Understanding the modes of heat transfer-conduction, convection, radiation and perhaps evaporation-can help you keep your horse cool and more comfortable this summer. Last month's article discussed three of the four ways that horses lose heat, focusing especially on convection-heat loss that occurs because of air moving over the body of the horse. Now let's discuss the ace-in-the-hole that both people and horses have for staying cool in hot weather-evaporation, heat transfer that occurs as perspiration is evaporated from the skin.

Heat loss by conduction, convection and radiation depends upon the difference in temperature between the animal body temperature and the ambient temperature-the greater the temperature difference, the greater will be the rate of heat loss. So when ambient temperature approaches body temperature, the rate of heat loss slows to the point that very little heat will be removed from the body by any of these means. That's when evaporation becomes important.

The driving force for evaporation is the difference between the moisture content of the air surrounding the animal and the moisture content of air when it is completely saturated. Technically, it is the difference between the actual pressure associated with the water vapor in the air and the pressure associated with water vapor in the air if the air were saturated with water vapor. The pace with which evaporation happens does not depend upon air temperature!

Evaporation is a very effective way transfer heat. The British Thermal Unit (BTU) is used to gauge the rate of heat transfer, a BTU is the amount of heat it takes to raise one pound of water one degree Fahrenheit. But to evaporate a pound of water requires just over 1,000 BTUs! The source of water, in this case, is perspiration from the body of the horse. Fortunately, horses (and humans) have the capacity to regulate rate of perspiration in response to surrounding temperatures and activity level in an effort to maintain a stable body temperature.

Because sweating adds to the water needs of the animal, providing an ample supply of clean drinking water becomes especially important during hot weather.

An animal begins perspiring when the heat generated from within the animal is greater than the amount of heat the animal is able to dissipate to its surroundings. Again, the purpose is to help control body temperature. As long as heat is dissipated by the animal as fast as it is being produced from within, body temperature will remain stable. Sometimes, however, more heat is generated from within than can be dissipated; e.g., when a horse is performing excessive exercise on a very hot day. In that event, body temperature will rise and, sometimes, body core temperature rises to dangerously high levels.

One way to cool a horse in a hurry is to soak the horse with cold water. First off, the cold water comes in direct contact with the skin. Heat is added to the water by conduction accomplishing some cooling-a temperature difference of, say, 95°F skin temperature relative to 55°F water temperature is the driving force. But, more importantly, some of the water will be evaporated from the skin surface, removing over 1000 BTUs for every pound of water that evaporates. Increasing air movement over the horse will make the heat loss from the combination of evaporation and convection occur even faster.

What if you are on a trail ride with only a limited amount of water available and your horse gets too hot? First, remove the saddle from the sweating horse to allow air to move over as much of the horse's skin surface as possible. This will enhance heat loss by convection and evaporation. Then move the horse to a place with both wind and shade and allow the sweat to evaporate. If limited water is available, soak the horse to the skin with just enough water so that it does not drip off and allow the water to evaporate. Then soak the hair again if more water is available.