What is humidity? Humidity is nothing more than airborne water vapour. Relative humidity is the percentage of water in the air at any given temperature (e.g. 50% RH at 20 °C) with respect to the maximum quantity that the air can hold at

that temperature (100% RH at 20°C). The need to humidify the air usually occurs in winter months,

when the outside air is cold and humid, for instance -5°C, 90% RH, and is heated in industrial environments at 20°C with a relative humidity that is lowered to 15% RH. As a result, the air can seem "dry".