Areal Frequencies of Hail and Thunderstorm Days in Illinois

Hail and thunderstorm statistics for the 1951-60 period obtained from 119 U.S. Weather Bureau stations in Illinois were conlbined n-ith crop-hail insurance data for this same period for performing a detailed climatological investigation of the frequencies of hail days and thunderstorm days in Illinois. In the crop-growing season thunderstorms occur on 1 out of every 2 days on the average and hail occurs somewhere in Illinois on 4 out of every 10 days. The hail-thunderstorm areal ratio for Illinois is 68 percent as compared with point ratios varying from 3 to 7 percent. It appears that some thunderstorms may not contain hail since 32 percent of all the thunderstorm days had no hail reported at the surface anywhere in Illinois. Thunderstorms on days wit,llout hail most frequently occurred in southern Illinois and were associated more frequently with air mass and warm frontal conditions than were the hail-thunderstorms. This research also has shown how "Days With" data from cooperative substatious of the Weather Bureau can be used to enlarge our knowledge of regional climatology.