Response of the Wet-Bulb-Globe-Thermometer Heat Stress Index to Selected Land Use Surfaces

RESPONSE OF THE WET-BULB-GLOBE-THERMOMETER HEAT STRESS INDEX TO SELECTED LAND USE SURFACES Richard J. Kopec INTRODUCTION. At one time or another most people are subject to heat stress. Whether they live in the tropics or in hot deserts, or reside in regions of seasonal change where summers are hot and heat waves common, their bodies are exposed to conditions which strain their thermoregulatory systems. Although the human body can withstand high temperatures for short periods of time, during extended exposure the heat-regulating system will deteriorate unless some voluntary action is taken to protect the body against the stress imposed on it by heat. Action to protect the body requires cognizance of the factors involved in producing heat stress. Heat generated by the human body must be transferred to the surrounding air in order to avoid internal heat buildup. Body heat is transferred away from the body by all the natural modes of heat transfer, namely conduction , convection, radiation, and evaporation. The rate at which body heat is removed is dictated by the surrounding environment. High temperatures, high humidities, high radiant heat loads, and low wind velocities are not conducive to rapid removal of body heat and thereby contribute to the creation of stressful conditions. Other factors which contribute to the problem of human response to heat stress include such physical and physiological variables as poor acclimatization , obesity, poor diet, strenuous physical activity, exposure, improper type and amount of clothing, and insufficient water and salt intake. If care is not taken to reduce the strain created by a highstress environment, failure of the thermoregulatory system may result and some form of heat disorder will follow. After years of disregarding the debilitating effects of high thermal conditions upon humans, authorities are now giving such weather phenomena their close attention. Military leaders, administrators in industry, and directors of athletics, for example, whose businesses are associated with vigorous physical exertion, are concerned with stressOr . Kopec is Professor and Chairman, Dept. of Geography, University of North Carolina at Chapel Hill, Chapel Hill, NC 27514. 134Southeastern Geographer ful weather, and they attempt to measure such weather conditions for the purpose of adjusting work loads to safe levels. Measurement of heat stress, however, has proven difficult. In the past 50 years many heat stress indices have been proposed and tested, but none has seemed entirely satisfactory. This was because special considerations often had to be taken into account which exceeded the utility of a particular heat stress index. Yet individuals responsible for deciding the degree of strain imposed by high thermal environments on active people under their charge find that an index, however general, is an invaluable input to decision-making. Of those indices which have proven useful, perhaps the one used more than any other is the wetbulb -globe-thermometer index or the WBGT index. Initiated by the United States Navy Bureau of Medicine and Surgery in 1945, it has since become a common heat stress index for regulating outdoor activities in the United States. The WBGT index is determined by combining the temperature readings of three thermometers and is usually expressed as follows: WBGT = 0.7Twb + 0.2Tg + 0.1Tab where Twb = the naturally ventilated wet-bulb temperature Tg = the Vernon globe temperature Tab = the ambient air temperature. The greatest weight in the formula is given to the naturally-ventilated wet-bulb temperature in recognition of the importance of evaporation in regulating body thermal balance. It was found that the psychrometric or aspirated wet bulb used for meteorological purposes provided biased values when used to determine the body's sensitivity to heat. With wind velocities under seven knots, the naturally-ventilated wet-bulb temperature will usually be higher than the psychrometric wet-bulb temperature. The Vernon globe is a blackened sphere six inches in diameter (Fig. 1). It is used to measure the solar heat load resulting from direct, diffuse, and reflected short wave radiation as well as long wave radiation produced from heated surfaces. The bulb of a thermometer is placed at the center of the globe, and the stem of the thermometer is insulated from the wall of the sphere, allowing the temperature to be read on the outside...