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What is relative humidity?

Relative humidity is a ratio, often expressed as a percentage, that shows how close the air is to saturation. It compares the amount of moisture in the air to the maximum amount it can hold at its saturation point. Other measures of humidity include absolute humidity (a straightforward representation of the density of water vapor in the air) and specific humidity (the ratio of the water’s mass to the air’s mass within a given volume of air).