Understanding dew points and the relative humidity

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Dear Tom,
Why does the WGN (and many others) web site only report the relative humidity? I tend to look at dew point. That tells me so much more.
—Andy E., Chicago

Dear Andy,
You are spot on regarding dew point in relation to relative humidity. Dew points are routinely mentioned on TV weather broadcasts. Relative humidity defines how close the air is to saturation. Dew point temperature offers a means of determining how much water vapor is actually in the air. As air temperature rises, its capacity to hold water vapor increases exponentially. This is why you don’t feel like a wet rag when the air is saturated at 10 degrees, but you do at 70. Saturated air at 10 degrees contains about 1.5 g of water vapor in a Kg of air. At 70, air contains 16.2 g/Kg, or nearly 11 times as much moisture. Increase the dew point to 78 degrees, and the air has nearly 22 g of water vapor.

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