Formation of Clouds by Heating From Below

The first step is to determine the convection temperature or the surface temperature that must be reached to start the formation of convection clouds by solar heating of the surface air layer. The procedure is to first determine the CCL on the plotted sounding and, from the CCL point on the T curve, proceed downward along the dry adiabat to the surface pressure isobar. The temperature read at this intersection is the convection temperature.

Figure 5-3 shows an illustration of forecasting afternoon convective cloudiness from a plotted sounding. The dewpoint curve was not plotted to avoid confusion. The dashed line with arrowheads indicates the path the parcel of air would follow under these conditions. You can see that the sounding was modified at various times during the day.

To determine the possibility of thunderstorms by the use of this method and from an analysis of the sounding, the following conditions must exist:

. The greater the excess, the greater the possibility of thunderstorms.

. The parcel must rise to the ice crystal level. 

. Generally, this level should be 10C and below.

. There must be sufficient moisture in the lower troposphere. This is the most important single factor in thunderstorm formation.

. Climatic and seasonal conditions should be favorable.

. A weak inversion (or none at all) should be present in the lower levels.

. An approximate height of the cloud top may be determined by assuming that the top of the cloud will extend beyond the top of the positive area by a distance equal to one-third of the height of the positive area.

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