relative humidity

The amount of water vapor in the air as a percentage of the amount needed for the air to be saturated at that temperature.

Example: Air at 75% relative humidity has dissolved in it seventy-five precent of the water vapor required for the air to be saturated at that temperature (raining).

Prescribed burning is typically not allowed in air whose relative humidity is below 20% because drier conditions make fuels too dry and easy to ignite for adequate fire control.