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 cirrostratus, may be observed in the lower stratosphere above the polar tropopause, but mainly below the level of the jetstream core. The cirrus clouds of the equatorial zone also generally extend to the tropopause. There is a general tendency for the mean height of the bases to increase from high to low latitudes more or less paralleling the mean tropopause height, ranging from 24,000 feet at 70to 80atitude to 35,000 to 4,000 feet or higher in the vicinity of the equator. The thickness of individual cirrus cloud layers are generally about 800 feet in the midlatitudes. The mean thickness of cirrus clouds tends to increase from high to low latitudes. In polar continental regions in winter, cirrus clouds are virtually based at the surface. In the midlatitudes and in the tropics, there is little seasonal variation.

A discussion of cloud types associated with the jetstream is contained in the AG2 TRAMAN, volume 1. In addition to this information, we will discuss a few studies pertaining to cloud types. All of these studies agree that most of the more extensive and dense clouds clouds are on the equatorward of the jet axis. The observed frequency of high clouds poleward of the jet axis can be accounted for as the upper reaches of a cold front, or cold lows, not directly related to the jetstream. In some parts of a trough, these high clouds may tend to be dense, and in other areas thin.

PREDICTION OF SNOW VS RAIN

LEARNING OBJECTIVES: Evaluate the surface and upper-level synoptic situations in determining the form of precipitation in your forecast.

Typically, an inch or so of precipitation in the form of rain will cause no serious inconvenience. On the other hand the same amount of precipitation in the form of snow, sleet, or freezing rain can seriously interfere with naval operations. In such cases, the snow versus rain problem may become a factor of operational significance.

Sleet and freezing rain, which often may occur in the intermediate period between snow and rain, are generally grouped with snow in our discussion. Any decision arrived at for the snow versus rain problem would, naturally, have to be modified, dependant on your geographical location. This should be easily accomplished through a local study of the optimum conditions. The various techniques and systems presented here will often complement each other. The approach used here is a discussion of the general synoptic patterns and the thermal relationship; that is, the use of temperatures at the surface and aloft, and the presentation of an objective technique to distinguish the types of precipitation.

GEOGRAPHICAL AND SEASONAL CONSIDERATIONS

The forecasting problem of snow versus rain arises, naturally, during the colder months of the year. In midwinter when the problem is most serious in the northern states, the southern states may not be concerned.

The type of precipitation that reaches the ground in a borderline situation is essentially dependent on two conditions. There must be a stratum of above-freezing temperatures between the ground and the level at which precipitation is forming, and this stratum must be sufficiently deep to melt all of the falling snow prior to striking the surface. Thus, a correct prediction of rain or snow at a given location depends largely on the accuracy with which the vertical distribution of the temperature, especially the height of the freezing level, can be predicted. On the average, it is generally satisfactory to assume that the freezing level must be at least 1,200 feet above the surface to ensure that most of the snow will melt before reaching the surface.

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