Science of Drying - Relative Humidity

11/16/2015 (Permalink)

Relative humidity is a term used to describe the amount of moisture in the air as compared with the amount of moisture the air would hold at saturation. For health reasons, indoor humidity should not exceed 60% (mold spores begin to grow).

Because heat causes moisture (vapor) molecules to move faster and longer distances, hot air holds moisture because the heat keeps the molecules separated and in a gas. Therefore, higher temperatures increase the rate of evaporation. Cold air can't hold a lot of moisture because the molecules stick together and condense. Since warm air holds more actual moisture than cold air before reaching saturation, the relative humidity changes with temperatures. If we heat the air, the relative humidity would decrease as the temperature increases.

The opposite is true of cooler air. As air cools, the relative humidity increases until the air reaches its point of maximum humidity (100%). The air will not hold any more water vapor and is saturated. If the air is cooled further, the water vapors begin to condense and deposit liquid. At saturation, all drying stops.

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