Is often used in drying applications and is the ratio of the mass of water vapor to the mass of dry air with which it is associated. Vaisala products give this output as grams of water per kilogram of dry air (g/kg).
An absolute measure of the amount of water vapor present in a given amount of dry air. The quantity is measured in grams of water vapor per kilogram of dry air. It is analogous to the relative humidity in that it measures the amount of moisture present in the atmosphere, but it is a more open ended quantity. Very dry (desert-like) air will have values in the 1g/kg range or lower, while very moist tropical air can exceed 20 g/kg at the ground.
The ratio between the weight ( mass) of water vapor (or some other gas) held in the atmosphere compared to the weight of the dry air in a given volume of air. Usually measured in grams water vapor (or gas) per kilogram of dry air.