The higher the work rate, the greater the heat production. Consequently, the core temperature of the body must increase during exercise. Core temperature increases of 5 to 6 degrees are common with intense, prolonged exercise. If the body is hotter than the environment it will loose heat to the environment by radiation. In direct sunlight, the body absorbs radiant heat from the sun.
Conduction is the process of heat transfer by direct contact with another surface.
If the skin touches a frozen piece of metal, heat is lost from the skin by conduction. Convection occurs when air or fluid passes over a heated service. When standing in front of a fan which is blowing air at the same temperature as the environment, you loose heat by convection. A great deal of heat is lost to the water by fluid movement across the body surface when swimming. When water evaporates from any surface it changes from a liquid to a gas.