The body sweats to cool itself through evaporative cooling. Water
requires heat to go from a liquid phase to a gas phase. In evaporative
cooling the water absorbs this heat from the skin ( and the lungs) thus
cooling the animal. However, there is another factor. It is the ability
of the air to hold more water vapor. This is measured by relative
humidity. Air at 100% humidity cannot absorb more water vapor so
evaporation will not take place - hence the ability of the animal to
cool itself through evaporative cooling is lost. The higher the
humidity the less water that can go from the liquid stage to the gas
stage - hence the less heat removed.
Since the body is still hot - it keeps sweating to cool itself. While
there is some cooling through the lungs (why a lot of horses in the SE
tend to develop the ability to pant) - it is minimal compared to the
heat load a horse produces. Basically the way to look at it is
evaporative cooling loses it efficiency as the humidity goes up to
being totally worthless in very high humidity. The body still keeps
pouring out the sweat since it has an automatic mechanism for cooling.
That's the reason in few words.
Truman
Kristen A Fisher wrote:
I have a question about fluid loss
in humidity. Heat causes
perspiration to cool the body. I didn't think that higher humidity
causes more perspiration, it causes slower evaporation of that
perspiration (making it look like more sweat than if it is arid/not
humid because the sweat then evaporates rather than clings to your
skin). So in effect wouldn't you/horse sweat the same amount at a
certain temperature, but just less efficiently *cool* yourself/your
horse in a higher humidity? I can't figure out why, then, there would
be worse dehydration in more humid conditions. Maybe more overheating
though.
