Just as the WBZ height is an important and reliable indicator of thunderstorm gusts, it is also a reliable preliminary indicator of hail occurrence. Research has shown that 84 percent of all cases of hail occur with the WBZ between 5,000 and 10,500 feet AGL. Furthermore, nearly all analyzed cases of hail larger than 1/2 inch occurred where the WBZ was near 8,000 feet AGL. Although many strong thunderstorms were studied where the WBZ was higher or lower than these values, most did not produce hail or strong gusts at the surface even though various indicators showed that the potential was present and that hail was almost certainly produced aloft. In the 16 percent of the cases where hail did occur at the surface with WBZ heights outside the 5,000- to 10,500-foot range, the hail fell only for a short period and was small. These cases occurred mostly in the states along the Gulf Coast.

Hail, like the maximum wind gusts, usually occurs in a narrow shaft seldom longer than a mile or two and less than a mile wide.

HAIL ALOFT. Since hail is normally associated with thunderstorms, the season for hail occurrence is the season for thunderstorm occurrence. When a thunderstorm is large and well developed, you should assume that it contains hail aloft. Below 10,000 feet, aircraft may encounter hail in the rain shaft under the storm, in the clear air within 2 miles of the storm cloud, and in the thunderstorm cloud itself, with equal probability. From 10,000 to 20,000 feet, 40 percent of aircraft hail encounters occur in the clear air under the overhanging tilted cumulonimbus column or the anvil, while 60 percent occur within the cumulonimbus column. Above 20,000 feet, 80 percent of hail encounters are within the cloud, while 20 percent are under the anvil. No computations are required to de-termine if hail is present aloft. Assume that it is.

HAIL AT THE SURFACE. Will the hail reach the surface? If the WBZ height is favorable (5,000 to 11,000 feet for most areas), proceed with the following objective computation for a simple "yes" or "no" determination. If you are stationed along the Gulf Coast or in areas climatically similar to the Gulf Coast, you should use the procedure even though the WBZ exceeds 11,000 feet.

1. Find EL, and the freezing level (FzL), in millibars.

2. Calculate the ratio of the depth, in millibars, of the freezing level to CCL, compared to the EL to CCL depth. This is the cloud depth ratio, It may be calculated as follows:

3. Enter figure 6-2-18 with the cloud depth ratio and the freezing level, in millibars, for a "yes" or "no" prediction of hail reaching the surface.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing