Assessment of Historical Thunderstorm Data for Urban Effects: The Chicago Case
- Cite this article as:
- Changnon, S.A. Climatic Change (2001) 49: 161. doi:10.1023/A:1010797013336
Altered incidences of thunderstorms resulting fromthe influence of large urban areas on the atmosphere can biasefforts to assess the long-term behavior of regional stormactivity. Past studies of selected cities have defined increasedthunderstorm 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 werecollected with areas with potential urban influences on stormactivity is important to identify quality records and to deletedata from questionable urban sites. An investigation of thisissue was conducted using unique, quality long records from twostations at Chicago. This study compared storm activity using40-years of data from a station in central Chicago and one on thecity's western boundary. The city site averaged 4.5 morethunderstorm days a year, an increase of 12%, than theadjacent rural site. In-city increases occurred in all fourseasons with the greatest differences in spring and summer andleast in fall, and enhanced storm activity occurred in all hoursbut was greatest in the afternoon and evening. The resultsestablished that sizable and statistically significant increasesin storm activity occurred over the central portions of Chicago. This has major implications for the selection of stations foruse in climate change assessments of storm activity. This studyrevealed that the official thunderstorm records that had beentaken within the city since 1896 were likely urban influenceduntil the station was moved to the city's western edge in 1959. The Chicago data should not be used in regional and nationalstudies of historical fluctuations of storm activity during the20th century. Results from Chicago and studies of othercities suggest that most storm enhancement occurs to thenortheast, east, and southeast of cities with populations of 1 to2 million, whereas in larger cities like New York and Chicago,storm modification also occurs well within the confines of thecity.