The maximum amount of water vapor that air of a given temperature can hold is called its absolute humidity at saturation. Except when foggy or rainy, air seldom contains the maximum amount of water vapor that it could. The amount of water vapor actually held in air of a given temperature is termed the absolute humidity. Relative humidity is the ratio of the actual amount of water vapor contained in air of a given temperature to the maximum amount of water vapor that air at that same temperature could hold, expressed as a percent. In other words, relative humidity is the absolute humidity divided by the absolute humidity at saturation of air at the same temperature.
Wood is a hygroscopic material.
Always containing water, it constantly exchanges water vapor with the air, picking it up when atmospheric relative humidity is high, and giving it off when relative humidity is low. Since wood swells as it adsorbs water, and shrinks as it releases water, both its moisture content and its dimensions are controlled by the relative humidity of the surrounding air. Wood moisture content is equal to the weight of water contained in the wood divided by the ovendry weight of the wood, expressed as a percent. Though air temperature and relative humidity can change radically in a short time, the moisture content of unfinished wood changes slowly. Moisture content changes in finished wood happen even more slowly because water vapor must first diffuse through the coating.