# Physical principles of the measurement parameter humidity

Atmospheric air always contains a certain quantity of water vapour. This content deviates with time and its location and is called air humidity (humidity). Like every other substance, air also has only a limited capacity to absorb water. This limit is referred to as saturation. Below the saturation level, humid air is indistinguishable from dry air with the naked eye. Above the saturation limit, the excess water is visible in the form of fine water droplets (condensation) - a mist forms.

#### Absorbtion capacity of humidity

The capacity of air to absorb humidity is dependent on the air temperature. The warmer the air, the more water it can absorb.

We can imagine a sponge which changes its absorbtion capacity depending on the temperature. At 0 °C, the sponge can absorb 4.8 g of water (corresponds to 100 % humidity). At more than 4.8 g, the sponge begins to drip. If the sponge has stored 2.4 g of water, this corresponds to 50 % relative humidity at 0 °C.

At 20 °C, the sponge can store 17.3 g of water (so the sponge has 100 % relative humidity at 17.3 g of water). Now, if the sponge has stored 2.4 g of water at 20 °C, this corresponds to approx. 14 % relative humidity.

If the temperature drops, the relative humidity increases (at the same water content)!

## Humidity parameters

#### Absolute air humidity

The absolute humidity (fabs) is calculated by dividing the weight of the water contained in the air by the volume of the air quantity in question. The absolute humidity is usually given in g/m³.

#### Relative air humidity

The relative humidity describes the ratio of the current actual moisture contained in the air to the maximum possible absolute humidity. Relative humidity is given in percent

### Max. air humidity and dewpoint temperature

#### Maximum air humidity

Is the maximum possible absolute air humidity at a certain temperature (fmax). It is reached when the water vapour partial pressure in the air is as great as the saturation vapour pressure of the water at the corresponding temperature. In this state, the relative humidity is 100 %.

If the maximum humidity is exceeded, the excess water vapour precipitates in the form of condensation (droplet formation). The saturation humidity is an absolute humidity and is given in g/m³.

#### Dewpoint temperature

The dewpoint temperature is defined as the temperature at which the current water vapour content in the air (100 % air humidity ) is the at maximum possible level. This means the water vapour pressure is the same as the saturation vapour pressure. The dewpoint temperature is therefore a parameter which is dependent on the current temperature.

The dewpoint temperature can be determined from the ambient temperature and the relative humidity, and the relative humidity from the ambient and the dewpoint temperatures. Additionally, the absolute humidity of the air can also be calculated from this.

#### Info

In nature, condensing water vapour precipitates as dew on the surfaces of solid objects. Humans feel most comfortable during low to medium activity in a range from approx. 30 % to 65 % relative air humidity.