On page 206 mention was made of the part played by perspiration in controlling bodily temperature. We shall now look into this a little more fully. The rate at which moisture evaporates from the skin depends on three factors, namely, the temperature, the amount of water vapour present in the atmosphere and the rate of movement of air over the skin.
On occasions when the air is still and its vapour content is near to saturation the rate of evaporation from the body slows up. Under these conditions even moderate exercise induces profuse perspiration, and the skin remains moist or clammy. The
atmosphere is described as being close or humid. Matters can be improved if the rate of evaporation is increased by setting the air in motion. Where there is no natural breeze an artificial one can be created by fans. This is often done in rooms where humid conditions frequently occur. At other times, when the air is comparatively dry, evaporation from the skin takes place more easily, and faster cooling of the body ensues. Thus, in hot continental climates, where the atmosphere tends to be dry, high temperatures can be tolerated more comfortably than in tropical regions, where heavy rainfall leads to nearsaturation conditions.
From the point of view of personal comfort and for certain industrial purposes to be mentioned later, the actual quantity of water vapour present in the air is not so important as its nearness to saturation or relative humidity.
There are a number of different methods available for measuring relative humidity by instruments called hygrometers. We shall describe one of the simpler and more common types in use.