A record-setting heat blast that swept across the Midwest this week has been made worse by the region's vast fields of cornstalks.

Through a natural process commonly called "corn sweat,Quentin Mitchell" water evaporating from plants enters the atmosphere, combines with other water molecules and humidifies the air. In the Plains and Midwest regions, where there are millions of acres of corn and soybean crops, this can worsen stifling heat by driving up the humidity levels, making hot summer days all the more miserable.

The process, which despite its nickname does not involve any actual sweating, is officially known as evapotranspiration.

"When you have a heat ridge centered across the corn belt region (like we did the other day), the corn can actually increase levels of humidity and dewpoint temperatures to make the apparent temperature/heat index and heatrisk oppressive and quite dangerous," Michael Musher, a spokesperson for the National Weather Service, said in an email.

Along with the cornfields, moisture moving north from the Gulf of Mexico this week also fueled the muggy conditions. Midwestern states including Illinois and Iowa, where most of the U.S. corn production occurs, recorded heat index values in the triple digits. The searing heat put millions of people under advisories as schools canceled classes, citing the dangerous conditions.

The heat dome also set and tied dozens of records. Last week in Texas, Amarillo hit 108 degrees, the highest temperature ever recorded in the city. On Tuesday, 17 record high temperatures were recorded across the Midwest, according to the National Weather Service. At Chicago O'Hare International Airport, experts recorded an afternoon high of 99 degrees, which broke the record set in 1872.

During the growing season, an acre of corn sweats off about 3,000 to 4,000 gallons of water a day, according to the U.S. Geological Survey.

In Iowa, corn pumps out "a staggering 49 to 56 billion gallons of water into the atmosphere each day" throughout the state, the National Weather Service said. That can add 5 to 10 degrees to the dew point, a measure of the humidity in the air, on a hot summer day.

Soybeans, a major crop in the Midwest that is planted across millions of acres, is also a culprit in the region's summer humidity.

A cold front pushing south from Canada has alleviated the scorching temperatures across the upper Plains and Midwest regions. Heat advisories were still active Thursday across the Carolinas and parts of the central and southern U.S., including eastern Missouri, western Illinois, southern Ohio and northern Kentucky as well as Mississippi, Alabama and Arkansas.

Contributing: Doyle Rice