Data and Approaches for Determining Hail Risk in the Contiguous United States

Abstract

Rapidly increasing hail damages to property have brought average annual losses to $1.2 billion (in 1997-adjusted dollars) during the 1990s; this rise in loss exposure has created great concern in the insurance industry and has led to efforts to define the hail risk across the nation. Since the property industry has not kept hail loss records, it must rely on available climatological data supplemented by data from field studies to assess hail risk. Hail risk at a point or over an area is a function of the target at risk (property or crop) and the hail frequency and intensity. Newly available climatological data based on historical hail records collected by the National Weather Service since 1900 have enhanced the ability to assess point risk. Some spatial risk assessments have combined point averages of hail-day frequencies with hailstone sizes to define risk, and others also employed hailstone volume (mass of ice) and wind associated with hail, based on data from field projects. Those seeking to define the spatial aspects of risk caused by very large hailstones, greater than 2.5 cm in diameter, have had to use area-based risk assessments since point data are too short to provide reliable frequencies of these rare events. Ongoing research is defining the hail damage characteristics for various structural surfaces and roofing materials for which most damage occurs. Crop insurance risk studies have combined that industry?s existing data on crop losses across the nation with long-term frequencies of hail days to generate crop?hail risk patterns for setting rates, and their data, which began in 1948, have been used to assess temporal variability of risk. These temporal assessments have relied also on long-term records of hail-day incidences, and both datasets show that the magnitudes of major features in average hail risk patterns fluctuate as much as 50% in any given 5- to 20-yr period, but that these features persist over time. Long-term trends of hail reveal increases in the High Plains and Southeast with decreases in the Midwest and West.