What was done
Thunderstorm data from an urban (Midway Airport) and near-rural (O'Hare Airport) site in Chicago were examined over the 40-year period 1959-1998 in an attempt to assess any urban-induced influence in thunderstorm frequency.

What was learned
Over the 40-year period, the urban station averaged 4.5 (12%) more thunderstorm days per year than the rural location; and thunderstorm days recorded at the urban site exceeded those at the rural site in all four seasons.  Significance tests showed these differences to be significant at the 1% level.

What it means
According to the author, knowledge of urban influences is "critical for assessing the quality of thunderstorm data in studies attempting to define the long-term behavior of thunderstorms."  Furthermore, he concludes, and we agree, that "thunderstorm data from any urban weather station must be assessed to detect any local influence, and if present, to delete the station's data from studies attempting to define and understand point and regional temporal fluctuations and trends."  Until such studies are conducted, we can put but little faith in assessments of temporal trends in thunderstorms and climate alarmist claims that global warming is causing the frequency of such storms to increase.