Frontal and cyclone models have been developed that embody an idealized cloud distribution. In these models, the cirrus clouds are lowering and thickening to form altostratus clouds, which indicates an advancing warm front.

Above 500-hPa the concepts of air masses and fronts have little application. Most of the fine cirrus clouds observed ahead of and above warm fronts or lows initially form independent of the frontal middle cloud shield, though later it may trail downward to join the altocumulus and altostratus cloud shields. With precipitation occurring in advance of a warm front, a 60-percent probability exists that cirrus clouds are occurring above. Cirrus clouds observed with the cold front cloud shield either originate from cumulonimbus along and behind the front or from convergence in the vicinity of the upper trough. In many cases there is no post cold front cirrus clouds, probably due to marked subsidence aloft.

One forecasting rule used widely states that "the ridge line at 20,000 feet, about 500 hpa, preceding a warm front marks the forward edge of the cirrus cloud shield."

For a typical 500-hPa wave pattern, the following information applies:

. No extensive cirrostratus clouds will occur before the surface ridge line arrives.

. Extensive cirrostratus clouds follow the passage of the surface ridge line.

. No middle clouds appear before the arrival of the 500-hPa ridge line.

. Middle clouds tend to obscure the cirrus clouds after passage of the 500-hPa ridge line.

. When the 500-hPa wave has a small amplitude, the cirrus cloud arrival is delayed and the clouds are thinner.

. The greater the 500-hPa convergence from trough to ridge, the more cirrus clouds between the surface and 500-hPa ridge lines.

This information is now available on CD in Adobe PDF Printable Format

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing