The weight of water vapor in each pound of dry air expressed in grains of moisture per pound of dry air, or pounds of moisture per pound of dry air. NOTE: 7,000 grains = 1.0 pound.Humidity of air mixtures is normally discussed in terms of grains of moisture per pound rather than the more common term of relative humidity because the grains of moisture in an air stream do not change when it is heated or cooled, unless condensation takes place. (also see: Grain)