Relative humidity is a measure of how much water vapor is in the air. Most people are familiar with the term from the evening news, where high relative humidity can make the difference between enjoying a comfortable afternoon outdoors and sweltering beneath a choking cloud of saturated air.
From a technical standpoint, relative humidity is defined as the ratio of the actual vapor pressure of water vapor to the saturation vapor pressure of water; in other words, how much water is in the air divided by the most water that could possibly be there.
Climate Research Unit, Univ. of East Anglia.
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