Data for thunder days during the 1901-1980 period from 227 stations around the globe were collected, evaluated, and then examined for temporal fluctuations. The densest and highest quality data were for North America, which was the focal point of the study. After three data quality tests, data for four North American stations, were rejected, leaving 86 stations in the United States and Canada for study. A 14% increase in thunder days occurred from 1901 to 1945 in North America, followed by an 8% decrease through 1980 with an anomalous high during 1971-1975. The temporal behavior is similar to the annual temperature trends of North America and relates well to historical cyclone frequencies. Major regional differences existed in the temporal distributions of thunder days and their variability for North America. North fo 45 ¿N, a general increase existed from 1901-1980 along with decreasing interannual variability; the southeastern United States had a general decrease in thunderstorms with increasing variability from 1930 to 1980; and the western third of the continental United States had a downtrend in both thunderstorm frequencies and their interannual variability from 1901 to 1980. Statistically significant periodicities of greater than 15 years were found throughout the United States, with shorter periodicities, less than 7 years, also present in the western third of the United States. Moderate length periodicities of 7 up to 15 years were found in the northern United States and Canada. Thunder-day trends at the 86 stations were used to delineate 14 areas that were consistent with standard climatic zones. There is a suggestion that inadvertent human influences may also relate to the decrease in thunder activity in the eastern United States. Discrete regions of uptrends or downtrends were found in the data of most stations in the other continents, but station density was too poor to allow definitive delineation of regions as in North America. However, all northern hemisphere stations above 50 ¿N exhibited uptrends in thunder from 1901 through 1980. The quality data are available to interested scientists.