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The humidity


The air around us is composed with dry air and water vapor: other elements, that are polluting components, are also present but not considered here. Dry air is a mixture of nitrogen (78%), oxigen (21%) and other gases (1%). Water vapour is water that has been transformed into gas, it is colourless, odourless, tasteless and invisible. Unlike the other gases in the atmosphere, it is present in extremely variable quantities, depending on many factors such as pressure, temperature and the nature of the surrounding environment. For example during the day the temperature in the desert reaches very high values up to 50 °C (122 °F), and the humidity drops very low (15 ÷ 20%  R.H.): in a tropical forest, on the contrary, with a very high temperature of 40 °C (104 °F) the humidity is also very high (80 ÷ 90 %R.H.). We could say that the air is greedy for water vapour, absorbing it from any material available, as well as from people, animals and plants: more water there is in the environment and higher is the humidity level. The air, however, cannot absorb water vapour indefinitely: if the amount of water vapour exceeds a given value, called  saturation point, no more water vapour can be absorbed by the air. In this case the exceeding water vapor will start to condensate turning from a gas into liquid water. Examples of this phenomenon are the clouds in the sky, the "smoke" coming out of a saucepan, the steaming up of glass, the breath we see coming from one's mouth on cold days etc... .
In general we can say that with all other conditions being equal, the higher is the temperature the more water vapour the air is capable of retaining. One chilogram (2,2 lb) of air at atmospheric pressure and at a temperature of 0 °C (32 °F) can only retain 3.8 g (0.0084 lb) of vapour, whereas the same quantity of air at 30 °C  (8,6 °F) is capable of storing up to 27.4 g (0.0604 lb) of vapour, i.e. 7.2 times more.

The humidity The humidity