Heat is transferred to or away from the body through radiation, conduction, convection, and evaporation. Radiation involves the transfer of heat from a body to the environment, and from warm objects in the environment, for example, the sun, to a body. Conduction involves the transfer of heat to solid or liquid media in direct contact with the body. Water immersion or wet clothing in contact with the body conducts significant amounts of heat away from the body. This effect facilitates cooling in a swimming pool on a hot summer day, or may lead to hypothermia despite moderate ambient temperatures on a rainy day. The amount of heat lost through conduction and radiation depends on the temperature gradient between skin and surroundings, cutaneous blood flow, and insulation, such as subcutaneous fat, hair, clothing, or, in lower animals, fur. Convection is the transfer of heat to the air surrounding the body. Wind velocity and ambient air temperature are the major determinants of convective heat loss. Evaporation is the process of vaporization of water, or sweating. Large amounts of heat are dissipated from the skin during this process, resulting in cooling. Ambient temperature, rate of sweating, air velocity, and relative humidity are important factors in determining how much heat is lost through evaporation. On a very humid day, sweat may pour off, rather than evaporate from a person exercising in a hot environment, thereby accomplishing little heat loss. In very warm environments, thermal gradients may be reversed, leading to transfer of heat to the body by radiation, conduction, or convection.

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