What does the term “relative humidity” actually mean?

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Dear Tom,
What does the term “relative humidity” actually mean? What is it relative to?
Anthony Sporlin, Barrington
Dear Anthony,
“Relative” in relative humidity refers to the maximum amount of water vapor that air can hold at a specific temperature. A relative humidity value of 50 percent means the air contains half of the water vapor it could hold at that temperature. The complication is that air’s capacity to contain water vapor increases dramatically as its temperature rises. Thus, relative humidity depends on the air temperature and its moisture content. A better measure of moisture content is the dew point: the temperature the air must be cooled to in order for its relative humidity to be 100 percent. Dew point temperatures depend only on the amount of moisture actually in the air.

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