THREE-DIMENSIONAL HUMIDITY ANALYSISTHE MOIST LAYER 

Using a single level (for example, the 500-hPa level dewpoint depression analysis) to find probable cloud areas does not indicate clouds above or below that level. For example, if the top of a cloud system reached only to 16,000 feet, and there was dry air above at 500-hPa (approximately 18,000 feet), you wouldnt suspect, from the 500hPa analysis, the existence of clouds below the 500-hPa level.

However, an analysis of the extension of the moist layers in three dimensions can be obtained simply by scrutinizing individual RAOBs. Those selected should be in the general vicinity of, and the area 500 to 1,200 miles upstream of, the area of interest, depending on the forecast period. The heights of the bases and tops can be indicated, though there is little advantage in indicating a dry layer 2,000 to 3,000 feet thick sandwiched between thicker moist layers. Usually, it is sufficient to indicate the entire moist layer, without bothering about any finer stratums. A survey of the cloud field is made easier by writing the heights of the bases and tops in different colors.

A moist layer for the sake of simplicity may be defined as a layer having a frost point depression of 3C or less (i.e., a dewpoint depression of 4C at 10C; 5C at -20C; 6C at 30C).

The type and intensity of precipitation observed at the surface is related to the thickness of the cloud aloft, and particularly to the temperatures in the upper portion of the cloud.

The results of a study relating cloud-top temperatures to precipitation type and intensity are as follows:

. From aircraft ascents through stratiform clouds, along with simultaneous surface observations of precipitation, it was found that 87 percent of the cases where drizzle occurred, it fell from clouds whose cloud-top temperatures were warmer than 5C. The frequency of rain or snow increased markedly when the cloud-top temperature fell below 12C.

. When continuous rain or snow fell, the temperature of the coldest part of the cloud was below 12C in 95 percent of the cases.

. Intermittent rain was mostly associated with cold cloud-top temperatures.

. When intermittent rain was reported at the surface, the cloud-top temperature was colder than 12C in 81 percent of the cases, and colder than 20C in 63 percent of the cases. From this, it appears that when minor snow (continuous or intermittent) reaches the ground from stratiform clouds, the clouds (solid or layered) extend in most cases to heights where the temperature is well below 12C, or even 20C.

This rule cannot be reversed. When rain or snow is not observed at the surface, middle clouds may well be present in regions where the temperature is below -12C or 20C. Whether or not precipitation reaches the ground will depend on the cloud thickness, height of the cloud base, and the dryness of the air below the base.

INDICATIONS OF CIRRUS CLOUDS IN RAOB

Cirrus clouds form at temperatures of 40C or colder. At these temperatures, as soon as the air is brought to saturation, the condensate immediately freezes. The ice crystals often descend in altitude slowly, to levels that have air temperatures of 30C, and persist if the humidity below the formation level is high enough to support saturation. In general, cirrus clouds are found in layers that are saturated, or supersaturated, with respect to ice at temperatures colder than 0C.

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