INFERRING CLOUDS FROM RAOB 

Theoretically, we should be able to infer from the humidity data of RAOBs the layers where the rawinsonde penetrates cloud layers. In practice, the determination that can be made from temperature and dewpoint curves are often less exact and less reliable than desired. Nevertheless, RAOBs give clues about cloud distribution and potential areas of cloud formation. These clues generally cannot be obtained from any other source.

DEWPOINT AND FROST POINT IN CLOUDS

The temperature minus the dewpoint depression yields the dewpoint, which is defined as the temperature to which the air must be cooled at a constant vapor pressure for saturation to occur. The FROST POINT (that is, the temperature to which the air has to be cooled or heated adiabatically to reach saturation with respect to ice) is higher than the dewpoint except at 0C, where the two coincide. In the graph shown in figure 4-15, the difference between dewpoint and frost point is plotted as a function of the dewpoint itself.

In a cloud with the temperature above freezing, the true dewpoint will coincide closely with the true air temperature, indicating that the air between the cloud droplets is practically saturated. Minor discrepancies may occur when the cloud is not in a state of equilibrium (when the cloud is dissolving or forming rapidly, or when precipitation is falling through the cloud with raindrops of slightly different temperature than the air); but these discrepancies are very small. In the subfreezing portion of a cloud, the true temperature is between the true dewpoint and the true frost point, depending on the ratio between the quantities of frozen

Figure 4-15.-Difference between frost point and dewpoint as a function of the dewpoint.

and liquid cloud particles. If the cloud consists entirely of supercooled water droplets, the true temperature and the true dewpoint will, more or less, coincide. If the cloud consists entirely of ice, the temperature should coincide with the frost point. Therefore, we cannot look for the coincidence of dewpoint and temperatures as a criterion for clouds at subfreezing temperatures. At temperatures below -12C, the temperature is more likely to coincide with the frost point than the dewpoint. The graph shown in figure 4-15 indicates that the difference between the dewpoint and frost point increases roughly 1C for every 10C that the dewpoint is below freezing. For example, when the dewpoint is 10C, the frost point equals 9C; when the dewpoint is 20C, the frost point is 18C; and when the dewpoint is 30C, the frost point is 27C. Thus, for a cirrus cloud that is in equilibrium (saturated with respect to ice) at a (frost point) temperature of 40C, the correct dewpoint would be 44C, (to the nearest whole degree).

We can state, in general, that air in a cloud at temperatures below about 12C is saturated with respect to ice, and that as the temperature of the cloud decreases (with height), the true frost point/dewpoint difference increases. Any attempt to determine the height of cloud layers from humidity data of a RAOB is, there fore, subject to error. It is possible to overcome some of these errors by a subjective interpretation of the Rawinsonde Observations (RAOBs), as discussed in the following sections.

INTERPRETATION OF RAOB LAYERS WITH RESPECT TO CLOUD LAYERS

The following diagrams (figs, 4-16, 4-17, and 4-18) illustrate the behavior of a rawinsonde during cloud penetration. These diagrams are correlated with aircraft observations or the heights of cloud bases and tops from aircraft flying in the vicinity of an ascending rawinsonde. The difference in time and distance between the aircraft and sounding observations was usually less than 2 hours and 30 miles, respectively. Some of the aircraft reported only the cloud observed above 15,000

Figure 4-16.-Example of inferring clouds from a RAOB with an active warm front approaching from the south.

feet; others reported all clouds. In figure 4-16 through 4-18, the aircraft cloud observations are entered in the lower left corner of each diagram under the heading cloud; the surface weather report is entered under the aircraft cloud report. Where the low cloud was not reported by the aircraft, the height of the cloud base may be obtained from the surface reports, Aircraft height reports are expressed in thousands of feet, pressure-altitude.

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