9th September 2005
Cirrostratus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide. Height: Above 16,500 feet. Cause: Saturation of a large air mass at high levels. Associated Weather: If the cloud is thickening, it may indicate the approach of a frontal system. Cirrostratus is an even layer of cirrus that covers a wide area of the sky. As with other kinds of cirrus, it is formed when a moist air mass is lifted to a level where it cools to saturation and forms ice crystals. In the case of cirrostratus, this lifting occurs on a large scale.
Meteorologists distinguish various types of cirrostratus. Among the most common are fibratus and nebulosis. The former consists of long, thin filaments, known as striations, that spread out across a wide area of the sky. The even texture of this formation results from the ice crystals being blown by strong, steady, high-level winds.
In the case of cirrostratus nebulosis, the uplift that gives rise to the cloud is very gentle, and the resulting ice-crystal layer is extremely thin, with vague edges that are difficult to discern and a lack of the texture or fiber common to other cirrus clouds. Frequently, the only sign of cloud formation will be a slight diminution of the intensity of sunlight.
When it forms in a very thin layer, cirrostratus nebulosis often gives rise to haloes, sun dogs, and iridescence.
Occasionally, snow showers fall from cirrostratus formations, but these usually evaporate before reaching the ground and are classified as virga. If a build-up of cirrostratus from one direction is taking place, this is a sign of an increase in moisture at upper levels and may indicate the approach of a weather system such as a cold front.
Cirrostratus formations may cause slight turbulence at cloud level, but this is unlikely to affect aircraft operations or discomfort passengers.
Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.
Check out Skyscapes for cloud photos taken from the aircraft.
