About air and water

The air which allow us to live is a mixture of invisible and impalpable materials, which we call gas. This air, which constituted what we call our atmosphere, has nevertheless a weight which weighs on each piece of surface that surrounds us with a force of about one kilogramme and which we call atmospheric pressure. It is the weight of all this air that there is from where we are to where the satellites are up there in the sky. In his mixture, there is a fifth of oxygen, if it was aont, the atmosphere would be five times lighter, we said that there is a partial pressure of a fifth of atmosphere, which is about 0.2 kg.
It is it that ensures the combustions which free thermal energy : the heat. There are slow combustions, the one that occurs in the living cells, animal or vegetal, do not forget that the man is an animal, and everywhere around us : rust, verdigris, the silverware, the pewters, the paint which lose their sheen.
There are vivacious combustions which burn the fossil matter : oil, gas, coal, wood, which are actually only concentrated residues of living cells.
The other four fifth of the atmosphere is essentially constituted of nitrogen which is a relatively neutral gas. There is also the residues of combustion : carbon dioxide and carbonique gas which is guilty of the greenhouse effect. There are scarce gas but very useful, which we call rare gas : helium, neon, argon, krypton..., and the gaseous wastes of our activities which constitute the pollution.

And finally, there is the water vapour. Beware that mist is invisible : the white smoke from the nuclear power plants, the mist, the fog, the clouds, what you see above you saucepan of boiling water, this is not water vapour, this is very tiny drpos of water in suspension in the hot aire, which comes from the water vapour which escapes from the hot water, and which is already recondensed to water.
The quantity, the weight, of water vapour that can be in the air, that is to say the partial pressure, is very variable and also depends on the temperature of the air.
At a given temperature, say 20°C, the air can contains up to a certain maximum of water vapour, and then the supplement of water vapour recondenses immediatly into water, into very tiny drops, which are visible. This maximum partial pressure which water vapour can reach without recondensing is called the partial saturation pressure. We will say that the hygrometric degree of that air is 100%.
This partial saturation pressure decreases when the temperature of the air decreases, and increases when the temperature increases. Thus, if this air is at 20°c saturated, heats when passing through a radiator at 45°C, it is no more saturated without the amount of water vapour that it contains being changed. Its hygrometric degree will be for instance down to 50%, which means that it can contain twice more water vapour before being saturated, which means that it became dry and can again absorb water, this means also that it became less conductor of heat. The hygrometric degree of the air is the ratio between the amount of water vapour that it contains and the saturation amount.

60% is the hygrometric degree of comfort which gives to the air a suitable conductivity to absorb enought caloris and not too much, because the conductivity of the air depends on the amount of water that it contains, it is actually the water that absorbs the calories. Beyond 60%, the air is too humid, it cools too much, and below it is too dry and not enough conductive.

Let's take now the air of the kitchen at 25°C and 80% of humidity and let's cool it down for instance by passing it throught the interchange of the VMC, at a certain temperature, let's admit 18°C, it will reach its point of saturation and a part of the water vapour will recondense. We call this point : point of dew, like the dew. If now this air is only at 60% humidity, will the dew point be the same ? No, because due to the fact taht it does not containt the same amount of water vapour, the saturation temperature will be lower.

If the temperature is below 0°C, indeed very negative, the saturation pressure is very weak, the air is very dry, that is what happens in high mountain.

If some air comes close to a cold surface, locally it will cools down and reach its dew point, and some vapour will recondense on the surface and form water. But for that reason the amount of vapour of the air in the room will decrease and the air wille become drier. That is what we call the principle of the cold surface : the hygrometric degree of a room is established by the dew point of the coldest surface. It is the ratio between the saturation pressure at the temperature of the coldes surface and the saturation pressure at the temperature of the ambient air.
Furthermore the condensation yield energy which is absorbed by the surface, and get lost outside, when the surface is a window or a thermal bridge.

The inverse phenomenon, the evaporation absorbs energy, that is why you have to heat water to produce vapour, for instance to make the turbines which produces electricity spin, be in the power plants said "thermal" or in the nuclear power plant that are also thermal.

In a way summarize and general : air transports water, and water transports the calories. Vaporizing water absorbs calories, and restitutes it when recondensing, it is the coolant cycle.