OBSERVATION AND FORMATION OF CIRRUS

Cirrus, or cirriform clouds, are divided into three general groups: cirrus (proper), cirrostratus, and cirrocumulus. Cirrus clouds, detached or patchy, usually do not create a serious operational problem. Cirrostratus and extensive cirrus haze, however, may be troublesome in high-level jet operations, aerial photography, interception, rocket tracking, and guided missile navigational systems. Therefore, a definite requirement for cirrus cloud forecasting exists. The initial formation of cirrus clouds normally requires that cooling take place to saturation, and to have temperatures near 40C. Under these conditions, water droplets are first formed, but most of them immediately freeze. The resulting ice crystals persist as long as the humidity remains near saturation with respect to ice. There is some evidence that the speed of the cooling, and the kind and abundance of freezing nuclei, may have an important effect on the form and occurrence of cirrus clouds. Slow ascent starts crystallization at humidities substantially below saturation; this is presumably the case in extensive cirrostratus clouds associated with warm frontal altostratus clouds. If slow ascent occurs in air that has insufficient freezing nuclei, a widespread haze may result, which at 30 to 40C is predominantly composed of water droplets. In the case of more rapid 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 30C) follows:

Visibility = 1/2 nm times dewpoint depression in degrees C. For example; temperature is 35C, dewpoint is 38C, 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.

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