Corn Sweat and Humidity: A Few Facts Explained

By Aaron B. Wilson and Alex Lindsey et.al

Every summer, as conditions heat up and humidity levels rise during mid-summer, we hear the phrase “corn sweat.” What is corn sweat and how much does it contribute to the oppressive heat in the summer? As you can imagine, this is a complex question to answer.

First, “corn sweat” refers to the natural process by which plants, in this case corn, move water through the plant to its surface, which then allows this water to evaporate into the atmosphere. For corn, this transition of water from plant to atmosphere typically occurs through the stomata in leaves as well as other surfaces of the corn. Also, when thinking of how much water is associated with a crop like corn, researchers consider two components: water losses from the soil (evaporation) and water losses from the crop (transpiration). Scientifically, we call this combined process evapotranspiration (ET). Some research conducted by USDA Agricultural Research Service suggests that corn can contribute between 3,500 and 5,000 gallons of water per acre to the atmosphere over the course of one to two days. A typical pool contains 18,000 - 20,000 gallons.

There are many factors at play when thinking of how much water in the atmosphere is tied to plants. Research conducted by the Illinois State Climatologist, Trent Ford, showed that local ET contribution plays a role in low level humidity, especially on extremely hot days[1]. However, when compared to moisture brought into the region, typically due to southerly or westerly winds (e.g., from the Gulf of Mexico), this local ET is order of magnitude smaller than the contribution from the large-scale atmospheric flow. This suggests that while the corn contribution to low level humidity is a factor, it may not be the most abundant source. Indeed, we need a more robust analysis of moisture budgets and the contribution from local ET vs. moisture advection (moisture brought in due to prevailing wind patterns). Additionally, even when precipitation deficits are present, water can still be present in mid to deeper layers of the soil and available for the growing crop depending on soil type. Likewise, having a cover on the surface (e.g., a crop) can play a positive role in reducing direct water evaporation losses from the soil, meaning less water moving from the crop to the atmosphere.

Tasseling (VT) and flowering (R1 stage) is the peak of corn water use, and in Ohio, we are well past that point now. From there, water use in corn only decreases (learn more about corn water use here). According to the last USDA Crop Progress & Condition report (week ending 8/25/24), about 90% of Ohio's corn acreage has reached dough stage (R4), 47% of corn was dented (R5), 9% was mature (R6), and 12% of corn silage has been harvested. Much of the national discussion on corn sweat is happening in August, a time when corn begins to significantly scale down its water use as it approaches maturity. Likewise, even when atmospheric demand of moisture is high (like during a hot stretch of weather), corn may not maximize the ET process, and therefore, may not be much of a contributor to the humidity. Recent published modeling work suggests high temperatures (above 86°F) paired with low relative humidity are more detrimental to crop yield as compared to high temperatures paired with high humidity, and suggests extreme temperatures decrease yields indirectly due to limited moisture availability[2].  

For Ohio in 2024, much of the state has seen precipitation deficits during the growing season, with only pockets of heavy rainfall. In fact, much of southern and southeast Ohio are experiencing the worst drought conditions in over a decade. Ohio just hit some of the warmest high temperatures of the summer with very little rainfall over the past 7-10 days, and drought conditions are expanding while corn is shutting down. While humidity levels have been high across the central plains and western portions of the Corn Belt, humidity has been much lower across the dry areas here in the east.

With all this information in mind, it is very unlikely that “corn sweat” is contributing in any meaningful way to the recent stretch of muggy days. Instead, the current humidity is driven by more significant climate patterns such as large-scale atmospheric flows and its moisture advections (e.g. regional wind patterns). Here’s to hoping for some relief (and rain) ahead.

Special thanks to Dennis Todey (Director of the Midwest Climate Hub) and Trent Ford (Illinois State Climatologist) for their insightful conversations on corn’s impact on ET and low level humidity.