The Mollier or Hx diagram describes the relation between temperature, humidity and energy content of air. It is a very useful tool to better understand the physics of air, and to predict what will happen in a given ventilation situation.

Air contains a certain amount of moisture in an evaporated form. The higher the temperature of the air, the more moisture it can hold. If the air is fully saturated with moisture, it holds the maximum amount of moisture possible. If the moisture content would increase, it can not be evaporated anymore and it will start to rain or mist.

Relative humidity is the relation between the amount of moisture the air actually holds and how much it can hold at that temperature. If the air can hold 10 grams per m3 but actually only holds 5 gram, the relative humidity is 5/10 = 50%.

If the temperature increases, the air can hold more water, and the relative humidity will decrease, although the actual moisture content of the air in grams per m3 will not change. However, as the air can take up more moisture, it will dry litter more easily.

If the temperature drops, less moisture can be held in the same air and the relative humidity will increase. If the temperature drops enough, the air can not hold the water anymore and it will start to rain (condensate). This is called the dewpoint of air, and is dependent on the actual amount of moisture in the air.

Evaporation of water takes a lot of energy, 2.26 kJ per g of water. This is why cooling of air by evaporation is very effective, for instance in a pad cooling system. At the same time, the energy content of the air will go up with increased moisture levels.