Why is relative humidity called “relative” humidity? What is it relative to?
The short answer: relative to the greatest amount of water vapor that can be contained in the air at a particular temperature. For example, a relative humidity reading of 50 percent means the water vapor present represents only half the amount that would be present if the air were saturated at that temperature.
Air can”hold” only a specific amount of water vapor at a given temperature. Relative humidity is the ratio, expressed as a percentage, of the current moisture level compared to the maximum amount possible at that temperature. Much more water vapor can be contained in air as temperatures increase — the carrying capacity doubles for approximately every 20-degree increase in temperature.