds cirrostratus.
Because cirrostratus clouds form high in the troposphere, they are made up of ice crystals. They are white or pale gray in appearance and may be precursors of precipitation.
CIRROCUMULUS (Cc)
Composed of ice crystals, these high, sheetlike clouds generally form around 20,000 feet above the earth's surface. So thin that light shines right through them, cirrocumulus clouds cast off a shimmering white light which is particularly beautiful in the moonlight. Oftentimes, the light creates a halo effect around the moon, which sometimes is an indication that rain or snow will be falling within 12 to 24 hours.
