Simply, humidity is airborne moisture. Relative humidity is the volume of airborne water at a certain temperature divided by its maximum volume that may be absorbed by air at this temperature. E.g.: at 21°C, 1kg of dry air may absorb up to 15.8g of water.100% relative humidity is reached if, at 21°C, 1kg of air contains 15.8g of water. If, at 21°C, the same volume of air contains 7.9g of water then, after dividing this value by the volume of water that can be absorbed by air until it reaches saturation at this temperature one obtains: 7.9/15.8 = 0.50 (50%). In this case, air’s relative humidity equals 50%. Volume of water that can be absorbed by air varies depending on the temperature - the higher the temperature, the more water can be absorbed. Consequently, if 1kg of dry air at 21°C can absorb up to 15.8g of water, but at -18°C, the same volume of air can absorb as little as 0.92g of moisture. That is why, at 21°C, 1kg of air has relative humidity of 50%, i.e. it can absorb 7.9g of water. When air is cooled down to -18°C, it will reach its saturation value (100% relative humidity) at 9.5°C and it will cause rain- or snow-fall. And vice-versa, if the temperature of 1kg of dry air at -18°C and 100% relative humidity, i.e. 0,92g is increased up to 21°C without adding water, the following result will be obtained: 0.92/15.8 = 0.06% (6% relative humidity). Such air is even more dry than in the Sahara Desert, where average relative humidity equals 12%! Dry air causes a number of problems referred to as the IAQ (Indoor Air Quality).