When the body temperature rises, the sweat glands are stimulated to secrete perspiration. It is nature's way to keep the body cool. During the process of evaporation of sweat, body heat is taken away, thus giving a sense of coolness.
Sweating is therefore the human body's primary cooling mechanism. Because this mechanism uses water, we need to replace lost fluids by drinking more fluids in hot weather. This is especially true after exercising or working in hot weather. When we sweat, our skin and clothing become covered with water. If the atmospheric humidity is low, this water evaporates easily. The heat energy needed to evaporate the water comes from our bodies. So this evaporation cools our bodies.