Some water in the form of invisible vapour is intermixed with the air throughout the atmosphere. There is a limit to how much water vapour the air can hold and this limit varies with temperature . When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated.
Warm air can hold more vapour than cold air.
In general the air is not saturated, containing only a fraction of the possible water vapour. The amount of vapour in the air can be measured in a number of ways. The humidity of a packet of air is usually denoted by the mass of vapour contained within it, or the pressure that the water vapour exerts. This is the absolute humidity of air. Relative humidity is measured by comparing the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature.