Natural Resources, School of


Date of this Version



1988 American Meteorological Society


Hourly precipitation data from 1967 to 1983 for the conterminous United States were harmonically analyzed in order to document the diurnal variability of several categories of heavy precipitation during winter, spring, summer, and autumn. The analysis revealed that the diurnal characteristics of hourly precipitation vary considerably with season, geographic region, and precipitation intensity. During winter and spring, a weak, later-morning frequency maximum prevails for the lightest (2.5-6.2 mm h-1) precipitation category. As intensity increases (to 6.3-12.6, 12.7-25.3 and >25.4 mm h-1), the amplitude of the diurnal cycle also increases, and a nocturnal maximum becomes apparent across much of the eastern and central United States. In summer, the diurnal cycle is strongly modulated for all categories. The nocturnal region decreases in areal extent at this time of year, as an afternoon maximum becomes established across the southern and eastern states. In autumn, the nocturnal region again increases in size, although the area it encompasses is smaller than that during winter and spring. Seasonal variations in the semidiurnal cycle are more ambiguous due to the dominance of the diurnal cycle at most locations, although secondary maxima and minima are most likely south of the Great Lakes and in eastern and central Texas. Comparison with the results of previous studies indicates that different definitions of "winter," "spring," "summer," and "autumn" can leas to divergent descriptions of the diurnal cycle of hourly precipitation.