Under what condition can a horse effectively cool itself by radiating heat into the air?

A horse can effectively cool itself by radiating heat into the air when the air temperature is cooler than its body temperature. This is because radiation is one of the ways in which heat is exchanged between the horse and its environment. When the surrounding air is at a lower temperature than the horse's body, the heat naturally flows from the horse to the cooler air, promoting a cooling effect.

This principle is fundamental to understanding how horses manage their body temperature, particularly in warm climates or after exercise. When the air temperature is higher or equal to the horse's body temperature, radiative cooling is less effective, as heat would not effectively move from the horse to the warmer air. Additionally, using methods like cold water or being indoors can aid in cooling, but they do not rely on the principle of radiative heat exchange, which is specifically addressed in this question.