Water vapour is one of several gaseous constituents of air, the other principal ones being nitrogen, oxygen and carbon dioxide. Each exerts its own partial pressure in proportion to the amount of gas present, the sum of the pressures making up the total or barometric pressure of the air. When there is a difference in concentration of one of these gases between two points, there will be a corresponding difference in partial pressure. This will cause a flow of that particular gas from the point of higher concentration to the lower. When a partial pressure difference exists between two sides of a material, the gas involved will diffuse through the material until the partial pressures of that gas are equalized.
The rate of diffusion will be determined by the partial pressure difference, the length of the flow path, and the permeability to the particular gas involved of the medium through which flow is taking place.
