This atlas addresses the climatology of hail in the United States. The information has been assembled from diverse sources from the past 80 years, and includes results of research conducted specifically for this document. Climatological descriptions of the various hail conditions that cause damages to crops and property also are presented, as well as assessments of hail-produced losses. The nation’s areas of greatest hail frequency are along and just east of the central Rocky Mountains where point averages vary between 6 to 12 hail days per year. The lee of the Rocky Mountains has the nation’s greatest hail intensity with the largest average stone sizes, the highest average number of hailstones, and the longest hail durations. The nation’s lowest hail intensities are found in the southeastern U.S. (Florida) and in the southwest (Arizona and California), areas where hail occurs only once every two or three years. Winds with hail tend to be strongest in the central and southern High Plains, the location where property-hail damage is the nation’s highest. Hail risk to crops and property is characterized by enormous variability in both space and time. Exceptionally large hailstones, those exceeding 2 inches in diameter, can occur anywhere it hails in the U.S., but are most frequent in southeastern Wyoming (once every five years) and least frequent in the low hail frequency areas (only once every 100 years or less often at a given point). The extent of hail damage results from hailstone sizes, the number of hailstones per unit area, and winds with hail. Hail causes considerable damage to U.S. crops and property, occasionally causes death to farm animals, but is only infrequently responsible for loss of human lives. The average annual frequency of days with crop-damaging hail in the U.S. is 158 days, and the average annual crop loss is USD580 million. The average annual frequency of days with hail damages to property is 123 days, and the average annual loss total is USD850 million. Each year, on average, the nation experiences 15 days with property losses exceeding USD1 million and 13 days with crop losses exceeding USD1 million. Hail is a threat in most parts of the nation. The risk of property damage across the nation varies from an index value of 1 in the southeast to a high of 50 (Colorado, Kansas), and the indices of risk of crop damage vary from a low of 1 in the eastern Midwest and East to a high of 20 in the western High Plains (Montana, Wyoming, and Colorado). Hail that is damaging to crops differs from that damaging to property. Various crops can be damaged by small stones, whereas property damage occurs only when hailstones exceed 3/4 inch in diameter. Distributions of hail damage vary considerably with most damages occurring in 5 to 10 percent of all storms, and most losses occurring in only a small portion of an area experiencing hail on a given date. Hail frequency and crop-hail intensity conditions change significantly over time with a tendency for low hail incidence in 60 to 80 percent of the years, and exceptionally large losses in 5 to 15 percent of the years. The temporal variability of hail loss is greater in the High Plains states than in states in the Midwest, East, or West. The magnitude and frequency of hail shifts up and down randomly over time, but the primary spatial features of hail (areas of extremely high or low incidence of hail in a region) persist from decade to decade. Hailstorms extremely damaging to property show an upward trend with time, and the two most damaging storms in the U.S. have occurred since 2000. Nationwide trends in crop-hail losses, in property-hail losses, and in the number of hail days are either flat or slightly downward for the 1950-2009 period, and do not suggest any climate change influence.
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