Relative humidity
Temperature & humidity RH relative air humidity
Relative humidity is the percentage that describes how close air is to saturation with water vapour at its current temperature. Heating or cooling alone can change the value.
What the percentage means
Like other gases, water vapour exerts a pressure. Relative humidity compares this vapour pressure with the saturation vapour pressure at the same temperature. At 50 per cent, it is half the saturation value. This does not mean that half the air is water. Even a very wet day does not go quite that far.
Why warming changes the reading
Saturation vapour pressure is higher at higher temperatures. Air is therefore not a sponge with a fixed capacity. Heating moist air lowers its relative humidity even if no water is removed.
For example, air at 10 °C and 100 per cent relative humidity has only about 53 per cent relative humidity after warming to 20 °C at the same pressure and without changing its water-vapour proportion. Cooling has the opposite effect.
What 100 per cent does not mean
- The air is saturated with water vapour, but rain is not inevitable.
- A cold, foggy day may contain less water vapour than a warm day with 60 per cent humidity.
- Dew point helps compare moisture content; temperature is essential when interpreting a relative-humidity percentage.