General term for the presence of water vapor in the air, commonly expressed as the amount of moisture in the air. Control of humidity is important in the printing process, and is a vital concern in the papermaking process. Because of paper's hygroscopic properties, it adds or loses moisture readily, depending on both its moisture content and the moisture content of the surrounding air. (See Dimensional Stability, Moisture Content.)
Humidity can be measured in terms of specific humidity (the ratio of the mass of the moisture vapor in the air to the total mass of air and water vapor), absolute humidity (the mass of the water vapor present in the atmosphere), or relative humidity (the amount of water vapor in the air expressed as a percentage of the total amount of water vapor the air could hold at that particular temperature). A related term is dew point, the temperature at which the air needs to be cooled for it to become saturated and for dew to form. Relative humidity is the most important measurement of the humidity as it relates to paper and its properties. (See Relative Humidity.)