What is Corn Sweat?

Have you ever wondered why Iowa has some of the highest humidity and heat index values in the nation? You may be wondering how that can be since they are so far away from a water source, such as an ocean. The answer may lie in corn and a phenomenon which is known as “corn sweat.” 

Corn sweat is a term used to describe the moisture that is released from the corn, causing more water vapor to be released to the atmosphere. The corn is able to draw moisture from the soil and into its leaves. At this point, the leaves “sweat,” meaning water is being released from the plant. This mechanism is also known as transpiration, when combined with evaporation, it is called evapotranspiration. This process has more of an effect the hotter it gets. This is because more water can be absorbed through the leaves of corn as the temperature of the surrounding air increases. Evapotranspiration is the main process that causes high humidity and heat index values for Iowa and even parts of Minnesota. 
 
​During the year 2011, people started looking at corn as a reason why some areas, such as Iowa and Minnesota, were suffering from extreme heat. On July 18th of that year, Knoxville, Iowa reached an astonishing heat index of 131°F. What might be just as impressive is the dewpoint that Minneapolis received that same day. The dewpoint reached a sweltering 81°F. That set a record for the highest dewpoint that the city has ever received up to that point. The following graphic is taken from July 18th, 2011. The dark green color on the image on the left is where most of the corn is located. The image on the right shows where the greatest heat was occurring. The pink areas are where excessive heat warnings were in effect and the orange areas are where heat advisories were issued. According to National Oceanic and Atmospheric Administration (NOAA), an excessive heat warning is issued when the heat index is at least 105°F for more than 3 hours per day for 2 consecutive days, or the heat index is more than 115°F for any period of time. A heat advisory is issued when the heat index is at least 105°F but less than 115°F for a 3-hour period. As you can see, the areas of large corn production coincide with the areas of the greatest heat. 

2016 was another year which brought a lot of attention to the idea of corn sweat. The image below shows the maximum temperatures. It’s interesting to note how much cooler the temperatures are in Iowa and Minnesota. This once again is due to the transpiration of the corn. The water vapor in the atmosphere will actually suppress the temperatures. The downside to this is that the humidity is much higher, so although the temperatures are cooler, the heat index will be greater. This can be seen in the second image which shows the heat index values. There is a noticeable 110-115°F swath of heat index values from Iowa into southern Minnesota. Once again, this is right where most of the corn production is located. As summer is quickly approaching, the term “corn sweat” will be used more frequently in much of the upper Midwest. 

Credit: UCAR, Washington Post

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@2019 Meteorologist Corey Clay