THE FREQUENCY OF DRY SPELLS IN THE SOUTHERN UNITED STATES

Abstract

A dry-spell climatology for the southern United States was developed using a definition of a dry spell of length × as × days without measurable precipitation. Data for 1950 to 1994 for 154 stations, evenly distributed over the area approximately bounded by 98°W and 37°N, were used. Separate analysis for winter (December-January-February) and summer (June-July-August) were undertaken. For the majority of stations in both seasons the modal dry period length is one day, although in winter approximately a third of the stations have two-day modes. There are commonly about 15 dry spells in winter, with little spatial variation. In summer, the number increases from east to west. In both seasons, the duration of the longest spells is greatest in the west. A cluster analysis revealed four regions in summer, three reflecting the east-west increase in dryness and the fourth isolating a separate regime in Florida. Winter had three regions that were indicative of an increase in the dominance of long dry periods northward of the Gulf of Mexico superimposed on the east-west trend. The data were reclustered using the same criteria and number of regions for the 1950–1969 and 1975–1994 periods separately. During the latter period in winter, there was an increase in the number of short dry spells at the expense of relatively long ones throughout the region. Summer changes were less marked, but the Mississippi Valley shows a similar trends to that of winter. [Key words: dry spells, cluster analysis, climatic regions, southern United States.]