The Temporal Trend in U.S. Hail Damage
Changnon, S.A. 2009. Increasing major hail losses in the U.S. Climatic Change 96: 161-166.
With respect to this finding, Changnon states that "two factors could have affected this increase." One of them, in his words, could have been "more frequent occurrences of major cases of strong atmospheric instability, leading to the development of supercell thunderstorms capable of persisting for many hours, covering large areas, and producing large hailstones." However, he says that this scenario "has not been measured and can not be verified." The second factor, as he describes it, "is the expansion of the nation's metropolitan areas, enhancing the target for hail damages to property," in support of which he notes that "urban population in the U.S. since 1960 increased by 56% and urban areas grew by 154%," as per data contained in the World Almanac (2008), all of which makes a pretty good case for the second of the two factors Changnon suggested.
World Almanac. 2008. The World Almanac and Book of Facts. U.S. Cities, States and Population. World Almanac, New York, New York, USA.