84 zyxwvut Volume 44, Number I, February 1992 zyxw A More Rational Climatic Moisture Index* zyx Cort J. Willmott University zyxwvuts of Delaware Johannes J. Feddema University of California, Los Angeles A zyxwvutsrqpo climatic moisture index (I,) used extensively by C. W. ThornthLvaite and others is examined, and a modified version of the index is proposed. Arbitrary limits zyxwvu [- 100 5 I, 5 100 (p - l)], combined with its dimensionless property, make the original index difficult to interpret. Our refined version also is dimensionless, but the limits have been recast so that - 1 5 I, 5 1. Wet climates have positive values of I, while dry climates are negative, and the index is symmetric about zero. The spatial distribution of our modified annual-average moisture index over the world's continents i5 mapped and described. Key Words: water balance, moisture index, global climate Introduction limate is largely characterized by the heat C and moisture states of the earth's surface and near surface (Willmott 1987) and, there- fore, concern for their (the states') spatial and temporal variability has commanded the atten- tion of climatologists for centuries. Moisture has received most of the attention, owing to its paramount role in biological processes (Carter and Mather 1966). Climatologists often have used simple indices to describe the mois- ture state of the earth's surface, and one such index-Thornthwaite's index- is the subject of this note. Moisture indiccs typically are functions of the ratio of time-averaged precipitation to time-averaged moisture demand by the atmo- sphere. Their value lies in their ability to char- acterize the relative wetness or dryness of places based solely on climatic data (Mather 1974). Perhaps the moisture-index concept can be traced to Linsser (Thornthwaite 1943), al- though it was Thornthwaitc who first replaced the early surrogates of moisture demand (pan evaporation and air temperature) with a cli- matologically meaningful moisture demand, termed potential evapotranspiration (Mather 1990). Thornthnaite and his colleagues' many hydroclimatological publications during the 1930s, 1940s, and 1950s served to disseminate widely their contributions, including the moisture index (Thornthwaite 1943, 1948; Thornthwaite and Mather 1955). The evaluation and mapping of moisture indices continue to be educational. Delworth and Manabe (1988), for instance, use a mois- ture index to illustrate relationships that might occur between climate and moisture availabil- ity at the earth's surface. As Thornthwaite's moisture index is well known and typical, our purpose within this paper is to examine several of its properties and present a more rational, modified version of the index. Thornthwaite's Moisture Index Thornthwaite (1948) specified his moisture in- dex as S - 0.6D I, = 100 [ E" ] (1) where 1, is the moisture index, S is the mois- ture surplus, D is the moisture deficit, and E" is the potential evapotranspiration; that is, the evapotranspiration that would occur if the veg- etation experiences no water stress. His mois- ture deficit (D) is merely (E" - E), where E is the actual evapotranspiration rate. Thornth- waite and Mather (1955) later dropped the 0.6 scaling coefficient. Thornthwaite's surplus can be written r - [E+(w*-w)], r > [E+(w*-w)] = io, r 5 [E+(w*-w)] * Thoughtful wggestiona un a preliminary draft of this note acre made by J K \lather. zyxwv li. Mink, and S. XI. Robeion. and we gratefully acknou ledge their assistance. \Ve also thanh I<. Alatwura for his help MI drafting the map A portion of this paper is based on work supported by S.AS 4 under grant 9AGIV-1884. Professional Geographer, 44( I) 1992, pages 84-87 zyxwvuts 0 Copyrighr 1992 by .Association of American Geographers Initial submission, June 1990; revision submitsion. July IYY I; final acceptance, August 199 1