What Are Cirrus Clouds Made Of?

Types of Cirrus Clouds

• There are two main subtypes of cirrus cloud: cirrostratus and cirrocumulus clouds. Cirrostratus clouds are thin and sheet-like, often covering large areas of sky. They are transparent and often hard to see at all. Cirrocumulus clouds are smaller and more rounded, giving them an appearance similar to that of scales on a fish. They often move across the sky from west to east.

Formation

• Both types of cirrus clouds form high up, at around 6000m (20,000 ft.) above sea level. They form when supercooled water droplets in the air freeze to make ice crystals. Cirrus clouds tend to be thin because of low humidity at their high formation altitudes.

  Cirrus clouds also form during hurricanes and thunderstorms, when water droplets in other cloud types freeze as temperatures drop. They can often be seen spreading out from the eye of a hurricane.

Composition

• Cirrus clouds are made up of ice crystals. The ice crystals come from supercooled water droplets that have frozen at very low temperatures. Supercooled droplets are liquid water which has cooled to below it's normal freezing point without becoming ice and require temperatures below -38ºC to freeze. The ice crystals in cirrus clouds come in different shapes and sizes and the attributes of an ice crystal are largely determined by the temperature and humidity inside the cloud in which it forms. Cirrus clouds in different regions tend to have a different composition. For example, in Antarctica cirrus clouds mainly consist of column shaped ice crystals.

Contrails

• As well as naturally forming cirrus clouds, condensation trails left by high flying aircraft are often cirrus-like and contribute to the number of cirrus clouds seen in the sky. A study of cirrus clouds over Salt Lake City showed a significant increase in the amount of cloud cover in 1965, when the usage of domestic jet engine fuel also increased. Similar increases were reported near major flight paths elsewhere in the US at the same time. Cirrus clouds made by aircraft have the same composition as natural cirrus clouds.