THE CIRRUS CLOUD FORECASTING PROBLEM

cooling, there is a tendency for the initial condensation to contain a higher proportion of water droplets, which leads to a “mixed cloud’ that will convert to ice or snow in time. Presumably, dense cirrus, fine cirrus, cirrocumulus, and anvil cirrus clouds are of this type. It is assumed that fine cirrus clouds (proper) are formed in shallow layers that are undergoing rapid convection due to advection of colder air at the top of the shear layer. On the other hand fine cirrus and cirrostratus clouds are so often associated, and cirrostratus clouds are so often reported by pilots as developing from the merging of fine cirrus clouds, that there is a question whether the process of formation in cirrus and cirrostratus clouds are essentially different. Nevertheless, the prevailing crystal types in cirrus and cirrostratus clouds seem to differ, though this may not be universal, or may merely represent different stages in cirrus cloud evolution. Horizontal risibilities within cirrostratus clouds are generally between 500 feet and 2 nm. Thin cirrus haze, invisible from the ground, often reduces the visibility to 3 nm. A rule of thumb for forecasting or estimating the visibility within thin cirrus or other high cloud (temperatures below –30°C) follows: Visibility = 1/2 nm times dewpoint depression in degrees C. For example; temperature is –35°C, dewpoint is –38°C, and visibility = 1/2 x 3 = 1 1/2 nm. This rule has been used successfully only in the Arctic where poor visibility in apparently cloudfree air is often encountered. THE CIRRUS CLOUD FORECASTING PROBLEM Many forecasters have attempted to forecast cirrus clouds by using frontal or cyclone models. This procedure is not always satisfactory. There are a number of parameters, both surface and aloft, that have been correlated with cirrus cloud formation. A few of the more prominent parameters are mentioned in the following text. Surface Frontal Systems 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. Fronts Aloft 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. Contour Patterns 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. l 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. Cirrus Clouds in Relation to the Tropopause Experiences of pilots have confirmed that the tops of most cirrus clouds are at or below the tropopause. In the midlatitudes, the tops of most cirrus cloud layers are at or within several thousand feet of the polar tropopause. Patchy cirrus clouds are found between the polar tropopause and the tropical tropopause. A small percentage of cirrus clouds, and sometimes extensive 4-20