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.) (1945) / Chapter Skim
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Solar Heat Load (Blum)
Solar Heat Load (Blum)
Pages 23-36
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From page 23... ...
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 visible spectrum must be absorbed in order that a man may appear to blend into the terrain.
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From page 24... ...
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 provide 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*
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From page 25... ...
water 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.
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From page 26... ...
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.
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From page 27... ...
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, provides 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.
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From page 28... ...
Because of this specific absorption of certain wave lengths the Stefan-Boltzmann law is not directly applicable 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.
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From page 29... ...
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.
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From page 30... ...
(j_) Schultze, W., Die Reflexion und Absorption der menschlichen Haut in Ultraviolett.
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From page 31... ...
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)
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From page 32... ...
. Table III REFLECTION OF VISIBLE AND INFRARED PORTIONS OF SUNLIGHT BY FABRICS Item.
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From page 33... ...
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.
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From page 34... ...
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)
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From page 35... ...
2 . Estimated radiation loss from a horizontal surface to the atmosphere.
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From page 36... ...
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 concerned in work on the clothed or naked human body are small compared to the absolute temperatures of either clothing or body surface and of the ambient air.
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