A Method for Forecasting Hailstone Size at the Earth's Surface*

Abstract

All upper-air soundings in the United States have been evaluated since January 1, 1950 and 274 selected as representative of air in which hailstones of known size formed. These soundings are summarized and several parameters which are or may be useful in forecasting are discussed. A graph is presented, which permits the translation of a forecast of air structure into one of hailstone size and two groups of forecasts are summarized to show the validity of this forecasting tool. The forecasts discussed were prepared in practical situations by meteorologists on regular forecasting duty and issued before the storms occurred. They therefore constitute a real test of the method under working conditions. Evidence is presented to show a very striking relation between the incidence of surface hail and the height of the wet bulb freezing level above the terrain. It was found that in 91.6% of the 274 representative soundings, the wet bulb freezing levels were between 5000 and 12,000 feet above the surface and the larger the reported surface hail, the greater the concentration of these levels near 8000 feet. It is emphasized that the method of hail forecasting discussed is not susceptible to machine methods, but depends on accurate prognostication of the thermodynamic structure of the air column as it changes in time and space.