Cirrus Cloud - Relation To Other Clouds

Relation To Other Clouds

Cirrus clouds are one of three different genera of high-étage (high-level) clouds. High-étage clouds form at 5,000 m (16,500 ft) and above in temperate regions. The other two genera, cirrocumulus and cirrostratus, are also high clouds.

In the intermediate range, from 2,000 m (6,500 ft) to 7,000 m (23,000 ft) in temperate regions, are the mid-étage clouds. They comprise two or three genera depending on the system of height classification being used: altostratus, altocumulus, and, according to WMO classification, nimbostratus. These clouds are formed from ice crystals, supercooled water droplets, or liquid water droplets.

Low-étage clouds, form at less than 2,000 m (6,500 ft). The two genera that are strictly low-étage are stratus, and stratocumulus. These clouds are composed of water droplets, except during winter when they are formed of supercolled waterdroplets or ice crystals if the temperature at cloud level is below freezing . Two additional genera usually form in the low altitude range, but may be based at higher levels under conditions of very low humidity. They comprise the genera cumulus, and cumulonimbus, which along with nimbostratus, are often classified separately as clouds of vertical development, especially when their tops are high enough to be composed of super-cooled water droplets or ice crystals.

The altitudes of high-étage clouds like cirrus vary considerably with latitude. In the polar regions, they are at their lowest, with a minimum altitude of only 3,000 m (10,000 ft) to a maximum of 7,600 m (25,000 ft). In tropical regions, they are at their highest, ranging in altitude from about 6,100 m (20,000 ft) to around 18,000 m (60,000 ft). In temperate regions, they range in altitude from 5,000 m (16,500 ft) to 14,000 m (45,000 ft)—a variation in contrast to low-étage clouds, which do not appreciably change altitude with latitude.