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Corn sweat and you: How large-scale agriculture is making life a little stickier

Yes, corn sweat is a thing. We sweat. Through a process called evapotranspiration, corn sweats as well. This process is not much of a thing around the City of Buffalo, but farther inland where corn is being grown, it is a bigger deal.

It’s long been known in meteorology that corn and other large-scale crops such as soybeans add significant moisture to air. Dew points in places like Iowa and Illinois during the recent heat wave exceeded 80 degrees at many locations. For those of you less familiar with the dew point concept, that is the temperature to which you would have to cool a parcel of air for saturation of the parcel to occur. An 80-degree-plus dew point is extraordinarily oppressive.

Even along the Gulf coast, dew points of 80 are generally rare. Large cornfields can add an extra 5 degrees to the dew point locally. A single acre of corn on a hot day adds up to 4,000 gallons of water to the atmosphere. The vast tracts of corn in the Midwest add many tens of thousands of gallons from a single farm, so one can visualize the contributions across such a large region. Generally, this extra water vapor doesn’t just stay in the cornfields. It mixes with the general circulation and adds extra humidity downwind as well.

The U.S. Geological Survey reports that studies show the transpiration of moisture drawn from the soil and released by plant leaves accounts for about 10% of all water vapor in the air, with the other 90% coming through evaporation from bodies of water, including the oceans. The processes of evaporation from the soil and transpiration from plant leaves is illustrated here by the USGS. USGS also provides a visualization of the water vapor released by a plant in these photos.

In dry spells and droughts, the amount of water transpired goes down as plants struggle to draw adequate moisture and nutrients from the soil. Evapotranspiration in a cultivated field of corn hastens the drying of the soil beneath. Obviously, the contribution of these fields to atmospheric humidity lessens during extended dry weather.

This year, after an excessively wet spring, the opposite has been occurring. Corn has had more soil moisture than normal to draw upon, and dew points over and downwind of corn fields have been even higher than normal. As recently as July 23, note the coverage of excessive soil moisture in place:


Last week, when Minneapolis suffered the highest heat index of any large city (115, still shy of their record high 119), the dew point reached 80 degrees. Even with other important factors such as moisture from past rainfall, such extraordinary conditions could not occur that far away from the Gulf of Mexico without the additional moisture supplied by regional corn and other crops.

As I said, an 80-degree dew point is relatively rare, even in Houston or Tampa. National Weather Service climate records clearly show an increase in the number of 80-degree dew point days, made more common by mean warming and the enormous acreage covered by the corn belt as seen in this graphic by web magazine Citylab.

While Western New York corn acreage isn’t comparable to the acreage in the Midwest and northern plains, it’s still significant.

The evapotranspiration process can be affected by other factors. Higher temperatures increase the amount of moisture transpired by plant leaves, and higher temperatures are more common in the face of our mean warming climate. Surrounding humidity matters as well. A dry air mass has more “room” for transpiring moisture to enter the atmosphere, and the increase in humidity from cultivated crops will be less noticeable. Gusty breezes will quickly mix out the transpired moisture near the plants and transport it in diluted concentrations downwind. Lower soil moisture will reduce the amount of transpired moisture.

Plant type matters, too. Wheat produces less evapotranspiration than corn or soybeans.

The focal point of this article was not to give you something else to worry about. It’s just an explanation of why life has gotten stickier due to the success of American corn farmers, with some help from climate change. The next time you’re enjoying corn on the cob, don’t hate. Just eat.

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