Portions of the Midwest, including Southeastern Wisconsin, have been stalked by “corn sweat” in recent days, which has contributed to the extreme heat and oppressive humidity.
The National Weather Service in Milwaukee issued an excessive heat warning and heat advisory for most of the area for Monday, with heat index values rising well above 100 degrees, actual air temperatures in the 90s and dew points climbing to the upper 70s, putting Wisconsin on par with Florida when it comes to sticky weather conditions.
Excessive Heat Warnings and Heat Advisories are in effect for most of the area for today, with heat index values expected to be between 100 and 108 this afternoon (actual air temperatures in the 90s). pic.twitter.com/WgteRJHhbS
— NWS Milwaukee (@NWSMilwaukee) August 26, 2024
It’s time to pick your Milwaukee favorites for the year!
The forecast called for much of the same on Tuesday, along with chance of severe storms.
Dangerous heat is expected again today, with gradually increasing chances for thunderstorms. One or two broken lines of thunderstorms may pass through the region this afternoon / evening, with a threat for damaging wind gusts, frequent lightning, and possibly hail. #wiwx pic.twitter.com/GKrk61KchM
— NWS Milwaukee (@NWSMilwaukee) August 27, 2024
The National Weather Service doesn’t formally include the term “corn sweat” in its glossary, but Michael Musher, a public affairs specialist and meteorologist at the NWS headquarters in Silver Spring, Maryland, described the current issue as being due to a heat ridge centered across the corn belt region, which includes Wisconsin.
“The corn can actually increase levels of humidity and dewpoint temperatures to make the temperature and heat index and heat risk oppressive and quite dangerous,” Musher said.
Brooke Brighton, meteorologist at Spectrum News 1 – Wisconsin, described corn sweat, or transpiration, as the process in which water evaporates from the leaves of corn stalks.
“When this moisture is released from the corn, it can lead to very localized increases in humidity and heat index temperatures,” Brighton said. “If you happen to walk past an area with cornfields, odds are it’s going to feel much more humid in that area due to the corn transpiration taking place. Since the Midwest has some of the highest population of cornfields in the country, more locations will experience this effect. For the most part, however, this process is hyper-localized, mainly impacting rural areas with farmland.”
Mark Baden, WISN 12’s chief meteorologist, described the term corn sweat as the “dumbed down” term for transpiration.
“All plants transpire moisture, corn just happens to be very prolific, especially mature corn,” Baden said. “Transpiration pushes out a gallon or more of moisture a day out of each corn stalk. That helps to increase the low-level moisture, especially when we have scenarios like this week with very little wind. Then, you can get a bubble of really high dew points that are enhanced by corn sweat.”
Corn sweat has contributed to the onerous weather conditions that first cropped up in the Milwaukee area on Sunday.
“Corn sweat is certainly much more prevalent in Iowa,” said Baden, who spent the first three years of his career at a television station that served the Des Moines market. “When I worked there years ago, we would actually get dew points into the 80s, which is just horrendous. The highest dew point I can remember was in Cedar Rapids, where it reached 85. There are very few places on earth that get dew points like that. That was all about being next to a huge number of cornfields. We don’t have as much corn in Wisconsin as they do in other spots, especially in Iowa, but part of what’s been going on here you can certainly attribute to corn sweat.”
According to the National Weather Service, 1 acre of corn can give off about 3,000 to 4,000 gallons of water to the atmosphere each day through evapotranspiration, the process by which water is transferred from the land to the atmosphere by water leaving the soil (evaporation) and water lost through plant leaves and stems (transpiration). This can increase dew points, making it feel more humid.
Monday’s 94-degree high temperature matched the highest mark of the year (reached also on June 17 and 18) and was just shy of the record-high for the date that was set at 96 degrees in 1953, according to Brighton.
“I don’t think we’ll be breaking any records on Tuesday, as the daily record high was set at 99 degrees back in 1973,” she said. “But it surely will be another hot and humid day with a forecasted high of 94, dew points in the mid-70s and heat indices in the triple digits.”
The dew point reached 75 in Milwaukee on Monday, which when combined with the 94-degree high temperature for the day, created a 104-degree heat index, Baden said. Temperatures in Southeastern Wisconsin can vary greatly in August, he noted.
“Go back five or six days and you remember how comfortable it was,” Baden said. “That’s what we can get here in Wisconsin, especially later in August. You get these cooler air masses that come in but you can also get the stagnant air masses where you get the big area of high pressure that parks itself to our south and that allows for the southerly wind flow, which can be fairly benign and light. That’s where you can get the corn sweat and the increase in the dew points. Last year, at about the exact same time in August, we had a very similar scenario and an excessive heat warning.”
At times of excess heat warnings, precautions must be taken, Baden warned.
“People don’t take heat seriously enough,” he said. “If you’re very healthy and you exercise regularly, this won’t affect you as much as an elderly person or a person who takes certain medications where their body doesn’t cool off as easily. The reason high dew points are rough on the body is because we sweat to cool off. It’s known as evaporative cooling. But if you have high dew points, your body still sweats but the evaporation rates are much different than when we have dry air.”
Baden also reminded residents to drink plenty of water and avoid alcoholic beverages, especially when outdoors. “And don’t forget about your pets in this weather. Just don’t let them out,” he said.
