High humidity makes evaporative cooling less efficient. High temperature and high humidity combined can lead to serious trouble quickly. Direct bright hot sunshine on a clear day intensifies the problem. Normally horses that are not being worked can tolerate high temperatures and humidity fairly well. Shade from large trees helps.
Flies appear to be more active in hot weather and add to horses' discomfort.
Foals, especially the very young, have very poor thermo-regulating ability. They can overheat simply standing in the very hot sun. Add to this some activity, such as following an upset mare up and down a fence line, and a serious problem can develop quickly. Many barns would benefit from cross ventilation or box fans hung on the stall fronts. Stall fronts need to be as open as possible, with grillwork or metal mesh to increase air movement.