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Good Question: What Is The Dew Point?

MINNEAPOLIS (WCCO) -- You have heard the saying a million times: It's not the heat, it's the humidity. That will certainly be the case Friday, when dew points are expected to be in the 70s.

So, what is dew point?

Humidity is moisture in the air and there are two ways to measure it. One is dew point and another is relative humidity.

"Dew point is the temperature at which the air needs to cool to reach saturation," WCCO Meteorologist Matt Brickman said.

Dew point is a concrete number and is the amount of moisture in the air. Relative humidity is a percentage and measures how close the air is to saturation.

The dew point is also the temperature the air must cool to for dew to form.

Dew points do no change with air temperature, but relative humidity does, so meteorologists prefer using dew point to demonstrate how muggy it will be outside.

Dew points under 55 are considered pleasant. Dew points in the low 60s are starting to get uncomfortable. By the time the dew point is in the low 70s, it is oppressive outside.

The higher the dew -- the more moisture in the air.

As for why we are so uncomfortable on humid days, it has to do with how our bodies cool down.

"When we've got moisture on our skin on a dry day, our sweat can evaporate quickly," Brickman said. "When the humidity is that high, it's difficult for that moisture to find somewhere to go, so it hangs on to us."

Why is it more humid here than out west? Brickman says one reason is we have a ton of water, and on very hot days some of that lake water is evaporating into the air.

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