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It’s not just the heat: Crops increase humidity

ST. PAUL -- It's not the heat, it's the corn. Well, it's the humidity, but that is increased by corn and other crops. Americans throughout the central part of the country cannot put all the blame on corn for muggy conditions they are experiencing...

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ST. PAUL -- It's not the heat, it's the corn.

Well, it's the humidity, but that is increased by corn and other crops.

Americans throughout the central part of the country cannot put all the blame on corn for muggy conditions they are experiencing this week, but crops "add an additional boost" to humidity that is making this heat wave worse than some, Minnesota State Climatologist Pete Boulay said Wednesday.

Thursday likely will be the worst in Minnesota this week, with high humidity and temperatures near 90 in the north to almost 100 in the Twin Cities.

Heat advisories and warnings were expected to expire overnight Wednesday in North Dakota and South Dakota, but they continue for the southern two-thirds of Minnesota and all of Iowa through Friday and into the weekend for Wisconsin.

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Humidity drove the discomfort level, known as the heat index, higher than those in the Upper Midwest are used to. But the figures will "definitely not be in record territory," Boulay said.

Twin Cities dew points, a measure of humidity, could reach 75 on Thursday. That is nowhere close to 88 Moorhead experienced five years ago this month, Boulay said.

The 2011 heat wave was extra-bad in Moorhead because a breeze took the air over corn, sugar beets, clover and standing water, producing a 130-degree heat index, Boulay said. Just across the Red River in Fargo, N.D., where those conditions did not exist, the heat index was 111.

A University of Minnesota study, released earlier this year, shows farm crops can increase dew points 5 degrees, which may drive up heat indices that much. The heat index factors in both the temperature and humidity, much like wind chills combine temperature and wind to provide a figure about how cold it is in winter.

Northern Illinois University climatologist David Changnon released a little-publicized study in 2002 showing that modern-day heat waves probably are worse than a century ago because of crops.

Unlike many plants, corn transpires, or sweats, both day and night. Keeping humidity and heat high at night means there is little chance for relief.

"Stand in any cornfield and you can feel the increased humidity," Changnon said.

Corn is planted denser than years ago, Changnon said, so there are more plants sweating in each field.

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Boulay said researchers do not know how far corn-fed humidity travels, but said air moving across Iowa no doubt is raising Minnesotans' discomfort this week.

Iowa and southern Minnesota have received lots of rain in recent days, Boulay said. "Lots of moisture is now sitting on the ground."

The heat wave covers an area from Arkansas north into Minnesota, with some of the highest density cropping in the world.

The heat is sitting over the Upper Midwest because of a large high pressure area in the eastern United States, Boulay said. That forces air up from the south.

"This happens every summer," Boulay said, but often without the high humidity. "If you look at straight air temperatures, there is nothing unusual."

Many Twin Cities organizations are opening public buildings to give residents places to be cool. The National Weather Service warns that people and pets left in vehicles could face dangerous temperatures within minutes.

The Minnesota Pollution Control Agency has issued an air pollution health advisory for portions of eastern Minnesota for 1 a.m. through 9 p.m. Thursday. The affected area includes the Twin Cities Metro north and east to Cambridge, North Branch, Forest Lake and Stillwater.
The agency says that air quality is expected to deteriorate over the next two days. Sunny skies, hot temperatures and light winds will combine to cause an increase in ground-level ozone.
People at risk from ozone pollution include people with respiratory or cardiovascular conditions, the elderly, children and people who participate in outdoor activities requiring extended or heavy exertion.
By 1 p.m. Wednesday, reporting stations from the Twin Cities and south reported heat index readings well into the 100s. Such indices are considered dangerous, the weather service says.


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Related Topics: WEATHERCORNCROPS
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