THE SOLAR HEAT LOAD: ITS RELATIONSHIP TO TOTAL HEAT LOAD AND ITS RELATIVE IMPORTANCE IN THE DESIGN OF CLOTHING 12

Abstract

This study was undertaken to estimate the relative effect of clothing in protecting men exposed out-of-doors from the heat load contributed by sunlight. Under such conditions, sunlight, both direct and reflected, forms a certain portion of the total heat load. This will be referred to herein as the solar heat load. If the influence of clothing on the total heat load is to be analyzed, this factor is best treated as separate from the heat load contributed indirectly by the sun through its influence on the temperature of the ambient air and the terrain. The evaluation of the effect of clothing on the solar heat load by direct experimental methods presents many difficulties. Sunlight cannot be closely simulated in the laboratory and, on the other hand, testing under outdoor conditions presents difficulties because numerous factors cannot be accurately evaluated and controlled. THE SOLAR SPECTRUM In order to view the problem properly, reference must be had to the spectrum of sunlight. Curve 0 in Figure 1 represents the spectral distribution of sunlight outside the earth's atmosphere. The spectral distribution is altered in passage through the atmosphere due to the fact that all wave lengths are not absorbed equally. The atmospheric constituents chiefly responsible for this alteration of the spectrum are ozone, which absorbs the short wave length ultraviolet end of the spectrum, and water vapor which absorbs the long infrared wave lengths. The latter is of particular importance with regard to the present problem. The quantities of both ozone and water vapor in the atmosphere vary at different times 1 The opinions or assertions contained herein are the private ones of the writer, and are not to be construed as official or reflecting the views of the Navy Departnent. 2This paper was originally prepared as a report for the Subcommittee on Clothing of the National Research Council. and places, and the spectral distribution of sunlight is altered accordingly. The other gases in the atmosphere absorb very little within the spectral range of sunlight. The spectral distribution is also altered by scattering by gas molecules, and by dust particles. Curves 1 and 2 in Figure 1 represent sunlight at the earth's surface when certain quantities of ozone (2.8 mm.), water vapor (20 mm.), and dust (300 particles per cm3) are present in the atmosphere. Curve 1 represents the spectrum when these atmospheric conditions pertain and when the sun is directly overhead, while curve 2 represents the spectrum under the same conditions when the sun is 600 from zenith, at which time the rays pass through twice as thick a layer of atmosphere: Considering all these factors, it is obvious that accurate predictions cannot be made about sunlight without direct measurements, or without knowledge of the atmospheric conditions and proper consideration of latitude, season, and time of day, all of which determine the angle of the sun with respect to the zenith. Figure 1 shows that the maximum of the solar spectrum occurs at about 0.48 u. Thermal emission having its maximum at this wave length would be given off by a black body at 6,000° K. (K. = Kel-vin, absolute temperature). Such a temperature is not attainable in the laboratory for a mass great enough to supply quantities of radiant energy comparable to sunlight. This presents an apparently insurmountable barrier to the simulation of sunlight in the laboratory. The curves R and C in Figure 1 indicate the spectral sensitivity of, respectively, scotopic vision (rods) and photopic vision (cones). The latter covers the approximate range 0.4 ,u to 0.7 u. This is generally referred to as the visible spectrum, shorter wave lengths being denoted ultraviolet, and longer wave lengths infrared. Measurements in which the human eye is used as the photo-712