THE FREQUENCY OF DRY SPELLS IN THE SOUTHERN UNITED-STATES

A dry-spell climatology for the southern United States was developed u sing a definition of a dry spell of length x as x days without measura ble precipitation. Data for 1950 to 1994 for 154 stations, evenly dist ributed over the area approximately bounded by 98 degrees W and 37 deg rees N, were used. Separate analysis for winter (December-January-Febr uary) and summer (June-July-August) were undertaken. For the majority of stations in both seasons the modal dry period length is one day, al though in winter approximately a third of the stations have two-day mo des. There are commonly about 15 dry spells in winter, with little spa tial variation. In summer, the number increases from east to west. In both seasons, the duration of the longest spells is greatest in the we st. A cluster analysis revealed four regions in summer, three reflecti ng the east-west increase in dryness and the fourth isolating a separa te regime in Florida. Winter had three regions that were indicative of an increase in the dominance of long dry periods northward of the Gul f of Mexico superimposed on the east-west trend. The data were reclust ered using the same criteria and number of regions for the 1950-1969 a nd 1975-1934 periods separately. During the latter period in winter, t here was an increase in the number of short dry spells at the expense of relatively long ones throughout the region. Summer changes were les s marked, but the Mississippi Valley shows a similar trends to that of winter.