Corn Sweat, Explained: How Cornfields Turn Up the Heat Index

An acre of corn doesn't look like a machine. But by late July, "machine" is close to the right word: a single acre can pour as much as 4,000 gallons of water into the atmosphere in one day, according to the National Weather Service office in Sioux City, Iowa. Multiply that by the millions of acres stretching from Nebraska to Ohio, and the Corn Belt starts to function less like scenery and more like a humidifier the size of several states.

Meteorologists have a nickname for it: corn sweat. It isn't a metaphor so much as a mechanism, and this time of year, it's a real contributor to how dangerous a Midwest heat wave actually feels.

What's happening in the field

The process is called evapotranspiration — water pulled up from the soil through a corn plant's roots, then released into the air through pores in its leaves called stomata. Corn plants are largely made up of water, and especially mid- to late-summer, mature corn crops have a tremendous surface area and large, broad leaves, Geoff Cornish, AccuWeather's assistant chief video meteorologist, told Newsweek. In late July and August when the plant is mature and surface area of leaves increases, there's a great deal of water vapor that's released, especially on a hot day.

Soybeans and other row crops transpire too, but corn's scale and leaf area make it the headline case. It isn't a plant working against farmers — it's ordinary plant physiology colliding with agriculture at industrial scale.

Why it makes heat feel more dangerous

The extra moisture doesn't raise the air temperature so much as it raises the dew point — the number that actually determines how humid, sticky and dangerous the air feels. Ricky Castro, a meteorologist with the National Weather Service's office in Romeoville, Illinois, said the timing tracks the corn cycle almost exactly. It's probably the beginning of where we start to see the effects of being in the Corn Belt, the effects of evapotranspiration, Castro said. It's not the peak season for that, but it does start to ramp up as the crops start to grow at the end of June into July.

Dew points in the 70s are already uncomfortable; forecasters consider anything above 80 oppressive. Castro pointed to a grim benchmark for what that range can mean: a 1995 heat wave that killed more than 500 people in Illinois carried dew points in the upper 70s and lower 80s — the same range corn sweat can produce on its own in a normal Midwest summer.

It rarely acts alone

This year, a wet spring left soil saturated across much of the region, adding its own evaporation on top of what the crops release. Areas of very moist soils or even leftover standing water from during the stretch of heavy rain can result in evaporation into the air that increases the amount of moisture in the air, Castro said. Layer a stalled "heat dome" — a high-pressure system that traps hot air in place for days — over that moisture, and the ingredients for a dangerous stretch of weather are largely homegrown.

Bob Oravec, lead forecaster at the National Weather Service's office in College Park, Maryland, has watched the same pattern play out across the eastern half of the country this summer. The Midwest is famous for high dew points from the vegetation, Oravec told Scientific American, adding that a stagnant weather pattern can keep both the humidity and the risk in place for a week or two at a stretch.

Who actually feels it

The effect isn't distributed evenly. Cropland tends to hold the highest dew points while nearby cities often run hotter air temperatures, and Castro said the combination can catch people off guard. You might actually get high enough dew points to have suburban areas and areas more out in the rural areas seeing higher heat index values just because of the dew points, he said — meaning a farm town on a city's edge can register a more dangerous heat index than the skyline a few miles away, even with a lower reading on the thermometer.

That distinction matters most for people already at risk in extreme heat: outdoor and agricultural workers, older adults, young children and anyone without reliable air conditioning. Humidity blunts the body's main cooling tool, since sweat evaporates more slowly when the surrounding air is already saturated — which is exactly why dew point, not temperature alone, drives heat-illness warnings. It's also the reason a stalled heat dome becomes so much more dangerous once a humid airmass settles under it.

Corn sweat typically peaks in late July and into August, right as the crop reaches full canopy — which means, for most of the Corn Belt, this homegrown humidity machine is only getting started for the season.