Relative Humidity and Dew Points

Weather Blog

Dear Tom,
Relative humidity and dew points are both measures of how much water is in the air, but are reported in different values. Why?
—Ron Parkard, Chicago
Dear Ron,
Both terms are related to the amount of moisture in the air, but they have very different meanings. Air can hold only a certain maximum amount of water vapor at any given temperature. Dew point is that temperature at which air becomes saturated and is used as a measure of how much water vapor is actually present in a sample of air. Relative humidity is a ratio comparing the actual amount of water vapor in the air to the maximum amount it can hold. This ratio is expressed as a percentage of saturation. For instance, if the RH was 50%, it means the water vapor present was at 50% of capacity.

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