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John Wheeler, WDAY, Published July 02 2012

Weather Talk: Relative humidity poorly understood by most folks

There is probably no weather element so poorly understood as humidity. Actually, we understand humidity when we feel sticky. But most people do not understand how humidity is quantified.

Humidity is actually “relative humidity,” which means it is relative to temperature. What we quantify as “humidity” is not actually the percentage of water vapor in the atmosphere (absolute humidity) because that number is always so ridiculously low (0.01 percent) so as to be useless to the general public. Relative humidity is the percentage of saturation that is relative to temperature. More water vapor can exist in the air when the temperature is warm than when it is cool. So this is why the relative humidity rises at night when it is cool. There isn’t more moisture in the air. Rather, the air is cooling so its capacity to contain water vapor is decreasing.

This is why we get dew in the morning. Dew point, however, is a way of quantifying humidity that is unrelated to temperature. The higher the dew point, the stickier the air feels.

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