But that's not news to you, even if you've stayed in the air conditioning. You've heard the warnings about taking it easy during the heat of the day and heard the reports about people who are dying because of the heat. Normally, heat stress occurs when a horse overexerts during performance in hot, humid weather. In this kind of weather the horse's ability of cool itself can be overtaxed. One of the major ways a horse cools itself is sweating.
High humidity interferes with evaporation of the sweat.
If the sum of the air temperature and the humidity is 180 or higher, the horse cannot naturally cool its body. At 120, the normal cooling mechanisms are satisfactory to cool the body, unless the horse is fat. Heat is naturally produced in the horse's body by metabolic processes. For example, the digestion of feeds produces heat. In winter, this aids in keeping the horse warm.