Cirrus clouds are the thin, wispy clouds that sometimes look like white feathers in an otherwise clear sky. The word cirrus comes from latin and means curl, comparing them to the tendril-like curl of hair or a feather. To see them, the sky needs to be relatively clear except for them, because they usually form in the upper troposphere, above other cloud formations. When particularly thick, they can present as a haze in the upper atmosphere.
Clouds, in general, are visible formations of liquid or solid water droplets held up against gravity by air pressure. They may be composed of micro-droplets of water, larger droplets formed around particulates in the air and/or ice crystals of varying size. Clouds are classified based on their composition, shape, altitude above the Earth's surface and their effect on the weather of the geographic locality below them.
The higher above the Earth's surface you get, the cooler the ambient air temperature becomes. Cirrus clouds usually form at altitudes of 8000 meters (26,000 feet) above sea level or higher. They encompass an altitude range of three to nine miles above the Earth's surface. At this altitude the ambient air temperature varies between minus 20 to minus 30 degrees Celsius or minus 4 to 22 degrees Fahrenheit. Because of this, cirrus clouds are predominantly composed of ice crystals of varying size, formed by the freezing of super-cooled water droplets.
The tendril pattern of cirrus clouds can occur as the ice crystals accrue additional water vapor, becoming heavier, so that gravity overcomes the air pressure holding them up. These ice crystals fall from their great height creating the tendril-like display, but melting and then evaporating well before reaching ground level. This form of aerial precipitation that never reaches the ground is called virga, appearing as streaks across the sky.
Cirrus clouds make up a significant proportion of the clouds in our skies and are abbreviated as Ci; varieties include cirrostratus (Cs) and cirrocumulus (Cc) clouds. They can form during any season and typically cover between 20 to 25 percent of the Earth's surface at any time. Recent study using infrared satellite monitoring indicates that cirrus coverage at the tropics may average in at approximately 70 percent of the sky.
Because they are composed mainly of ice crystals, they reflect rather than refract or focus light, as liquid water droplets do. This means that they both reflect incoming sunlight back into space and return light reflected from the Earth's surface back down towards the Earth. Whether this results in cirrus clouds reducing or increasing global temperatures overall is a subject that is engendering much debate in climatological circles at this time, especially since global warming has become such a hot topic both politically and publicly.