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.

When water changes state from a liquid to a gas, as it does when it evaporates into the air, the water molecules in the vapour expand. Just as air pressure is directly related to the number of gas molecules per cubic metre of space, so vapour pressure results from the number of water vapour molecules per cubic metre. The greater the moisture vapour content of air, the greater the vapour pressure. Thus vapour pressure is linearly related to absolute humidity and runs along the same axis of the Psychrometric Chart.

Figure 1 - Lines of constant vapour pressure on the Psychrometric Chart.

Vapour pressure directly affects evaporation rate. If the vapour pressure in the air is already very high, it is more difficult for water molecules to break free from a liquid surface and enter the air as vapour. That is why there is very little evaporation in humid environments. The point at which absolutely no more evaporation will occur because the air is already saturated is called, interestingly enough, saturation pressure and coincides with the saturation point.

Measuring Vapour Pressure

Measuring vapour pressure is very difficult without complex laboratory instrumentation. However, it can be relatively easily derived from more measurable properties. The ratio between the weight of water vapour actually present in the air and the weight it can contain when saturated at the same temperature is called the relative humidity of the air. It is usually expressed as a percentage. As the vapour pressures are set by the quantities of vapour in the air, the relative humidity is also given by the ratio between the actual vapour pressure and the saturation vapour pressure at the same temperature. Thus, if the temperature and relative humidity are known, the actual vapour pressure can be calculated from the product of the relative humidity (expressed as a decimal) and the saturation pressure.

These saturation vapour pressures and the corresponding quantities of water in the air can be read directly off the psychrometric chart or from tables.

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