Water capacity of the air
Relative humidity = --------------------------- x 100
Maximum possible amount of
water in the air
The maximum possible amount of water in the air depends on air temperature. Warm air can hold more water than cold air. If you take a hot shower the warm air is saturated with water. When you open the window the air cools down and cannot hold the water any longer. The bathroom mirror steams up.
A relative humidity of 50 per cent means that the air could contain twice as much water than it contains actually.