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Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Page 27
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
×
Page 28
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Page 29
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Page 30
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
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Page 31
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
×
Page 32
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
×
Page 33
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
×
Page 34
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
×
Page 35
Suggested Citation:"Solar Heat Load (Blum)." National Research Council. 1945. Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.). Washington, DC: The National Academies Press. doi: 10.17226/18651.
×
Page 36

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24 CLOTHING TEST METHODS for a mass great enough to supply quantities of radiant energy comparable to sun- light. This presents an apparently insurmountable barrier to the simulation of sunlight in the laboratory. The curves R and C in Fig. 1 indicate the spectral sensitivity of, respec- tively, scotoplc vision (rods) and photopic vision (cones). The latter covers the approximate range 0.4|i to Q.fV-. 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 photosensitive in- strument (this includes all "Photometric" measurements) give inaccurate informa- tion as to the. intensity of the ultraviolet or infrared radiation or of total sunlight. In the present problem we are concerned with the heat load contributed by sunlight, which is made up of wave lengths ranging from approximately 0.29H to 2.2H. A certain portion of the radiation falling upon the body is absorbed, and the remainder reflected. If the body were covered with a surface which reflected a large proportion of all these wave lengths, as for example, with aluminum paint, the solar heat load would be reduced to a negligible quantity. Even white clothing would reduce the load, but camouflage requirements limit the amount of sunlight that can be reflected in certain regions of the spectrum, for a part of the visi- ble spectrum must be absorbed in order that a man may appear to blend into the terrain. Thus, any attempt to improve the reflecting power of clothing must be limited by the requirements of camouflage so far as visible wave lengths (0.4n to 0.7H) are concerned. Since photographic reconnaisance using infrared sensitive photographic emulsions must also be considered, similar restrictions are placed on the amount of reflection allowable in the near infrared, to which these emulsions are sensitive. If the more common infrared sensitive emulsions are used, the long wave length limit is about 0.911; if it is possible to use the most sensitive emulsions, this must be extended to 1.2|i. In Table 1 the amounts of solar radiant energy falling on a horizontal surface are shown for various spectral regions for different conditions. RELFECTION OF SUNLIGHT FROM FABRICS Aldrich has recently measured the reflection of sunlight by a number of military fabrics. His measurements, quoted by Wulsin (2_), are reproduced in Table II, together with a few older measurements (2.). Earlier measurements by Coblenz (4.) are in general agreement with these, but refer to only limited por- tions of the spectrum. Aldrich's data include measurements of transmission of sunlight by the fabrics, which in no instance is high. It may be assumed that most of the transmitted radiation is eventually absorbed either at the skin sur- face or by the fabric, so it has been included in the percentage contributing to the heat load in the first column of Table II. There is considerable difference in reflection by the different fabrics. As would be expected, white fabrics reflect more than colored fabrics, but the total reflection need not parallel too closely the apparent darkness to the eye. Aldrich has estimated the per cent of radiation reflected in the "visible" (.3H to .7H) and In the "infrared" (.7H to 2.5H), for the items described in Table II, and these data are reproduced in Table III. In general these fabrics reflect in- frared radiation to a greater extent than visible. This is contrary to a wide- spread, erroneous belief that all substances absorb infrared radiation almost completely. Improvement of the reflecting power of clothing within the limits imposed by military field requirements would depend chiefly upon finding dyes which, while

SOLAR HEAT LOAD 25 presenting appropriate colors to the eye (or contrasts to the photographic emul- sion) compatible with camouflage requirements, permit greatest reflection of the total radiation. This would entail mainly the reflection of infrared radiation. Texture of the fabrics is also of importance, since some will be better diffuse reflectors than others. Th'ese will probably be minor factors, however, and the absorption spectra of the dyes can be regarded as placing the limits of attaina- ble reflection. The absorption spectra of dyes, and hence their reflecting properties, depend upon their chemical constitution. As a rule they do not give sharp spectral cut offs. It would probably be difficult to predict the appropriate- ness of particular dyes without laborious study of their absorption spectra, in- cluding ranges outside the scope of the usual spectrographic equipment. Thus, the selection of dyes to improve the reflecting powers of military fab£±cs would be a difficult task, and the degree of success to be expected is not great. Under the field conditions clothing becomes soiled and this may alter both the total reflection and the reflection in different spectral regions. REFLECTION BY HUMAN SKIN The reflection of sunlight by human skin provides a basis of comparison with the reflection by fabrics. Martin (3_) found 43 per cent reflection pf total sunlight from average blond human skin. Brunet skin showed 35 per cent reflec- tion, and negro skin 16 per cent. The values for white skin are in general agree- ment with those of Adolph (5_) for reflection of total sunlight, and compatible with those of others who have measured the reflection of visible, ultraviolet, and infrared wave lengths (6.,3_,8) • THE SOLAR HEAT LOAD AND ITS RELATIVE IMPORTANCE The total solar heat load, L, impinging upon a man exposed directly to the sun may be divided into three portions, D, the direct radiation which strikes the profile exposed, H, the reflected radiation from the sky, and T, the radiation re- flected from the terrain. Thus, L = D + H + T (l) While a great many data have been collected on the direct and "sky" radiation falling upon a horizontal surface, there is little information available as regards the sunlight reflected from the earth, or the total energy from these three sources which falls upon a solid object such as the human body. The rela- tive importance of the three factors, direct, sky, and earth radiation, varies with the position of the man exposed to them. Hence, integrated measurements of the energy from the three sources by means of a physical instrument such as the Vernon sphere are not directly interpretable in terms of the solar heat load re- ceived by a man exposed to the same conditions. The following estimates in which the human body is treated as though made up of simple geometrical surfaces give an idea of the variations of the solar heat load with various conditions, and pro- vide approximate values for comparison with the metabolic heat load. The direct radiation.—Let us designate as S, the total energy of all wave lengths contained in sunlight (approximately 0.29J* to 2.2[i) falling on unit area of a surface normal to the sun's rays in unit time. Let F represent the fraction of sunlight diffusely reflected by a fabric or by human skin; the portion

26 CLOTHING TEST METHODS of the incident energy absorbed by the clothing or body is then (l-F)2. The direct component of the solar heat load, D, is then D = S(l-F)P (2) Where P is the profile exposed, i.e., the projection of the body shadow in a plane normal to the sun's rays. With the sun directly overhead and the man stand- ing erect, P is equal to about 7 per cent of the body surface or about 0.12 m2 for a man of average body surface, 1.7 m2. For a man lying prone, P is equal to about JO per cent of the body surface or for an average man, .0.51 m2. As the sun moves away from the zenith, P approaches 0.51 m2 for a man facing the sun, ap- proximately as the sine of the zenith angle. At 15° from zenith (one hour) the profile presented should be about 0.15 m, i.e., about the same as when the sun is at zenith. At 60° from zenith (four hours), however, the profile should be about *51 x sin 60° = 0.42 m2. For a man lying prone P decreases as the cosine of the zenith angle, so that when the sun is at 60° the profile presented is only one -half as great as when the sun is at zenith, i.e., 0.255 m2. Direct solar heat loads have been cal- culated on the above basis for 0° and 60° zenith angle, and these appear in Table IV. In all calculations in Table TV the values of solar radiation for 20 mm. wa- ter vapor, which appear in Table I have been used; and the reflection factor F taken as 0.45, the value found by Martin for blond human skin. Rough as these estimates are, they show clearly that the direct solar heat load must vary considerably with the position of the man and the time of day; and that the maximum direct load may be received in one position at one time of day, and in another position at another time. The "sky" radiation. --Direct measurements show that about 15 per cent of the radiation falling on a horizontal surface when the zenith angle is between 0° and 60° is reflected radiation from the sky (see 9> P- 60). The proportion of sky radiation increases rapidly for zenith angles greater than 60°, but between 0° and 60° the sky radiation falling on a horizontal surface of unit area should be equal to approximately ( ' ^ ) S cos z, where z is the zenith angle. The J_ • UU "~ • -L.5 sky radiation falling on a vertical surface will be only one-half that striking a horizontal surface since the former presents itself to only one-half the heavens. This makes it difficult to estimate the amount of sky radiation striking an ir- regular body such as a man. In calculating the heat load we have used one-half the total body surface for both the erect and prone positions, on the assumption that about half the body is presented horizontally to the sky 'when prone and that the greater part of the body surface is presented vertically to the sky when erect. Estimates based on these assumptions are presented in Table IV. 2. •X2.2fi and 'X.29Ii when Sx and FX are, respectively the solar energy, and the reflection for wave length X . Since Sx and FX vary independently with X the numerical value of D in equation (l) depends upon a given set of conditions throughout which the sunlight spectrum and the -reflection spectrum remain unchanged. Since the solar spectrum varies and the reflection spectrum is different for each fabric, such estimates are only approximate; but the error is certainly not greater than errors introduced by other assumptions that must be made in such an analysis.

SOLAR HEAT LOAD 27 The terrain reflection. — Estimation of the heat load reflected from the terrain is yet another problem. It is necessary, first, to know the albedo, A, or fraction of the solar radiation that is diffusely reflected by the terrain.3 A good many determinations of the albedoes of terrains have been made by visual photometry, and hence can only be accepted as approximate values for total sun- light. For our' estimates in Table IV, 25 per cent diffuse reflection has been as- sumed. This value was obtained for a desert sand by Mr. Irving F. Hand (personal communication). Hulburt (10) obtained somewhat higher values fo? beach sands. Coblenz found JO per cent diffuse reflection from the leaves of the tulip tree, but lower values for other foliage. Some high albedos have been obtained for snow, in the ultraviolet and visible, but the infrared is largely absorbed (see 10 ) . For approximate estimates it may be assumed that the terrain is a surface of infinite area, which reflects 25 per cent of the solar radiation falling on it. A horizontal plane facing this surface will receive per unit area that quantity of radiant energy reflected from a similar area of the reflecting surface; whereas a horizontal plane facing upwards will receive none of the reflected radiation. A vertical plane will receive one-half the radiation received by a horizontal plane facing the reflecting terrain. If we assume that in the erect position most of the body surface is ex- posed vertically, we may write T = M(1"F) A <s C03 z + 1.QQ-.15 3 C03 z> . ._ _ where M is the portion of the body surface exposed to the diffusely reflected radiation from the terrain. Assuming that all the surface is exposed vertically the value 1.7 m2 may be assigned to M. Since any part of the body exposed hori- zontally facing the earth's surface will receive twice this much reflected radia- tion from the terrain whereas those that face upward will receive none at all, this assumption seems not too unreasonable. In the prone position, the surface presented to the terrain is relatively small. Assuming that a profile 0.5 m2 is in contact with the ground and another equal profile is presented to the sky, 1.0 m2 of the body surface will receive no appreciable amount of reflected radiation from the terrain. The remainder of the body surface, 0.7 m2 may be regarded as presenting a vertical surface, and hence may be substituted for M in equation (3). In Table IV estimates of the direct, sky, terrain, and total heat loads for the erect and prone positions and for 0° and 60° zenith angle, are presented. Reference to this table indicates that, even though considerable errors may have been introduced in estimating the heat loads from the sky and from the terrain, these factors cannot be neglected in the estimation of the total solar heat load. They also show that these factors may be expected to have very different relative importance under different conditions. This alone throws doubt on the possibility of obtaining satisfactory estimates of the solar heat load by means of experiments in which men are exposed to sunlight out of doors. / X.29H A = S hence the same qualifications apply as for S and F, see footnote (l).

28 CLOTHING TEST METHODS THE RELATIVE IMPORTANCE OF THE SOLAR HEAT LOAD The relative importance of the solar heat load may best be evaluated by comparing it with the heat load of human metabolism. The metabolism 'of a man of average height and weight is about 96 kilocalories per hour when seated and about 265 kilocalories per hour, when marching at 3 miles per hour. For comparative pur- poses the average of all the values for the total solar heat load presented in Table IV may be used. This is roughly 4 kilocalories per minute or 240 kilo- calories per hour. This is 2 to 3 times the resting metabolism, and about equal to the marching metabolism. It would be necessary to evaporate approximately 420 gms. of water per hour to take care of the solar heat load of 240 kilocalories. This is about one-half the water requirement of a man marching in the desert in the middle of the day under average summer conditions (see %j 11). To what extent may this heat load be decreased by choosing clothing with the best reflection characteristics? The values for the heat load calculated in Table IV are based on reflection of 43 per cent. If the reflection were 71 per cent, as measured by Martin for white cloth, the solar heat load would be about one-half or 120 kilocalories per hour. This would seem to be about the best achievable condition, but would not be compatable with military field requirements. On the other hand, if the reflection were 12 per cent, as found for dark flannel suiting, the solar heat load would be increased to about 370 kilocalories per hour. In terms of evaporation of water, this means a difference of about 420 gms. per hour as the range between the best and the worst conditions. Considering the limits entailed by the requirements of camouflage, and the nature of fabrics and dyestuffs, the difference between field uniform fabrics in terms of the sav- ing of water by reflection of the solar heat load would probably be much less than this.4 Reference to Table I will show that if, because of camouflage re- quirements, the saving must be made principally from the longer wave length in- frared,- it could not be very great in any case. EXPERIMENTAL DETERMINATION OF THE EFFECTS OF CLOTHING ON THE SOLAR HEAT LOAD Physiological measurements. It is generally assumed that when the air is relatively dry and the ambient temperature is near that of the body's surface, the amount of water evaporated, as measured by the decrease of body weight, pro- vides a measure of the amount of heat which the body has dissipated within a given time. This is true only when surrounding surfaces and objects are also at the temperature of the body's surface; it does not imply that it is possible accurately to estimate the solar heat load by comparing evaporative losses for men in the sun and in the shade, as has been attempted. In the first place, the establishment of adequate shade for such an experiment is difficult, since reflec- tion of sunlight from the sky and from the terrain, which remain when the direct sunlight ia eliminated, are difficult to evaluate. Furthermore, the use of any object for shading the body introduces another factor, the radiation reernitted by by that object, and there are still other factors which need to be taken into con- sideration. Under conditions in which the ambient air temperature is below body tem- perature, heat is lost by convection and conduction, which thus interfere with estimates of the solar heat load. Convection, provided by wind or simply by body movement, may be a factor even when the ambient temperature is above that of the I*, fhe solar heat Iced may be easily estimated for the types of military clothing described in Table H. Die values of L presented in Table IV need only be multiplied by percentage reflec- tion 143.

SOLAR HEAT LOAD 29 body since it may affect the rate of evaporation on the body surface in the case, of porous clothing. The estimation of these factors is beyond the scope of this paper, but they should be considered in any calculation of the total heat load. Another factor sftldom taken into account is the exchange of radiation of longer wave lengths than those found in sunlight, between the body and its sur- roundings, i.e., the terrain and the atmosphere. To appreciate this phase of the problem let us first consider the exchange between the body and the terrain. For the purpose, the terrain may be assumed to be a diffusely radiating surface of in- finite extent, in which case the same geometry applies as for the reflection of sunlight from the terrain (see p. 8). On the basis of the assumptions made above, a man standing erect would present his body surface vertically and would receive one-half the radiation from the terrain. We may thus treat the problem as the exchange of radiation between two surfaces of area equal to one-half the body sur- face or 0.85 ms. If the air were absolutely dry, these two surfaces might be treated as black body radiators, and the Stefan-Boltzmann law applied. This law states that the exchange of radiation between two auch bodies ia proportional to the fourth power of the difference between their absolute temperatures. The mag- nitude and direction of this heat exchange would depend, upon the temperature of the body surface and that of the terrain. By way of example, if the body surface were at 37°C. and the terrain at 6o°C. the body of a man standing erect should gain 128 calories per hour from the terrain, a sizeable addition to the total heat load. If the terrain were cooler than the body, the latter would lose heat. When water "vapor is present a certain fraction of this radiation will be absorbed by the atmosphere lying between the body and the terrain. Black bodies at the temperatures of the human body and the terrain, emit radiation over a broad range with a maximum at about 10 |i. Water vapor is transparent to a wide spectral band at about this wave length, but strongly absorbs wave lengths on both sides including a large fraction of that radiated by such bodies (12, 13). The other gases of the atmosphere do not absorb'in the spectral region to which water vapor is transparent, with the exception of ozone which is only present in important concentration in the upper layers of the atmosphere. Because of this specific absorption of certain wave lengths the Stefan-Boltzmann law is not directly appli- cable when water vapor is present in the atmosphere, and the estimation of the heat load emitted by the terrain thus involves considerable uncertainty under these conditions. However, since most of the radiation from the terrain which strikes the body comes from relatively near regions, the effect of absorption by water vapor may not be great. It is improbable that the heat load received by such radiation from the terrain can be reduced appreciably by choice of fabrics. Aldrich has made meas- urements of the reflection by those military fabrics listed Table II and III of radiation from a body at'6o°C.; these are presented in Table V. Very little of such radiation is reflected by any of the fabrics. Radiation exchange exclusive of sunlight, between the body and the atmos- phere involves the same factors, but is even more complex. The^transparency of water vapor permits some of the radiation from the body to pass to higher layers of the atmosphere which are cooler than the ambient layers. This is a channel of heat loss usually disregarded. Accurate estimate of this radiant energy loss is difficult, but an idea of its relative magnitude may be gained from an analysis made by Simpson (14, 15) for an entirely different purpose. In considering the heat loss from the earth, this investigator (15) estimates maximum and minimum values for atmospheric transmission taking differences in amount of water vapor into account, and arrives at mean and limiting values for the long wavelength

50 CLOTHING TEST METHODS radiant energy lost to the heavens by a horizontal surface at a given temperature (figure 2). This is generally known as the "nocturnal" radiation because it is usually measured at night. The outgoing radiation measured independently of solar radiation during the day is comparable, and is dependent chiefly on the tem- perature and humidity (16). Measurements by different methods (17, 18) give val- ues falling withing Simpson's estimates. Extrapolating Simpson's mean curve (figure 2) we see that a surface at 37°C may be expected to lose about 2.5 Kilocalories per m2 per minute by this channel. Using the same treatment as for solar radiation reflected from the sky (p.7) a man standing erect would present 1.7 m2 vertically to the heavens and should lose about 128 Kilocalories per hour as long wavelength radiation. This might be considerably higher or lower depending upon the amount of water vapor in the atmosphere. For purely illustrative purposes, a thermodynamic balance sheet has been attempted in Table VI, for a hypothetical set of conditions, namely; sun at zenith, temperature of the terrain 60°C, ambient air relatively dry and at a temperature somewhat above that of the body, the man erect marching at 3 miles per hour. The evaporation factor is based on the loss of 882 gms. of water per hour, an average figure obtained by Adolph et. al. (11) for men walking in the desert. Convection and conduction losses are assumed to be small because the temperature of the am- bient air is near that of the body, but represent an unknown value. The radiation values are those calculated in this paper. The close over all balance obtained is fortuitous, as is the exact balance between radiation from the terrain and to the heavens. Had the ground temperature been taken as 10° lower or the assumption made that the sun had warmed the cloth- ing to a temperature 10° higher than the chosen, the balance would be considerably upset. It should be pointed out that for a man at rest, the long wavelength radia- tion exchange would be more important relative to the metabolism, and it might be interesting to explore other possibilities. However, Table VI shows clearly that a balance is possible with values of these magnitudes, but that none of the vari- ous items estimated therein can be neglected in drawing up a balance .sheet. The evaporation factor tends to adjust itself due to sweating so that the body temperature does not rise excessively. Thus this factor may be expected to vary to compensate when the other factors shift with various conditions. When the magnitude and variability of the other factors are considered, it does not seem surprising that Adolph and his coworkers (^, 11), should have obtained dif- ferent values for evaporative heat loss under the various conditions they explored, nor on the other hand that these values display the general consistency they do. The whole problem of radiant exchange with outdoor surroundings is, thus, quite complex, and cannot be accurately simulated in an enclosed room. Moreover, all these factors render physiological measurements out of doors subject to con- siderable variability, not only insofar as the solar heat load is concerned, but with regard to the heat load as a whole.

SOLAR HEAT LOAD 31 CONCLUSIONS Since the amount of saving of solar heat load to be anticipated by im- provement of the reflecting properties of military uniform fabrics is not great, it would seem wise to concentrate effort on the evaluation of properties of fabrics that can be studied in the laboratory, and which are of importance under all condi- tions of hot environment, namely, their effect on cooling by conduction, convec- tion, and radiation at ordinary temperatures. Where reflecting properties of clothing are to be considered, they should be determined by direct physical measurement. Necessary data are lacking for evaluation of the thermal relationships of ' man with an outdoor environment, some of which lie in a domain that is generally left to the physicist, the meteorologist, or the astronomer. It would seem im- portant to obtain some of these data with the express problems of the environ- mental physiologist in mind, if human climatology is to be properly understood, in relation either to military or civilian problems. ACKNOWLEDGEMENTS Most of the values for reflection of sunlight by military fabrics used in this report were obtained by Dr. L. B. Aldrich of the Smithsonian Institution of Washington at the request of Dr. F. B. Wulsin of the Military Planning Division, Office of the Quartermaster General. Albedo measurements of terrain made by Mr. Irving F. Hand of the U. S. Weather Bureau have also been employed. It is a pleasure to acknowledge the cooperation of these men in placing their material at my disposal. REFERENCES (1) Moon, P., Proposed standard solar-radiation curves for engineering use. J. Franklin Inst. (1940), 230; 583-617. (2) Wulsin, F. R., Responses of man to a hot environment. Report (24-500054), Climatic Research Unit, Research and Development Branch, Military Planning Division, O:Q.M.G. (1943), i-59- (^5) Martin, C. J. , Thermal adjustment of man and animals to external conditions. Lancet, (1930), 219; 673. (_4) Coblenz, W. W., The diffuse reflecting power of various substances. Bull. Bureau of Standards (1913), £: 283-325- (5_) Adolph, E. F., Heat exchanges of man in the desrt. Am. J. Physiol. (1938), 123: 486-499- (6) Schultze, W., Reflexion und Absorption Hautim sichtbarem Spektrum. Strahlentherapie (1926), 22; 38. (j_) Schultze, W., Die Reflexion und Absorption der menschlichen Haut in Ultraviolett. Strahlentherapie (1930), 35: 369. (8>) Hardy, J. D., and Muschenheimer, C., The radiation of heat from the human body IV. The emission, reflection, and transmission of infrared radiation by the human skin. J. Clinical Investigation (1934), 13: 817-831. (£) Laurens, H., The Physiological Effects of Radiant Energy (1933). New York, Chem. Catalogue Co. (10) Hulburt, E. 0., The ultraviolet, visible, and infrared reflectivities of snow, sand, and other substances. J. Optical Soc. America (1928), 17: 23-25. (11) Adolph, E. F., Rahn, H., Gosselin, R. E., Goddard, D. R., Brown, H. H., Kelly, J. J., and Wolf, A. F., Water losses from man in the desert. Interim Report No. 1. To the Committee on Medical Research, concerning Water Metabolism in Desert Troops. (March 20).

CLOTHING TEST METHODS (12) Humphreys, C. E., Physics of the Air, Chapter VI (1940) New York, McGraw- Hill. (13) Kulper, G. P., Stellar temperatures, in Temperature, its measurement and control in science and industry. (l94l), New York, Reinhold Publishing Corp. Some studies in terrestrial radiation. Mem. Roy. Meteoro- 69-95. (ii) (15) Simpson, G. C. logical Soc. (London) (1928) 2: Simpson, G. C. rological Soc, (16) Angstrom, A.: Further studies in terrestrial radiation. Mem. Roy. Meteo- (London) (1928) 3_: 1-26. Measurement and registration of the out-going effective tem- perature radiation. 22B: 1-6. Arkiv. fur Mathematik Astronomi och Fysik. (1929) (17) Dines, W. H., and Dines, L. H. G.: Monthly-mean values of radiation from various parts of the sky at Benson, Oxfordshire. Mem. Roy. Meteorological Soc. (London) (1927) 2: 1-8. (18) Angstrom, A.: A study of radiation of the atmosphere based upon observations of the nocturnal radiation during expeditions to Algeria and to California. Smith. Mis. Call. (I9l5) 6jj: No. 3: 1-159. Table I Zenith angle ENERGY OF SUNLIGHT1 Energy of sunlight Kilocalories per m2 per minute All wave lengths Exclusive of 0.7n to 0.9H to visible (all 0.9H 1.2n except 0.4n to 0.7H) 20° 14.7 8.7 5.0 2.7 a0o 13.2 7.3 3.9 1.9 360° 10.6 5.9 3.3 1.5 1. Estimated from the data of Moon (l) 2. Dry air, 2.8 mm. ozone, 300 duet particles/cm3. 3. 20 mm. H2O, 28 mm. ozone, 300 dust particles/cm3.

SOLAR HEAT LOAD Table II REFLECTION OF TOTAL SUNLIGHT BY VARIOUS FABRICS Item Per cent contribut- ing to the heat load1 Per cent reflected Per cent transmitted Data of Aldrich 1. Shirt, Mock Leno, slightly permeable 55-9 2. Cotton, khakiA-8.2 oz. 43-7 3. Cotton, percale, white 33.2 4. Cotton, percale, O.D. 51.5 5. Cotton, tubular balbrig- gan 37.6 6. Cotton, twill, khaki 48.3 7. Cotton, shirting worsted, 0. D. 61.1 8. Cotton denim, blue 67-4 9. Cotton, herringbone twill 73-7 10. Cotton,' duck #746 92.8 Data of Martin (3_) 11. Cotton shirt, white un- starched, 2 thicknesses 29.0 12. Cotton shirt, khaki 57-0 13. Flannel suiting, dark gray 88.0 14. Dress suit 95.0 44.1 56.3 66.8 48.5 62.4 51.7 38.9 32.6 26.3 07.2 71.0 43.0 12.0 5-0 5.1 0.0 0.5 2.5 3.2 0.2 0.1 0.0 0.1 0.0 1. The transmitted radiation is considered to be absorbed by the akin (see text). Table III REFLECTION OF VISIBLE AND INFRARED PORTIONS OF SUNLIGHT BY FABRICS Item. Data of Aldrich Per cent reflection of sunlight 0.3(i to 0.7n "Infrared" .7n to 2.5n "Visible" 1. 24.1 53.7 2. 27.8 64.5 3. 69-3 60.2 4. 28.8 55.0 5. 62.7 58.3 6. 25.8 58.9 7. 72.1 49.0 8. 12.1 49.0 9. 13.3 30.2 10. 6.6 7.5

CLOTHING TEST METHODS Table IV ESTIMATED SOLAR HEAT LOAD UNDER VARIOUS CONDITIONS Solar Heat Load1 Kilocalories per min. Position of man Zenith angle Direct (D) Sky (H) Terrain2 (T) Total (L) Erect 0° 0.90 1.13 1.88 3.91 60° 2.67 0.45 0.75 3.87 Prone 0° 3.84 1.13 0.78 5.75 60° 1.54 0.45 0.31 2.30 1. Under the following atmospheric conditions, 20 mm. HgO, 2.8 mm. ozone, 300 dust particles per cm3, and assuming that 43 per cent of the total solar radiation is reflected by the body. 2. Albedo of terrain assumed to be 0.25. Table V REFLECTION BY MILITARY FABRICS OF RADIATION FROM A BLACK BODY AT 60°C. Data of Aldrich Item Per cent con- tributing to the heat load1 Per cent transmitted Per cent reflected 1. 87.0 4.6 13.0 2. 90.0 0.6 10.0 3. 74.8 0.6 25.2 4. 75.0 2.4 25.0 5. 90.5 1.5 9.5 6. 88.5 0.0 11.5 7. 90.4 0.0 9.6 8. 90.0 0.0 10.0 9. 81.0 0.0 19.0 10. 90.9 0.0 9.1 1. The transmitted radiation is considered to be absorbed since it will be largely absorbed by the skin.

SOLAR HEAT LOAD 35 Table VI Attempted thermodynamic balance sheet for a man marching at 3 miles per hour; ambient dry air with temperature about 37°C, terrain at 6o°C, and body surface at 37°C. Sun at zenith. Metabolism Total Solar heat load Long Wavelength radiation exchange with terrain Long Wavelength radiation exchange with heavens Evaporation Convection and Conduction Kilocaloriea per hour + 265 + 234 + 128 - 128 - 5061 + 9 Total - 7 + ? (this close apparent balance is fortuitous) 1« Based on average value from Adolph et.al. (ll), 882 gms. of water lose per hour. IS U WAVE LENGTH. ZT Fig. 1 Spectral distribution of sunlight: 0, outside the atmosphere; 1, with the sun at zenith; 2f with the sun at 60° from zenith. Curves 1 and 2 are for 20 mm. H2d, 2.8 mm. ozone, and 300 dust particles per cm3. From the data of Moon (l). Curves K and C indicate, respectively, the spectral sensibility of the human rods, and cones; the ordinate units are arbitrary.

36 CLOTHING TEST METHODS U 3.5- J7 C X z / 2 / E ^ s *' / c X s / ^r (0 X ^r CALORIE M b X > [MAXIMUM m ' X f / ^ ,' ^^^ 2 1 1 < L* W ^ ^MEAN ^,'' >x _l ^s •^ ^9 3' r*c * 1.0- „,'* ^( r*^* •"- MINIMUM e< $0 £' ro 2 )0 29 0 3( )0 31 0 TEMPERATURE, °K Fig. 2 . Estimated radiation loss from a horizontal surface to the atmosphere. From the data of Simpson (15).

TRANSFER OF HEAT TO THE AMBIENT AIR, AND THE THERMAL INSULATION OF THE AMBIENT AIR A. C. Burton Heat is transferred from a warmer surface to the cooler surrounding air by two routes. (a) By Radiation, which, by incontrovertible laws of thermodynamics, is proportional to the difference of the fourth powers of the temperature of the surface and that of the air. (b) By Convection, including a small contribution by conduction. This is known to be proportional, to a close-approximation for small differences of temperature, to the difference of the first powers of the two temperatures. The effect of wind, or air movement, on the transfer of heat convection is known. The magnitude of the effect depends on the size and shape of the surface, but the dependent on wind velocity is the same in each case. In human calorimetry, as in the "Clo determination" we are, however, more concerned with the total heat loss, which is the sum of the losses by (a) and (b) above, rather than on the partition. Long experience in the labora- tory of physicists, engineers and physiologists has shown that the total heat loss is, to the first approximation, proportional to the difference of tempera- ture between surface and ambient air. Two theoretical difficulties arise from this statement, (l) How can the linear proportionality hold for the total heat loss when a considerable portion of the heat loss, namely, that by radiation, follows not a linear but a fourth power law? ' The answer lies in the fact that the differences of temperature con- cerned in work on the clothed or naked human body are small compared to the ab- solute temperatures of either clothing or body surface and of the ambient air. For example, this difference of temperature will rarely exceed 10°C., for if it did the heat loss would so greatly exceed possible heat production, that life could not be maintained. Even at -40°F. (-40°C.), the absolute temperature is 253°K. and a difference of 10°C. is small compared to this. In this case the differential calculus applies, and: (Tj - T|) = 4 T3 (Tj. - T2) (l) The linear proportionality is quite closely approximated. In cases where equilibrium is very far from being reached, the difference of temperature between clothing surface and air may be so great that this approximation no longer holds, but such cases are outside the field of practical interest. (2) Evan though this difficulty is resolved, another arises. Will not the constant of proportionality in the ab'ove .equation for radiation exchange (i.e., 4 H3) be very different at low temperatures from what it is at ordinary room temperatures? How then can a standard coefficient for the total heat loss (for, say, a 5°C. difference of temperature) be used in experiments at widely different temperatures? For example, if the clothing surface temperature be 25°C. (298°K.) in an ambient temperature of 20°C. (293°K.), the difference of the fourth powers of absolute temperature is 516 x 10s, whereas if the clothing surface were at -35°C. (238°K.') at an ambient of -40°C. (233°K.) the difference of fourth powers would be only 262 x 108. The loss of heat by radiation for the 37

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Clothing Test Methods, Edited by L.H. Newburgh (Physiological Tests) and Milton Harris (Physical Tests) of Subcommittee on Clothing of the National Research Council (U.S.A.) Get This Book
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