Near the close of the last century, Kunkel et al. (1999) analyzed historical trends in several different types of extreme weather events, together with their societal impacts. They found that most measures of the economic impacts of weather and climate extremes over the past several decades do indeed reveal increasing losses. However, they found that "trends in most related weather and climate extremes do not show comparable increases with time." This observation led them to conclude that the increasing economic losses "are primarily due to increasing vulnerability arising from a variety of societal changes," and in this regard they found that "increasing property losses due to thunderstorm-related phenomena (winds, hail, tornadoes) are explained entirely [our italics] by changes in societal factors."

One year later, Changnon and Changnon (2000) analyzed hail-day and thunder-day occurrences over the century-long period 1896-1995 in terms of 20-year averages obtained from records of 66 first-order weather stations distributed across the United States. This effort revealed that thunder-day frequency peaked in the second of the five 20-year intervals, while hail-day frequency peaked in the third or middle interval. Thereafter, both parameters declined to their lowest values of the century in the final 20-year period. Hail-day occurrence, in fact, decreased to only 65% of what it had been at mid-century, dropping so low that there actually was a decline in national hail insurance losses over the final two decades of the study.

Focusing on the same hundred-year time period in Poland, Bielec (2001) analyzed thunderstorm data obtained at Cracow, which is described as being "one of the few continuous records in Europe with an intact single place of observation and duration of over 100 years." Over the entire length of this record, there were 2470 days with thunderstorms, or an average of about 25 days per year. The highest annual number of thunderstorm days was 37, recorded in 1968 and again in 1975. The lowest annual number was 9 in 1904. Close analysis of the data revealed a slight but non-significant linear increase of 1.6 storms per year from the beginning to the end of the record. From 1930 onward, however, the trend is negative, revealing a linear decrease of 1.1 storms per year. Most germane to this review, Bielec reports there has also been a decrease in the annual number of thunderstorms with hail over the hundred-year period.

In the final study we have reviewed that touches on the subject of hail, Changnon (2003) investigated trends in both severe weather events and changes in societal and economic factors over the last half of the 20th century in the United States. In doing so he found that trends in various weather extremes over this period were mixed, noting that "one trend is upwards (heavy rains-floods), others are downward [including "hail, hurricanes, tornadoes, and severe thunderstorms"], and others are unchanging flat trends (winter storms and wind storms)." We might also add, however, that had the analysis of heavy rains-floods been extended back to the beginning of the 20th century, the longer-term behavior of this phenomenon would have been found to be indicative of no net change, as recently demonstrated by Kunkel (2003).

So why did United States insurance losses due to most extreme weather events (remember, hail was the exception) rise so rapidly over the past several decades? Changnon reports that "the primary reason for the large losses [was] a series of societal shifts (demographic movements, increasing wealth, poor construction practices, population growth, etc.) that collectively had increased society's vulnerability."

Yes, global warming -- CO2-induced or otherwise -- has had absolutely nothing at all to do with the increasing damages caused by extreme weather events in general; and it has had absolutely no tendency to increase the occurrence of hail storms, at least in the United States and the portion of Poland for which pertinent data have been properly analyzed.

References
Bielec, Z. 2001. Long-term variability of thunderstorms and thunderstorm precipitation occurrence in Cracow, Poland, in the period 1896-1995. Atmospheric Research 56: 161-170.

Changnon, S.A. 2003. Shifting economic impacts from weather extremes in the United States: A result of societal changes, not global warming. Natural Hazards 29: 273-290.

Changnon, S.A. and Changnon, D. 2000. Long-term fluctuations in hail incidences in the United States. Journal of Climate 13: 658-664.

Kunkel, K.E. 2003. North American trends in extreme precipitation. Natural Hazards 29: 291-305.

Kunkel, K.E., Pielke Jr., R.A. and Changnon, S.A. 1999. Temporal fluctuations in weather and climate extremes that cause economic and human health impacts: A review. Bulletin of the American Meteorological Society 80: 1077-1098.