Frequency of point and areal mean rainfall rates

Use of special recording raingages enabled recording of one‐minute rainfall amounts from a 50‐gage network on a 100‐sq mi area in central Illinois during the 1952–53 thunderstorm seasons. One‐minute amounts from 19 storms during this period have been used to study the frequency of rainfall rates in shower‐type storms under warm season conditions, on both a point‐rainfall and areal‐mean‐rainfall basis. Within the 100‐sq mi area, smaller areas of 10, 25, and 50 sq mi were used to obtain an indication of the effect of changing area on the distribution of areal mean rates. The expected tendency for the mean rate to decrease with increasing area was found. For example, the median rate for all 19 storms combined ranged from 1.45 inches per hour for point rainfall to 0.96 inch per hour for the 25‐sq mi area to 0.66 inch per hour for the 100‐sq mi basin. The degree to which averaging suppresses details of high rates, as well as the general effects of the averaging process on the distribution curve, was also studied by comparing the distribution of point rates when the rates were based on 1‐, 5‐, 10‐, 15‐ and 30‐minute averages. Results showed the maximum observed rate ranging from 13.80 inches per hour for one‐minute to 2.45 inches per hour when based on a 30‐minute average. Similarly, when all 19 storms were combined, the median rate ranged from 1.45 inches per hour for one‐minute averages to 0.56 inch per hour for 30‐minute averages.