Assessment of historical thunderstorm data for urban effects: The Chicago case

Altered incidences of thunderstorms resulting from the influence of large urban areas on the atmosphere can bias efforts to assess the long-term behavior of regional storm activity. Past studies of selected cities have defined increased thunderstorm activity downwind (generally east) of urban areas, but few have documented in-city changes in storm activity. Comparison of the sites where historical thunder records were collected with areas with potential urban influences on storm activity is important to identify quality records and to delete data from questionable urban sites. An investigation of this issue was conducted using unique, quality long records from two stations at Chicago. This study compared storm activity using 40-years of data from a station in central Chicago and one on the city's western boundary. The city site averaged 4.5 more thunderstorm days a year, an increase of 12%, than the adjacent rural site. In-city increases occurred in all four seasons with the greatest differences in spring and summer and least in fall, and enhanced storm activity occurred in all hours but was greatest in the afternoon and evening. The results established that sizable and statistically significant increases in storm activity occurred over the central portions of Chicago. This has major implications for the selection of stations for use in climate change assessments of storm activity. This study revealed that the official thunderstorm records that had been taken within the city since 1896 were likely urban influenced until the station was moved to the city's western edge in 1959. The Chicago data should not be used in regional and national studies of historical fluctuations of storm activity during the 20th century. Results from Chicago and studies of other cities suggest that most storm enhancement occurs to the northeast, east, and southeast of cities with populations of 1 to 2 million, whereas in larger cities like New York and Chicago, storm modification also occurs well within the confines of the city.