9th September 2005
Altostratus Undulatus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide. Height: 6500 to 16,500 feet. Cause: Lifting of a large air mass, followed by condensation combined with wind shear at cloud level. Associated Weather:
NoneAltostratus undulatus usually occurs in a thin layer of altostratus, and its distinctive, undulating appearance is due to wave motion in the air mass. This motion is normally a result of wind shear, which occurs when one layer of air slides over another layer moving at a different speed or in a different direction (or both). This creates vertical eddies, or waves, of air between the layers, and if sufficient moisture is present, cloud will form where the wave rises and dissipate where it falls.
Depending on the moisture content of the air mass and the degree of wind shear, undulatus may occur as fairly continuous waves across the sky with thin cloud connections at the base, or it may break up into unconnected wave peaks.
Undulatus produces no significant weather, but, because the cloud is produced by wind shear, it is regarded as a sign of local turbulence. In most cases, however, this turbulence would be only slight and would not be a concern for aircraft pilots.
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.
