Long-term fluctuations in the number of lightning deaths from 1900 to 1991 have been examined by analyzing lightning death data compiled by the Bureau of the Census and the Public Health Service for the contiguous United States and the District of Columbia. The effect of the varying population during the period was removed by dividing the yearly values of reported deaths by the corresponding yearly population of the reporting states. The resulting population-normalized series revealed the presence of an overall exponential trend in the number of deaths per million people producing a decrease from more than 6 deaths per million to less than 0.5 during the 92 years of data. This exponential trend is also present in the decrease of the rural US population for the period. The two data sets agree very well and this suggests that the downward trend in deaths resulted to a large extent from a migration of people from rural to urban areas, or from the progressive urbanization of rural areas.
Superimposed on the overall downward trend there were fluctuations in the number of lightning deaths of two or three decades in duration. These fluctuations have been isolated by removing the exponential decrease trend from the lightning deaths series. There are 3 major oscillations that decrease in amplitude with time throughout the period. From a comparison of the series of lightning death departures from the exponential trend with two different climatological variables, these oscillations appear to be climate related. The patterns of these fluctuations were parallel to nationwide changes in thunder-day frequencies and average surface temperature values