Humidity is a summer foe that Midwesterners know all too well. Some take it in stride, and some blame corn. Yes, that’s right — corn. More specifically, “corn sweat.”
But what is “corn sweat,” and is it really a chef in the humidity kitchen?
In order to take in carbon dioxide to undergo photosynthesis, corn — and plants in general — have to open small holes in their leaves called stomata. These holes allow the plant to bring in CO2, but at the same time allow water vapor to escape.
This process is vital for plants, especially in high temperatures. According to Jeffrey Andresen, Michigan’s state climatologist, most of the water a plant uses isn’t to help it grow, but rather for temperature regulation.
This temperature-regulation strategy is somewhat similar to humans’ sweat mechanism (hence the name “corn sweat”). When humans sweat, liquid on the skin evaporates due to the heat, taking thermal energy away from the body and cooling it down in the process.
But “corn sweat” is a colloquial term.
“[Agroclimatologists] refer to it as evapotranspiration. It’s classic: My students say ‘corn sweat,’ I say ‘no.’ It’s like ‘soil’ and ‘dirt.’ You don’t say dirt to a soil scientist, and it’s the same with an agroclimatologist,” Andresen, who is also a professor of meteorology and climatology at Michigan State University, said.
The name “corn sweat” can be misleading because evapotranspiration happens in all land plants, not just corn.
Of the 127 million acres of agricultural land in the Midwest, around 75% of that area is used to produce corn and soybeans. The U.S. Department of Agriculture estimates that one acre of corn can release 3,500 to 5,000 gallons of water into the atmosphere in one or two days. For comparison, the average in-ground residential pool in the U.S. holds between 18,000 to 20,000 gallons.
So it seems natural that in places with more corn, the air would be more humid. But it turns out the science is more complex.
“The overall evapotranspiration piece is part of the puzzle, but it’s not the driver of how we feel when we are walking outside,” Maninder Singh, associate professor of cropping systems agronomy at Michigan State University, said.
The quantity of water vapor released does not happen equally across corn’s growing stages. The greatest amount of evapotranspiration tends to be in the second half of July and the first half of August when the corn plant reproduces. Outside of that window, corn isn’t releasing large enough quantities of water to increase the humidity in the surrounding areas.
So what is the real culprit behind those muggy Midwestern summers?
In the summertime, the jet stream moves from south to north and takes the humidity — but also the heat — from the Gulf to the Midwest, according to Illinois State Climatologist Trent Ford.
“Estimates over the last 20 years or so of heat waves in [the] Midwest suggest about anywhere between 65% and 90% of the humidity in the air during heat waves comes from the Gulf of Mexico,” Ford said.
Holding 2.4 million cubic kilometers of water as the ninth largest body of water in the world, the Gulf of Mexico is a plentiful source of water for atmospheric movement north to the Midwest.
High humidity can be more than just uncomfortable — it can be dangerous. Warmer air can hold more moisture than cooler air, which increases the effects of humidity. When the air is so saturated with moisture, sweat can’t evaporate, decreasing its effectiveness in cooling the body, Ford said. With prolonged heat exposure, the body will continue sweating without the cooling benefit and dehydrate more quickly.
“[Heat] is an all-encompassing hazard, and it’s one that we don’t respect as much as we should,” Ford said.
Heat-related mortality rates have increased in the United States, especially since 2016. This trend is expected to increase as the effects of climate change intensify.
Increasing humidity doesn’t just affect humans. Too much heat negatively impacts livestock, causing overheated cows to eat less and produce less milk.
Humidity can also impact broader weather and climate patterns.
According to Andresen, the Midwest is seeing its nighttime temperatures rise in the summer, but not its daytime temperatures. This means that the area isn’t cooling down as much once the sun goes down, but it’s also not increasing as much once the sun comes up.
He attributes this to the large amounts of evapotranspiration happening in the Midwest from the abundance of corn fields.
When there are a lot of water molecules in the air, like those from “corn sweat,” the air absorbs and stores more heat. Daytime temperatures aren’t skyrocketing because the water in the air is increasing the amount of energy required to get hotter. However, the water stores and releases this heat slowly, making it more difficult to cool down at night and causing nighttime temperatures to rise.
The large agricultural landscape of the Midwest can have a moderate cooling effect on temperature and simultaneously make life uncomfortable. But with most of the region’s humidity coming from the Gulf of Mexico, the simple corn plant can’t take all the blame.
“A lot of times I hear those complaints towards [the] end of August,” Singh said. “And corn is actually not at its peak at that time. It’s actually starting to slow down.”
Great article! Loved reading it, will pass it around!