9th September 2005
Cirrus
Source: Excerpt from The Book " Weather "
Distribution: Worldwide. Height: Above 16,500 feet. Cause: Saturation of air mass at upper levels. Associated Weather: If extensive, may indicate an approaching frontal system; may also be a sign of a decayed thunderstorm. The Latin word cirrus, meaning wisp of hair, is the name given to high-level clouds that stretch across the sky in delicate strands. These formations indicate the presence of moisture at high levels of the atmosphere. At these levels, the temperature is normally below freezing, and any air mass that cools to saturation will produce ice rather than water droplets.
Cirrus clouds therefore consist of many millions of ice crystals, and these are blown about by upper-level winds, producing characteristic white streaks.
Among the most distinctive cirrus shapes are the irregular twists and tangles of cirrus intortus and the hook shapes of cirrus uncinus. Less common, but equally dramatic, is cirrus radiatus, which forms in long, parallel lines that seem to radiate from a point on the horizon.
Cirrus clouds may form in isolated patches or cover a wide area of the sky, depending on the distribution of moisture. Isolated patches rarely have any great significance, but an extensive deck, increasing from one direction, may indicate an approaching front.
Cirrus may be the result of local thunderstorm activity. The anvil that often forms above a cumulonimbus cloud is actually a cirrus cloud. It is generated as the thunderstorm pumps moisture up to the very top of the troposphere, where it freezes into ice crystals. After the storm has completed its life cycle, high-level winds may disperse the anvil across the sky, producing extensive cirrus formations far downwind.
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.
