'Corn sweat' and why it's heaping on the humidity

ILLINOIS (WGN) — Regions blessed by large swaths of farmland can also be cursed with seasonal spikes in humidity thanks to "corn sweat." As corn and other crops grow, they give off moisture through their leaves. This happens throughout the growing season but reaches its peak as crops mature.

According to the U.S. National Weather Service Central Illinois, the scientific term for the release of moisture is evapotranspiration. This is the combined process by which water is transferred from the land to the atmosphere through evaporation from soil and other surfaces along with transpiration from plants.

Transpiration happens when water vapor is lost mainly from the stomata (tiny openings) on leaves of plants. For areas dominated by agriculture, the total amount of water pumped into the atmosphere can be staggering.

In Illinois, for instance, the mature corn crops can give off more than 35 billion gallons of water daily. "This is enough water to fill over 52,500 Olympic-size swimming pools!!!" said NWS Central Illinois.

As you might imagine, this amount of moisture added into the air has a direct impact on humidity levels and thus the heat index. Why, though, do plants transpire, or "sweat," at all?

According to an article in the National Institutes of Health's Library of Medicine, plant transpiration provides evaporative cooling for the leaf while also acting as the driving force to transport water and nutrients from its roots up into the rest of the plant.

The articles says that, through transpiration, a plant loses about 97% to 99% of the water it absorbs. The rate of evaporation is regulated by several factors, including light, the most important factor since it causes the stomata to open.

The more light there is, the higher the level of transpiration. For a scientific deep-dive on transpiration, check out this 2021 article posted by CID Bio-Science.