Hail in Illinois

Jim Angel, state climatologist

Hail is a major weather hazard in Illinois, causing damage to crops and property. Unlike temperature and precipitation, there are very few hail observations made in Illinois. Hail is very difficult to measure because it occurs very locally (requiring a high-density network) and remains in its solid form for only a few minutes before melting. The National Weather Service does report instances of hail at its primary sites in Illinois over time. These sites include Chicago, Springfield, Rockford, Peoria, Moline, and St Louis. A limited amount of information is also available from cooperative observer sites across Illinois.

A recent study (Changnon 1995) used the above mentioned data to examine the limited hail records available back to 1900. Findings from that study include:

• Long-term averages vary only slightly across the state. The northern third of the state experienced about 2 days with hail per year, while the southern two-thirds experienced about 2.5 days with hail per year.
• No long-term trends were detected in the hail records.
• The period of 1946-1965 showed the highest frequency of hail days per year in the entire record.

Figure 1 shows the spatial pattern of total hail days during a 14 year period from 1981 to 1994. To get the average number of hail days per year, divide those numbers by 14. The numbers are lower, reflecting the most recent relatively quiet period.

While the long-term average occurrence of hail was about 2 to 2.5 days, some years were much more active (see Table 1).

Table 1. Average number of hail days and peak values for a year from stations with 94 years of record.
Station	Average	Peak
Chicago	2.1	10 in 1947
Moline	2.3	9 in 1954
Peoria	2.0	7 in 1950, 1927, and 1954
Urbana	2.2	9 in 1955
Springfield	2.5	8 in 1975
St Louis	2.4	7 in 1909
Marion	2.6	7 in 1907, 1917, 1927, 1945, and 1970

From: Changnon, S. A., 1995: Temporal Fluctuations of Hail in Illinois. Illinois State Water Survey Misc. Publication 167, pp.19.