The heat wave responsible for breaking more than 100 heat records across the U.S. since Thursday — including a heat index of 122 in Baltimore — is finally in the process of diminishing. But perhaps the most stunning of all the records challenged in recent days were the humidity levels in the Upper Midwest.
The dew point is a measure of how much moisture is in the air; the higher the number, the more humid it is. On Friday, the dew point in Wisconsin reached virtually unprecedented heights for the U.S. — exceeding even the steamiest of spots like Miami or Houston. The reason, experts say, is a phenomenon called corn sweat.
Late Friday parts of Central Wisconsin, near the Wisconsin River, may have experienced dew point numbers into the middle 80s to near 90, pushing the "feels like" temperatures over 130 degrees.
The reporting stations on the above map are not official NOAA sites, but at least 5 separate sites in this local area reported dew points above 85, lending credibility to the observations.
Dennis Todey, director of the Midwest Climate Hub for USDA, is somewhat skeptical of the highest numbers, citing various inconsistencies in how each station measures moisture and the location and calibration of the sensors. "Some of the more extreme numbers I would question very carefully," he said.
Even so, the region certainly sweltered. You might expect to find record moisture in tropical locations like Miami, but the highest dew points there rarely exceed 80. Instead the most "tropical" climate in the U.S. late last week was more than 1,000 miles north of the tropical belt.
That's because tropical humidity was transported north from the Gulf of Mexico by southerly winds. The hot winds blew over vast fields of corn across the Midwest, absorbing moisture along the way. The result: corn sweat.
It's no coincidence that the highest dew point ever recorded in the U.S., 90 degrees, was also nearby, in Appleton, Wisconsin, in 1995.
Here's how corn sweat works. In an effort to cool down, humans perspire by sweating liquid water, but plants transpire by sweating gaseous water vapor. Corn stalks suck up water from their roots and then release that moisture in the form of water vapor through small pores, called stomata, on the surface of their leaves. The technical term for release of moisture from both the soil and the crops themselves is evapotranspiration.
That lofted vapor moistens the atmosphere spiking dew points near and downwind of the cornfields. Al Marinaro, a weather research analyst for Maxar Technologies, says the corn can have quite an impact. "I'm a local that lives in the area," he said. "Parts of Northern Sauk County [Wisconsin] have very healthy corn, allowing for better evapotranspiration."
In the summer of 2016, the National Weather service in Des Moines, Iowa tweeted that corn can add a staggering amount of water to the atmosphere and boost dew points a full 5 to 10 degrees.
Joe Lauer, a professor and corn agronomist at the University of Wisconsin, says although corn produces the excess humidity, if it were not corn, other crops like soybeans would accomplish the task. "What is different is the irrigation practices used to produce crops, especially in the Midwestern Corn Belt. Irrigation increases the amount of water available to the plant to cool its leaf surfaces," Lauer said.
Late last week, that extra humid air combined with maximum afternoon temperatures well into the 90s, making for an oppressive mix. The impact can be seen in the heat index temperatures. For example, take a temperature of 95 and a dew point of 80; the heat index is a sultry 115. Theoretically, corn sweat can push that dew point as high as 90, making for a heat index of an unbearable 141 degrees, which is literally off the charts.