Relates to the concentration of water vapour in the atmosphere.
Two factors control: temperature of air, and availability of water.
Mass of water vapour per unit volume of air is known as absolute humidity (expressed in grams per cubic metre)
Saturated air refers to maximum amount of vapour at a given temperature.
Relative humdity refers to total amount of water vapour present in a mass of air, expressed as a percentage of the total amount of water which would be held if the air was saturated at that temperature.
Condensation occurs when warm moist air cools and reaches the dew or condensation point. Water vapour condenses. Air can be superaturated: it can hold water vapour below the condensation point, and have a relative humidity of over 100%. Condensation relies on presence of hygroscopic nuclei.