The Frequency of Large Hail over the Contiguous United States

One of Stan Changnon’s abiding interests is the frequency and pattern of large hail occurrence. Stan started publishing papers in AMS journals on the occurrence of large hail in 1966. After that initial effort, Stan has published 27 papers in AMS journals alone. When other journals, such as those of the AGU, Conference Preprints, and Technical Reports are added, Stan has an amazing record of doing productive research about a single phenomenon. The 1966 paper (Changnon, 1966) was “Note on Recording Hail Incidences.” In it, Stan showed how weighing-bucket rain gauges with their evaporation funnels removed could be used to record hail occurrence. Since that initial paper, Changnon has devoted considerable effort to finding segregate data sources for hail observations. A summary discussion of techniques he developed to use insurance loss data to infer hail characteristics appears in Changnon (1999). The Storm Prediction Center (SPC) has taken a more traditional approach to exploring the pattern and frequency of large hail. The SPC maintains a database of reported severe thunderstorm events over the contiguous United States (Schaefer and Edwards, 1999). Information on “severe hail,” i.e., hail of 190 mm (3⁄4 inch) diameter or greater, is available back through 1955. Since 1972, there has been a concerted effort to maintain agreement between the SPC data and the entries in the NCDC publication Storm Data. Prior to that, data were obtained from real time reports collected by the U.S. Air Force. It should be noted that the wind and hail data for the transition year, 1972, are incomplete. Each hail record contains the time, latitude, longitude, diameter, county, and the number of fatalities and injuries caused by each event. For storms that occurred before 1996, the property damage is simply listed as a logarithmic category that ranges from “0" (< $50) to “9" (> $500,000,000). From 1996 on, property damage estimates are given in millions of dollars. Hail reports typically come from specific observers, so no information on the path and extent of the hail swath is available. As an arbitrary artifice to quantify the data, hail reports within any given county are required to be separated by at least 16 km or 15 minutes (Grenier and Holmstead, 1986). 2. HAIL REPORT COLLECTION PROCESS