Sweating is the primary way your body stays cool when you exercise in the heat. The energy used is taken from the heat of the skin. Thus, evaporation cools the skin.
Environmental conditions can cause two major problems with regard to evaporation.
Humid air will not easily accept more moisture, which means sweat will evaporate slowly. And if humidity is very high, sweat won't evaporate at all. Thus, be especially wary in humid weather, regardless of the temperature. This can cause you to underestimate the amount of fluid you are losing, which can be considerable. But when sweating is extreme and prolonged, and dehydration occurs, the loss of interstitial fluid reaches a critical stage, and another source of fluid must be found.