The Comfort Day Index and Its Use as a Measure of Temperature Stress in the Southeastern United States

Southeastern Geographer Vol. XXXVIII, No. 2, November 1998, pp. 142-149 THE COMFORT DAY INDEX AND ITS USE AS A MEASURE OF TEMPERATURE STRESS IN THE SOUTHEASTERN UNITED STATES Peter T. Soulé This study introduces a new measure of temperature stress called the Comfort Day Index (CDI) that is easily calculated using region-specific temperature thresholds for operational heating and cooling degree days. Temporal trends and spatial variability of the CDI are examined for a portion of the southeastern United States centered on Atlanta, Georgia, for the period 19601989 . Temporal variability is examined through simple time-series and regression analyses. Maps of 30-, 15-, 10-, and 5-year climatic normals for CDI are presented. Weak and insignifi­ cant upward trends for regional-mean CDI values are found for the region during the 30-year study period and the mapped patterns show a high degree of spatial stability through the four different averaging periods. CDI calculations for stations outside the study area show that the index operates in a logical and predictable fashion as a measure of temperature stress. One of the mainstays of bioclimatology has been the classification of climate in such a way as to show where humans are “comfortable.” Bailey (1964) devel­ oped an index of temperateness, with those climates experiencing the least vari­ ability in temperatures (tropical highlands) receiving the highest scores. In the United States, Bailey found most southeastern climates to be temperate and ex­ ceeded on his temperateness scale only by areas along the Pacific coast. In his classic work on physiologic climates of the United States, Terjung (1966, p. 1731) concluded that “one can hardly find the comfortable climate. If one looks for tem­ perateness in terms of yearly variations in sensations, then the Pacific littoral comes close to this ideal.” Since variations of climatic elements beyond “comfort­ able” ranges affect physiologic comfort by forcing modifications of the human body’s heat balance (Ayoade, 1978), it follows that climates with minimal tem­ perature stress rank highest on temperateness scales. Even in popular publica­ tions, such as the Places Rated Almanac (Savageau and Boyer, 1993), a large emphasis is placed on temperature stress in determining the climate rankings for cities. Cities with minimal temperature variability and a lack of extremes score highly, as nine of the top-ten-ranked cities are found in California (Savageau and Boyer, 1993). Despite their historical attention in the literature and occasional use in popular publications (e.g., Places RatedAlmanac), indices of temperateness do not have high name recognition or use. Dr. Soule is Associate Professor o f Geography in the Department o f Geography and Planning at Appalachian State University, Boone, NC 28608. VOL. XXXVIII, N o. 2 143 The goals of this study are: (1) to present an easily calculated measure of tem­ perature stress and (2) to demonstrate, through example, how this measure can be used to study spatial and temporal variability in the thermal (temperature only) climate ofa region. The Comfort Day Index (CDI) is derived using practical, region-specific temperature thresholds. Specifically, the base “comfortable” temperatures for op­ erational heating and cooling degree day calculations are used. Heating (HDDs) and cooling degree days (CDDs) are regularly calculated and are the most wellknown measures of cumulative thermal stress. They have practical applications in climatology as the larger the accumulated degree days, the more energy consumed for space heating and cooling (Quayle and Diaz, 1980; Sanderson, 1983; Suckling and Stackhouse, 1983; Downtonetal., 1988; Soule, 1988; Lehman and Warren, 1994). In the United States, HDDs and CDDs have traditionally been computed as de­ viations from a base “comfortable” temperature of 18.3°C (65°F) (Downton et al., 1988). Because of regional variability in the response of humans to climatic con­ ditions, different base temperatures are often used by utilities to calculate degree days for load prediction purposes. For example, base-18.3°C calculated degree days are not the most accurate predictors of electrical demand for heating and cooling purposes in the southeastern United States (Meentemeyer et al., 1990). Oglethorpe Power Corporation of Tucker, Georgia, often uses 12.8°C (55°F) for its HDD and 22.2°C (72°F) for its CDD calculations because there...