Once the forecaster has determined that the probability of hail exists, as previously outlined, the next logical question will relate to the size of the hail that may be anticipated. The following text discusses the method that uses the Skew T Log P diagram.

The first step in forecasting hail is to determine the convective condensation level (CCL). This parameter is evaluated on the adiabatic chart by finding the mean mixing ratio in the moist layer of the lowest 150 hPa. and following this saturation, mixing ratio line to its intersection with the sounding dry-bulb temperature curve. Next, the moist adiabat through the CCL is traced up to the pressure level where the dry-bulb temperature is -5C. This pressure level, the dry-bulb temperature curve, and the moist adiabat through the CCL form a triangle, outlining a positive area. Figure 5-9 illustrates this procedure. The horizontal coordinate in figure 5-9 is the length of the horizontal side of the triangle in degrees Celsius. The length is measured from the pressure at the base of the triangle.

Figure 5-9.-Example of hail size forecast sounding.

These computations for the horizontal length in degrees and altitude in degrees are used on the graph in figure 5-10, and for the forecast of hailstone diameter.

EXAMPLE OF TECHNIQUE. In the sounding shown in figure 5-9, the CCL is point A. The moist adiabat from the CCL to the pressure level, where the temperature is -5C, is the line AB. The isobar from the point where the air temperature is 5C to its intersection with the moist adiabat is the line BB. The dry adiabat from the isobar BB through the triangle to the pressure of the CCL is the line HH. The base of the triangle in degrees Celsius is 6C (from plus 1 to minus 5). The length of the dry adiabat through the triangle is 21C (from minus 4 to plus 17).

EVALUATION. The value on the graph in figure 5-10, with a horizontal coordinate of 6 and a vertical coordinate of 21, is a forecast of 1-inch hail.

Figure 5-10.-Fawbush-Miller Hail Graph showing forecast hailstone diameter in inches.

To more accurately forecast hail size in conjunction with thunderstorms along the Gulf Coast or in any air mass where the Wet-Bulb-Zero height is above 10,500 feet, it is necessary to refer to the graph in figure 5-11. The hail size derived from figure 5-10 is entered on the horizontal coordinate of figure 5-11, and the corrected hail size read off is compatible with the height of the Wet-Bulb-Zero temperature.

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