A report issued by the Illinois State Water Survey concludes that annual values of thunder days for North America exhibited a general increase of about 15% from 1901 to 1945, followed by a general decrease of 10% from 1945 to 1980. A study of the variability of thunder days across North America showed a general decrease with time, particularly after 1940. A major finding of this study is that frequencies of thunderstorms over areas as large as the North American continent show major long-term trends.
The report, “Temporal Distribution of Global Thunder Days,” summarizes the results of a 1-year study by Stanley A. Changnon, Jr., and Chin-Fei Hsu of the temporal variations of thunder-day records during 1901–1980 using quality weather records from weather stations scattered around the globe. A thunder day is recorded when one or more peals of thunder are heard anytime during the 24-hour period from midnight to midnight, which is consistent with the definition of a thunderstorm used at first-order weather stations since 1897. They found most stations in the northern hemisphere north of 45° latitude exhibited a general increase in thunder activity from 1901 to 1980. The project was funded by the National Science Foundation.