The amount of water vapour in the air varies from place to place and from time to time, and is an important factor in determining the nature of the weather. The amount of water vapour that the air can hold in an invisible state is limited and varies with the temperature.

Warm air can hold more water vapour than cold air. When the air contains as much water vapour as it can hold at a given temperature, it is said to be saturated. If the temperature is lowered further, or more water vapour is added to the air, the surplus condenses into minute but visible drops of water, which constitute cloud, fog or mist.

Humidity is measured with a hygrometer. The smaller the difference between wet and dry bulb temperatures, the more humid the air.