By Jason DeRusha

MINNEAPOLIS (WCCO) — It’s not too hard to find the words to describe the Twin Cities’ weather over the past few days. “Sticky,” said one person. “Hot and steamy,” said another. “Clammy,” another said.

It’s humid. But meteorologists rarely talk about relative humidity. Instead, they tend to discuss dew points. Why?

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What’s the difference between dew point and humidity?

They are both different measures of the water content of the air, but humidity has fallen out of favor by scientists because “it hinges on the word relative. It’s so relative,” said WCCO-TV chief meteorologist Chris Shaffer.

Relative humidity is the amount of moisture in the air compared to what the air can “hold” at that temperature, Shaffer said. The problem is, “warmer air can hold more water vapor,” he said, so the relative humidity can give people a skewed sense of the sticky-factor.

Dew point is a more concrete number, he said.

“I can tell you if dew point is 60 degrees you’re gonna feel uncomfortable. If it gets to 70, you’ll be very uncomfortable,” Shaffer said.

Dew point is a temperature at which dew droplets would form. So if the dew point is 75, and the air temperature is 75, you’d see fog or dew.

But look what happens with relative humidity. Assume a constant dew point of 72: when the air temperature is 90, relative humidity is just 55 percent.

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“That seems almost comfortable, but you walk out here and we’re sweating,” Shaffer said.

If it’s 100 and the dew point is 72, relative humidity would be 40 percent.

In the winter, you could have a 40 degree day with a dew point of 38, and relative humidity would be 93 percent.

“But who steps outside and says ‘It’s so humid!’ Nobody, because it’s so cool outside,” Shaffer said.

So meteorologists prefer the consistent message that comes with dew point.

Many people think that moist, humid air is heavier than dry air. In fact, the opposite is true.

The average molecular mass of air (80 percent nitrogen, 20 percent oxygen, a mix or N2 and O2 with molecular mass 28 and 32 respectively) is about 29. The molecular mass of water vapor (H2O) is 18. When water vapor is introduced it displaces some of the air and lowers the overall molecular mass per unit volume. In fact, the difference is so great that clouds (which contain droplets of moisture as well as water vapor) readily stay in the air.

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Liquid water is heavier, or more dense, than air. But liquid water isn’t what makes the air humid, that’s water vapor – a gas that is lighter than nitrogen and oxygen.

Jason DeRusha