Hail Frequency as Related to Storm Intensity and Height

Fifty percent hail frequencies may be expected in storms exceeding 46,000 feet, based on radar reports. With a maximum echo height of 52,000 feet, a 67 percent hail frequency can be expected; and only 33 percent at 35,00 feet, Mean echo heights are 42,000 feet for hail and 36,000 feet for rain.

The technique presented in this section of the chapter is an objective method of hail forecasting. It uses the parameters of the ratio of cloud depth below the freezing level and height of the freezing level. The data used in this study were derived from 70 severe convective storms (34 hail-producing and 36 nonhail-producing storms) over the Midwestern States. Severe thunderstorms, as used in the development of this technique, were defined as those thunderstorms causing measurable property damage due to strong winds, lightning, or heavy rain. Severe thunderstorms with accompanying hail were defined as those thunderstorms accompanied by hail where hail was listed as the prime cause of property damage, even though other phenomena may also have occurred. All tornadoes were excluded from consideration to avoid confusion.

METHOD OF ANALYSIS. Plot representative upper air soundings (0000 UTC and 1200 UTC) on the Skew T diagram. Then analyze the following parameters:

. Equilibrium level (EL). The EL is found at the top of the positive area on the sounding where the temperature curve and the saturation adiabat through the CCL again intersect. This gives a measure of the extent of the clouds vertical development, and thus an estimate of its top or maximum height.

. Freezing level. This is defined as the height of the zero degree isotherm.

NOTE: All three heights are expressed in units of hectopascals (hPa).

1. The ratio of the cloud depth below the freezing level (distance in hectopascals from the CCL to the freezing level) to the clouds estimated vertical development (distance from the CCL to the EL in hectopascals) is defined as the cloud depth ratio. For example, if the CCL was at 760 hpa, the freezing level at 620 hPa, and the EL at 220 hpa, then the cloud depth ratio would be computed as follows:

2. Height of the freezing level in hectopascals (620 hpa).

With step 1 as the vertical axis and step 2 as the horizontal axis, use figure 5-8 for occurrence or nonoccurrence of hail. In this case, hail would be forecast if the value falls in the hail forecast area.

EVALUATION. Using the data in 70 dependent cases, the correct percent for prediction of hail or no hail was 83 percent. This technique combines the two parameters relating hail to convective activity into a single predictor. Although the data used in this study were from the Midwest, the application need not be confined to that area. With some modification of this diagram, this method could serve as a basis for a local forecasting tool for other areas.

Figure 5-8.-Scattered diagram showing the distribution of selected hail occurrences at certain Midwestern stations during the spring and summer of 1994 and 1995. The freezing level is plotted against the cloud depth ratio. (X = hail reported. 0 = no hail reported.)

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