Evapotranspiration Increases Humidity, Making Heat Waves Slightly Worse: But ‘Corn Sweating’ is a Misnomer Since It is a Minor Contributor

     The phenomenon of ‘corn sweating’, increasing humidity in June and July in the Northern Hemisphere, is well-known. The process is simply plant transpiration or evapotranspiration (ET). One might think of it as plants breathing, except they expire oxygen. This is how plants absorb nutrients and other substances as well. According to an article in Ag Daily, the process is as follows:

The process happens in three main steps:

1) Plants absorb water from the soil through their roots.

2) Water is transported through plant tissues, where it plays a role in metabolic and physiological processes.

3) Leaves release water vapor into the air through their stomata.

     Evapotranspiration is a vital part of the Earth’s hydrological cycle. It involves the movement of water from plants to the atmosphere through the processes of transpiration and evaporation. It is estimated that 60-75% of land precipitation is returned to the atmosphere via evapotranspiration. Three factors influence ET rates: available moisture, heat energy, and humidity (ability of the atmosphere to take up water).

     The corn and other plants are not exactly sweating, but the added atmospheric moisture makes us sweat more. Evapotranspiration is similar to sweating in that the plants release vapor through their stomata, like humans excrete liquid water through their pores.

     All plants and grasses transpire. Corn is actually a minor contributor overall. Among farm crops, soybeans are also a contributor. The main influence on summer humidity in the Midwest is warm, moist air coming from the Gulf of Mexico and other waterways. Some research suggests that soybeans contribute more than corn. Thus, the term ‘corn sweating’ is more of a misnomer. Corn farmers are not the main cause of humidity increases, as seemingly suggested in some articles.

     Satellite remote sensing is often used to estimate evapotranspiration rates. Plant sweating usually indicates healthy plants since it reflects water availability. When there is drought, the stomata close up somewhat to preserve water, which also can stunt growth. Thus, plant sweating should be seen as a good thing. One direct way to measure evapotranspiration is via a pan lysimeter, as shown below. There are other indirect ways to measure it through water and energy balance equations, and often satellite-based remote sensing models.

     In places where a lot of corn and soybeans are grown, such as Iowa, where 24 million acres of corn and soybeans are grown, they are larger contributors.

     An article in National Geographic notes that evapotranspiration rates are affected by the amount of plant cover and are influenced by deforestation and reforestation. Vegetation is more dense in forests. Thus, ET can change over time in an area. As forests continue to grow in places like North America, the overall rates of evapotranspiration increase.

References:

 

‘Corn sweat’—and other weird weather phenomena—explained. The Midwest’s massive corn fields are making the region more humid as it battles a sweltering heatwave. Kieran Mulvaney. National Geographic. July 23, 2025. ‘Corn sweat’—and other weird weather phenomena—explained | National Geographic

What is corn sweat? Understanding healthy plants and humidity. Braeden Coon. Ag Daily. June 21, 2023. What is corn sweat? Understanding healthy plants, humidity | AGDAILY

Evapotranspiration. Wikipedia. Evapotranspiration - Wikipedia