Heat management

Fabrics of a very open texture will allow micro convection and ventilation to the atmosphere as well as some radiation to pass through, but some of the radiant heat is absorbed by the fabric and warms it and some is reflected and has no effect on the temperature of the fabric. The colour of the fabric also influence light absorption, black fabrics absorb more light radiation than white fabrics and the chemical composition of the dye may also influence the amount of heat absorbed. Thin layers of aluminium foil can also be used to reflect radiant heat.

The ease and speed with which the body temperature is regulated by evaporation under hot conditions or during strenuous exercise depends on the dampness of the skin and the amount of moisture contained in the surrounding atmosphere, i.e. the relative humidity. In cool conditions the skin is kept dry and cool mainly by dry heat transfer: conduction, convection and radiation. As soon as the heat loss by dry heat transfer falls below the heat production level of the body, the other method of temperature control comes into effect, sweating. For it to cause cooling of the skin, and hence the core of the body, the sweat must leave the skin as vapour. The human skin exudes a small amount of insensible perspiration even when inactive in a cool environment. In impermeable clothing, evaporative cooling cannot take place and overheating results. At the cool end of the climatic scale sweat wetted clothing loses up to 70% of its dry thermal insulation. During periods of low activity, chilling and rapid cooling can occur, leading to hypothermia and finally death.