Convection

The exchange of heat with the environment by both conduction and evaporation also depends on the bulk movement of the air around the skin surface, which is referred to as convection. When the skin is surrounded by motionless air, the air molecules form an insulating layer of sizable thickness (L) around the body, which minimizes the heat exchanged by conduction, as shown in Fig. 3. The effect of the still air layer is to act like a second skin and further insulate the body. Air, because it is a gas, is a relatively good insulator and has a low thermal conductivity, kcond. As shown in Fig. 3, the

Layer of " Stagnant Air

Skin Surface o°c

Skin Surface

Skin Surface o°c

Air Currents o°c

Figure 3 The effect of convection on heat loss to the environment. The air adjacent to the skin forms a stagnant layer that serves as insulation. When the air is still, this insulating layer is relatively thick (large L), and the temperature gradient between the surface of the skin (30° C in this example) and the air farther away from the body (0° C) is shallow. When air currents reduce the thickness of the insulating layer of air, the temperature gradient is steep, and conductive heat transfer increases. Convection also increases evaporative heat loss by mixing drier air into the layer next to the skin.

mixing of air by bulk movement such as a breeze markedly reduces the thickness of the insulating layer of air adjacent to the skin. This reduction in L results in a proportionally larger conductive heat exchange, as indicated by Eq. [2]. This effect of convection increases geometrically with an increase in wind velocity. For example, heat loss with a wind velocity of 8 mph is four times greater than that at 4 mph. On the other hand, convection has no effect on heat exchange by radiation because this exchange depends only on the temperature difference between the actual surface of the skin and the surfaces of objects in the environment.

Convection also increases the rate of evaporation from the skin surface. When the air is still, the layer of air adjacent to the skin becomes nearly saturated with water vapor, resulting in a decrease in evaporation. Convection supplies fresh air with a lesser water content and thus enhances evaporation.

Because of the effects of convection on the rate of heat loss by both conduction and evaporation, much more heat is lost to the environment on a cold, windy day than on an equally cold but still day. This effect gives rise to the wind-chill factor used by meteorologists to indicate the equivalent temperature that would produce the same body cooling on a still day.

Clothing acts to reduce heat exchange with the environment because of its low thermal conductivity (kcond) and its thickness (L). The insulating effect of clothing is due both to the properties of the material of which it is composed and to the air that is trapped in the fabric and serves to increase the layer of still air surrounding the body. The insulating properties are reduced when clothing becomes wet because water, with a higher thermal conductivity, replaces the air. The insulating properties are also reduced by wind, depending on how loosely the material is woven.

Clothing has only a limited ability to reduce heat exchange by radiation. Body heat radiates from the skin surface to the clothing and from the clothing to the environment. Radiation from the skin to the clothing can be reduced in clothing designed for extreme environmental conditions, such as in the arctic or outer space, by layering the inside of the clothing with a highly reflective layer such as gold sputtering.

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