The temperature, frost point, and dewpoint curves are indicated by T, Tf , and TD respectively. In figure 4-16, a marked warm front is approaching from the south. Moderate continuous rain fell 2 hours later. At 1830 UTC, an aircraft reported solid clouds from 1,000 to 44,000 feet (tropopause). The 1500 UTC sounding shows an increasing dewpoint depression with height and no discontinuity at the reported cloud top of 15,000 feet. A definite dry layer is indicated between

Figure 4-17.-Example of inferring clouds from a RAOB with a middle layer and no precipitation reaching the surface

Figure 4-18.-Example of lnferring clouds from a RAOB showing layer clouds with their Intermediate clear layers not showing in the humidity trace.

18,300 and 20,000 feet. The second reported cloud layer is indicated by a decrease in dewpoint depression, but the humidity element is obviously slow in responding. The dewpoint depression at the base of the cloud at 21,000 feet is 14C and at 400-hPa; after about a 3-minute climb through the cloud, it is still 10C. From the sounding, clouds should have been inferred to be from about 4,500 feet (base of the rapid humidity increase) to 500-hPa and a second layer from 20,000 feet up. In view of the rapid falling of the cloud free gap between 15,000 and 21,000 feet that followed as the warm front approached, the agreement between reported and inferred conditions is good. Figure 4-17 shows a middle cloud layer with no precipitation reaching the surface. This is a case of a cloud in the 500-hPa surface with no precipitation reaching the surface; the nearest rain reaching the surface was in Tennessee. The evidence from the sounding for placing the cloud base at 12,200 feet is strong, yet the base is inexplicably reported at 15,700 feet. The reported cloud base of 15,000 feet was probably not representative, since altostratus, with bases 11,000 to 14,000 feet, was reported for most stations over Ohio and West Virginia.

Figure 4-18 shows layered clouds with their intermediate clear layers not showing in the humidity trace. There is good agreement between the sounding and the aircraft report. The clear layer between 6,000 and 6,500 feet is not indicated on the sounding. Thin, clear layers, as well as thin cloud layers, usually cannot be recognized on the humidity trace.

Comparisons between soundings and cloud reports provide us with the following rules:

1. A cloud base is almost always found in a layer, indicated by the sounding, where the dewpoint depression decreases.

2. You should not always associate a cloud with a layer of decreasing dewpoint, but only when the decrease leads to minimum dewpoint depressions from 6C to 0C. However, at temperatures below -25C, dewpoint depressions in clouds are often higher than 6C.

3. The dewpoint depression in a cloud is, on the average, smaller in clouds that have higher temperatures. typical dewpoint depressions are 1C to 2C at temperatures of 0C and above, and 4C between -10C and -20C.

4. The base of a cloud should be located at the base of the layer of decreasing dewpoint depression, if the decrease is sharp.

5. If a layer of decreasing dewpoint depression is followed by a layer of a stronger decrease, the cloud base should be associated with the base of the strongest decrease.

6. The top of a cloud layer is usually indicated by an increase in dewpoint depression. Once a cloud base is determined, the cloud is extended up to a level where a significant increase in dewpoint depression starts. The gradual increase of dewpoint depression with height in a cloud is not significant.

In addition to the above analysis, another study was made to determine how reliable the dewpoint depression is as an indicator of clouds. The results are summarized in figure 4-19. Each graph shows the percent

probability of the existence of a cloud layer in January for different values of dewpoint depression. On each graph one curve shows the probability of clear or

Figure 4-19.-Percent probability of existence of cloud layer bases for different values of dewpoint depression (degrees C). Solid lines represent probability of clear or scattered conditions; dashed lines, the probability of broken or overcast conditions with the cloud layer bases between 1,000-hPa and 600-hPa.

scattered conditions as a function of the dewpoint depression; the other curve shows that of broken or overcast conditions. Separate graphs are based on 1,027 observations, which are enough to indicate the order of magnitude of the dewpoint depressions at the base of winter cloud layers. Minor irregularities in the curves were not smoothed out because it is not certain that they are all due to insufficient data. The graphs are applicable without reference to the synoptic situation.

For a given winter sounding, you can estimate from the graph the probability of different sky cover conditions with cloud bases between 1,000-hPa and 600-hPa for layers of given minimum dewpoint depressions.

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