9th September 2005
Altostratus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide,
common in middle latitudes.Height: 6500 to 16,500 feet. Cause: Lifting of a large air mass; followed by condensation. Associated Weather: Extensive areas of rain and snow. Hazard Warning: Ice accretion on aircraft. This cloud is found in the middle levels of the atmosphere and is always a sign of the presence of significant amounts of moisture in those layers. It is typically featureless, ranging from a thin, white veil of cloud through which the Sun is plainly visible, to a dense, gray mantle that may block out the Sun completely.
Altostratus is the result of the lifting and condensation of a large air mass, usually by an incoming frontal system. This can result in an extensive deck of cloud, which may extend over thousands of square miles. If sufficiently thick, altostratus can produce rain or snow over a wide area.
When stratus cloud covers the entire sky, it can be difficult to determine whether it is a low or middle-level formation. As a rough guide, if you can discern a texture in the cloud deck, it is more likely to be low-level stratus; if it appears smooth and structureless, it is more likely to be an altostratus formation.
For pilots, a thick deck of altostratus can be a cause for concern if temperatures within the cloud are below freezing, because ice may build up on parts of the aircraft as it passes through the cloud, altering the plane's aerodynamics. Fortunately, most aircraft are equipped with de-icing devices that eliminate this problem.
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.
