Humidity vs. dew point — what's the difference?

A steamy soup of high temperatures, high dew points and rising humidity spread heat warnings across much of Minnesota Wednesday and raised those sticky questions of summer:

Just what is the difference between dew point and humidity? And does it really matter when the air clings to you?

Neither Kathryn Green nor Kendra Kohn, who pulled up a table outside the Capella Building in downtown Minneapolis Wednesday morning, knew the difference for sure. What they did know was that the air was muggy and uncomfortable — what many people generally call humid.

So, for the record:

A dew point is an actual measurement of how much water is in the air. Humidity is the percentage of air that is filled with water.

If the temperature and the dew point are the same, it creates a humidity reading of 100 percent, said Pete Boulay of the Minnesota State Climatology Office.

When the air temperature drops below the dew point, water vapor comes out of the atmosphere, usually in the form of fog or precipitation.

In short, when the temperature goes up, the relative humidity goes down. When temperatures go down, humidity levels go up. Assuming the water vapor content stays the same, of course.

Got it? If not, you're in good company. It's even tough for the weather folks to explain.

"It's one of the most confusing things when the weather gets hot," said Michelle Margraf, a meteorologist with the National Weather Service in Chanhassen.

As air gets warmer, it can hold more water. Dew points in the 50s seem comfortable. As dew points rise into 60s, it begins to feel juicy. Cross over into the 70s and it feels tropical. And in the 80s, well, downright oppressive.

The combination of dew points, humidity and temperature form what's called a heat index, or what it feels like to be outside.

The long and the short of it is, if you want a real judge of how "dry" or "humid" it will feel outside, look at the dew point instead of the humidity level. The higher the dew point, the muggier it will feel.

On Wednesday, the weather service was predicting heat indexes in the 95 to 105-degree range, conditions ripe for heat illnesses such as cramps, strokes and exhaustion.

"Drink plenty of fluids, stay in an air-conditioned room and out of the sun, and check up on relatives and neighbors," the weather service's heat advisory read.

The metro area recorded its highest heat index 52 years ago on July 11, 1966, at 119 degrees. The highest dew point of 82 degrees occurred at 3 p.m. on July 19, 2011.

The state record for the highest heat index is 127 degrees. That was set on July 30, 1999, in Red Wing, when the dew point reached 84 degrees and the temperature hit 97 degrees, the Minnesota State Climatology Office said.

Wednesday marks the 12th time this year the mercury has hit 90 degrees or higher in the metro, the weather service said.

The dangerous heat will subside slightly Thursday and Friday as temperatures slide back to 89 degrees and 85 degrees, respectively. But "it will be feeling downright tropical through this weekend," the weather service said.

More comfortable air will move in by early next week, with northwest winds helping cool temperatures to around 80 degrees by Monday and Tuesday.

For Joe Eichden, another downtown worker, a few muggy days are just fine.

"Enjoy it while it's here," he said. "We don't get a lot of it."

Tim Harlow • 612-673-7768