A review of recent research in the United States concerning urban effects on precipitation has revealed that relatively few studies have been performed. The lack of densely spaced precipitation stations with good historical records, inadequate instruments for airborne measurements of the mechanisms that affect precipitation systems, and the difficulties associated with separating orographic, maritime, and gage-exposure effects are the primary reasons for so little research.

However, certain climatological studies of four variously sized cities in the midwest and two large cities in the east have shown apparent urban-produced increases ranging from 5 to 16% in annual precipitation and rain days, with 7 to 20% increases in summer thunderstorm days. Substantially greater increases in precipitation, thunderstorms, and hailfalls, 31 to 246%, have been shown in a recent study of an area downwind from a major steel mill complex. The available results show little evidence of urban effects on the occurrence of excessive rainfall rates or on the amount of snowfall, although little study of these conditions has been performed.

The recent development of airborne nuclei measuring instruments has led to selected measurements of condensation and freezing nuclei over several urban areas. These furnish evidence that urban-induced nuclei concentrations are high and probably sufficient to produce the observed changes in precipitation, whereas other American studies have indirectly shown the importance of the urban thermal effects. The study of inadvertent precipitation increases from urban areas has particular significance for planned weather modification since the amounts of the inadvertent increases approximate those confirmed for planned experiments. The results of the urban studies may indicate the effectiveness of ground-based seeding, the possibility of successful increases in all seasons, the likelihood of thunderstorm and hailstorm increases with rainfall increases, and the need for dense raingage networks to adequately determine the area and amount of increase.

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