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An investigation was made of urban effects on precipitation in and downwind of St. Louis through use of long-term climatic records within a radius of 50–75 mi of the city. Based upon radar climatological studies, two potential urban effect areas and two no-effect (control) areas were designated. Analyses were then performed to assess the urban effect on 1) total monthly and seasonal precipitation; 2) the frequency and intensity of daily precipitation; 3) the frequency of thunderstorms, hailstorms, and severe rainstorms; 4) wet-dry period rainfall; and 5) weekday-weekend precipitation (indicator of industrial-induced effect). Evidence of urban effects was found in all seasons, but appear to he strongest in late spring and summer. Results indicated an urban-induced increase in average summer rainfall ranging from 6 to 15% for distances up to 25 mi downwind of the city. The urban-increase mechanism apparently is most active on days of moderate to heavy intensifies in the natural rainfall. The urban effect was most pronounced in relatively wet summers, and indications of a suppression effect were found in dry summers. Evidence was found of an urban-induced increase in thunderstorm days within the city and of hail-day maxima downwind of the city. Overall, results appear to support thermal effects as the most important causative factor for urban modification of the natural precipitation distribution.
Huff et al. (Tue,) studied this question.