Abstract
The atmosphere consists of a mixture of dry air and water vapor. Air is itself a mixture of several elemental gases, mainly oxygen and nitrogen, but the proportions of these are consistent throughout the atmosphere, and it is convenient to consider air as one gas. If air and water are present together in a confined space, a balance condition will be reached where the air has become saturated with water vapor. If the temperature of the mixture is known, then the pressure of the water vapor will be the pressure of steam at this temperature. If the percentage saturation of an air sample is less than 100, there will be a difference in vapor pressures. As a result, some of the water will evaporate. The latent heat required for this change of state will be drawn from the sensible heat of the water, which will be slightly cooled. This drop in the water temperature provides a temperature difference, and a thermal balance will be reached where the flow of sensible heat from the air to the water provides the latent heat to evaporate a part of it. The effect can be observed and measured by using two similar thermometers, one of which has its bulb enclosed in a wet wick. The drier the air passing over them, the greater will be the rate of evaporation from the wick, and the greater the difference between the two readings.
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