There are four avenues of heat loss: convection, conduction, radiation, and evaporation. If skin temperature is greater than that of the surroundings, the body can lose heat by radiation and conduction. But, if the temperature of the surroundings is greater than that of the skin, the body actually gains heat by radiation and conduction. In such conditions, the only means by which the body can rid itself of heat is by evaporation. So, when the surrounding temperature is higher than the skin temperature, anything that prevents adequate evaporation will cause the internal body temperature to rise.Guyton & Hall (2006), pp.891-892 During sports activities, evaporation becomes the main avenue of heat loss.Wilmore, Jack H., & Costill, David L. (1999). Physiology of sport and exercise (2nd ed). Champaign, Illinois: Human Kinetics. Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss.Guyton, Arthur C. (1976) Textbook of Medical Physiology. (5th ed). Philadelphia: W.B. Saunders
