Humidity is a measure of the amount of (invisible) water vapour or the degree of dampness in the atmosphere. If the relative humidity and temperatures are high the air feels damp and murky. It is the condensation of this vapour which gives rise to clouds, rain, snow, dew, frost and fog The limit to how much water vapour the air can hold varies with temperature. Warm air can hold more vapour than cold air. When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated. Usually the air is not saturated, containing only a fraction of the possible water vapour.

The amount of vapour in the air can be expressed in a variety of ways. Absolute humidity indicates the actual amount of water vapour present in a sample of air, or the vapour concentration, the mass of water vapour in a given quantity of air. 1kg of air might hold up to 25 grams of water vapour in the tropics, but almost nothing in cold polar regions.

Relative humidity is the ratio of the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature. For example, air at 10°C contains 9.4 g/m3 (grams per cubic metre) of water vapour when saturated. If air at this temperature contains only 4.7 g/m3 of water vapour, then the relative humidity is 50%.

When unsaturated air is cooled, relative humidity increases. Eventually it reaches a temperature at which it is saturated. Relative humidity is 100%. Further cooling leads to condensation of the excess water vapour. The temperature at which condensation sets in is called the dew point. The dew point, and other measures of humidity can be calculated from readings taken by a hygrometer.

Specific humidity is the ratio between the amount of water vapour in air and the total mass of the mixture of air and water vapour.