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Dewpoint vs. Humidity

Dewpoint vs. Humidity

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While we were living in Maine the weather forecasters would always talk about the dew point. It is seldom mentioned here in Utah. I have often wondered what the difference is between dew point and the humidity %.


Ross O.


Great question Ross. You hear a lot about dewpoint in other parts of the country where it plays a very big role. When dewpoints in the summer creep up to the 60 or 70 degree mark in Northern New England it gets uncomfortable and very sticky. In the midwest, dewpoints get high like that and you start to see some serious moisture available for thunderstorms to develop.

Here in Utah, we have very dry air. It's pretty seldom that we have high dewpoints up into the 60's, when we do though, it's not fun here either. Swamp coolers don't work as well and it's very humid outside. The temperature to dewpoint spread is very wide meaning we have very dry air all summer long and actually for most of the year.

The dewpoint is the temperature of air which is needed for condensation or dew (at that particular temperature). If you take a glass of ice water and it develops condensation on the glass surface, the air on the glass has condensted to its dewpoint and created dew. Dewpoint actually measures how much water vapor is in the air.

Relative humidity is a bit difference. Relative humidity is a measure of how much water vapor the air actually could "hold" at a certain temperature. The relative humidity represents how close the air is to saturation. Saturated air will have an RH of 100 %. You need the RH of 100% to have rain form in clouds.

They are both useful tools to understand, you'll hear both on the news but dewpoint probably more regularly than humidity. We don't mention dewpoint as often here in the dry state of Utah as other states where it becomes a huge comfort issue.

Answered by KSL Meteorologist Dina Freedman.

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