Humidity is the proportion of water vapour in the air. There are two kinds of humidity, absolute humidity and relative humidity.

The absolute humidity is a measurement for the maximal possible content of water vapour in the air. It is given in grams per cubic meter (g/m³).

This value depends on the temperature of the air, since warm air can absorb more moisture than cold air can. This is why the maximal air moisture level is higher in the summer than in the winter.

The relative humidity (rF) is the ratio of water vapour content to the maximal possible water vapour content. The relative moisture is given as a percentage. When the air is saturated, the relative humidity is therefore 100%.

The following table and figure show how much water (absolute value) can be absorbed by air at various temperatures.

Example:

At 20°C, the air has a maximal water vapour content of 17.3 g/m³. This means that during the colder times of year in particular, during which there is a major difference between the temperature of the air indoors and outside, water must be actively added to the air. In a room heated to 25°C, the air can absorb 23 g/m³ of water, while 0°C cold air cannot absorb much moisture and only contains 4.84 g/m³ of water. Opening the windows during the colder times of year makes the air inside even dryer, since the cold air from outside hardly contains any water and requires more moisture once it has been heated.