Evaporation is the most common and usually the most easily explained form of heat loss. When the temperature of the air and nearby objects exceeds skin temperature, the body actually gains heat. This gain may be dangerous to the aviator. When the temperature increases to about 82 to 84 degrees Fahrenheit, sweat production increases abruptly to offset the loss of body cooling through radiation, convection, and conduction. By the time the temperature reaches 95 degrees Fahrenheit, sweat evaporation accounts for nearly all heat loss.
Many factors affect the evaporation process.
Availability of drinking water. Relative humidity above 50 percent. Although the body continues to sweat, it loses only a tiny amount of heat. For example, a person can function all day at a temperature of 115 degrees Fahrenheit and a relative humidity of 10 percent if given enough water and salt. If the relative humidity rises to 80 percent at the same temperature, that same person may be incapacitated within 30 minutes.