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Air is a mixture of gases including water vapour. The water vapour exerts a partial pressure, which increases proportionally as the water concentration increases. At any given temperature there is a limiting maximum vapour pressure, called the saturated vapour pressure, beyond which the air cannot hold any more water vapour.
If air at a given temperature, containing water vapour, comes into contact with a cooler surface the air is cooled locally to the temperature of the surface. If the saturated vapour pressure of the cooled air is less than the original vapour pressure, the excess moisture is deposited on the surface as condensation. The surface temperature at which condensation forms is called the dew point.
The ratio of the water vapour pressure at room temperature to the saturated water vapour pressure at that temperature is known as the Relative Humidity. Many domestic activities, such as cooking and washing which liberate water vapour, increase the relative humidity and therefore the potential for condensation.
In all cold and temperate climates, condensation can occur on the inside surface of windows when the external temperature falls significantly below internal temperature and cools the glass below the dew point. The onset of condensation can be controlled either by reducing the humidity, which lowers the dew point - or it can be controlled by raising the inside glass surface temperature, through the use of double glazing which provides an insulating layer of air, or better still Argon gas, between the two panes of glass.
