What Is Relative Humidity?

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By By Steve Pool

SEATTLE - Relative Humidity is generally used as an indicator of how humid it is outside. But the measurement, given as a percentage, is actually a measurement of the ratio between water evaporating from liquid water to water vapor and condensating from water vapor to liquid water at any given time.

A relative humidity of 50% means that twice as much air is evaporating at any given time, than going from air vapor to liquid water. As the humidity drops, the evaporating increases, and vice versa. If the humidity is 100%, you have saturated air -- equal evaporating and condensation, at which point a body of water won't evaporate. Thus, low humidities signal dry air, and vice versa.

If you have more condensation than evaporation, you have a humidity of higher than 100% (called super-saturation), which is how clouds form.

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