Sweat, Page 4

Sweat glistening on the skin, dripping to the ground, or soaking a shirt does no good. For cooling to occur, sweat must change to water vapor. Through this transformation, heat is taken on by the air (the heat of vaporization), and the athlete's heat is reduced.

Dalton's Law of Partial Pressures describes how substances partition between their liquid and gas phases. Hydrogen is a liquid only at very low temperatures, mercury is a liquid at room temperature, silver becomes a liquid at high temperatures. When liquid, each of these substances will also have some molecules in the gas phase. As the temperature of the environment increases, more and more of the liquid will become a gas. At a high enough temperature, all of the liquid will be turned into a gas. At a low enough temperature, virtually all of the gas will be turned into a solid.

Where does water fit into this scheme? Water becomes ice at 32°F and it becomes a gas at 212°F. Between those two temperatures, some molecules of a quantity of water will be liquid and some will be a gas, water vapor. This relationship exactly describes the evaporation of sweat. For any given temperature only a certain amount of water vapor may be present. This quantity defines the saturation vapor pressure. As temperature increases, the amount of water vapor can increase. As the temperature falls, less water can be present.

Maximum amount of water one cubic meter of air can contain at various temperatures:
32°F	60°F	80°F	104°F
0.163 ounces	0.448 ounces	1.03 ounces	1.79 ounces

The amount of water vapor present in the air divided by the maximum amount which could be present is called the relative humidity.

So here's what happens at the runner's skin surface: In response to increasing body temperature, a sweat gland is stimulated to secrete a tiny drop of sweat. This sweat particle is expressed onto the skin surface where it "sees" the environment outside the runner's body. Depending on air temperature and how much water vapor is already present in the air, this tiny drop of sweat will be partitioned between the liquid phase and the gas phase, evaporating. The more of it which goes into the gas phase, the more energy is taken up by the air, and the more the runner's body cools. Now here's the really important part: If the amount of water vapor already present in the air adjacent to the drop of sweat is such that the gas portion of the environment is already filled (i.e., relative humidity is 100%, gas portion of the partition is maximized), no evaporation will occur, and no cooling will take place. The sweat just piles up as glistening, useless pools of water.

Nielsen has plotted this ability of sweat to evaporate at various temperatures and relative humidities in this graph:

As the relative humidity increases, the amount of sweat which will evaporate falls progressively. Clearly, if the amount of heat loss by sweat evaporation needed to maintain thermal balance exceeds that which the environment will permit, the runner's sweat will just glisten on his skin and his core temperature will rise.