Secular Trends in Thunderstorm Frequencies

Abstract

Thunderstorm day frequencies during the 1901–70 period at a series of 53 weather stations located around the world were studied 1) to examine for the existence of trends, and if trends existed, 2) to explain their causes. All stations examined in the central and eastern United States, Japan, South Africa, and in parts of southern Europe in the latitudes of 30° to 45° were found to have their lowest frequencies in the most recent period, 1951 to 1970, ranging from 10% to 30% below their 70-year averages. Furthermore, this recent minimum is the result of a general downward trend in thunderstorm frequencies that began at most locations in the 1930’s. Stations in the tropical areas and in latitudes above 45° exhibit no trends or upward trends since the 1930’s. The sizable trends of the last 35 years can be partially ascribed to inadvertent, man-made atmospheric alterations which are supported by a secular change in atmospheric conductivity and aerosols found in the North Atlantic. However, most evidence suggests these recent trends in thunderstorms are due to natural climatic change. Observational errors are also evaluated as a possible explanation but considered negligible. Regardless of the causes, such changes have pertinence in gaining a better understanding of possible regional shifts in atmospheric electricity, severe storm climatology, and in properly interpreting climatic records.