RELATIVE HUMIDITY is the ratio of the weight of water vapor in a sample of air to the weight of water vapor the same sample of air would contain if saturated at the existing temperature. This ratio is usually stated as a percent age. For example, when air is fully saturated, the relative humidity is 100 percent. When air contains no moisture at all, its relative humidity is 0 percent. If air is half-saturated, the relative humidity is 50 percent. The normal comfort range of relative humidity for humans is between 30 percent and 70 percent. The deciding factor in human comfort is the relative humidity- not the absolute or specific humidity.

Just as heat flows from a higher temperature to a lower temperature, moisture always travels from areas of greater wetness to areas of lesser wetness. If the air above a liquid is saturated, the two are in balance and no moisture can travel from the liquid to the air; that is, the liquid can-not evaporate. If the air is only partially saturated, some moisture can travel to the air; that is, some evaporation can take place.

When the temperature of the air is 76F and the relative humidity is nearly 90 percent, the air is nearly saturated. At such a relative humidity, the body may perspire freely but the perspiration does not evaporate rapidly; thus a general feel-ing of discomfort results.

However, when the temperature of the same air is 86F, the relative humidity would then be only 64 percent. Although the absolute amount of moisture in the air is the same, the relative humidity is lower, because at 86F the air is capable of holding more water vapor than it can hold at 76F. The body is now able to evaporate its excess moisture and the general feeling is much more agreeable, even though the temperature of the air is 10 higher. (The cooling effect on the body is brought about by the absorption of latent heat during the evaporating process.) In both examples, the specific humidity is the same, but the ability of the air to evaporate liquid is quite different at the two temperatures. The ability to evaporate moisture is directly indicated by the relative humidity. This is the reason the control of relative humidity is of extreme importance in air conditioning.