What is Evapotranspiration?

Posted: August 2, 2016, 2:15 pm by mikem

From the big metro areas of Chicago, Indianapolis and Cincinnati to the farmland surrounded cities of Peoria, IL and Lafayette, IN, the late summer months can be quite oppressive. While urban heat islands provide plenty of heat, you might be sweating it out even more in the open fields. And with corn growing tall and strong in the Midwest in August, we believe we have found the right person (err...crop) to blame.

Corn is the largest US crop in terms of production with about 12 billion bushels grown each year. In fact, 80 million acres of land is exclusively dedicated to the crop with the majority of this land in the Midwest. With that much corn being grown, plenty of water from irrigation and spring time storms is needed in the soil and utilized by the crop. As the corn grows, a process called evapotranspiration takes place, which basically is the sum of the sun evaporating water from the soil and the additional water vapor being released from the corn stalk leaves (transpiration). With an acre of fully grown corn releasing up to 4,000 gallons of additional water into the air, you can see why it can be very humid near fields! 

So why does increased humidity make it feel so sticky anyway? Well, sweating is our natural cooling mechanism. When the sweat evaporates off our skin, it removes heat since evaporation is a cooling process. On low humidity days, our bodies cool down quickly and the process is efficient because there is not much moisture in the air. On humid days, the air is more saturated and it's harder for the atmosphere to accept more water vapor. So sweat will evaporate slowly, making you feel hotter, longer. Let's take a look at an example of higher humidity levels caused by corn fields:

During the week of July 18th, a strong area of high pressure was anchored across much of the country and hot, humid conditions pumped into the Midwest from the Gulf of Mexico. By the 21st, temperatures reached into the 90s with lots of sunshine. In Chicago and Indy, dewpoints (we like to measure moisture with dewpoint, see dewpoint vs. humidity here) were in the low to mid 70s with heat index values in the upper 90s to low 100s. However, in the corn belt with all the evapotranspiration occuring, dewpoints were in the upper 70s and even low 80s. This increased moisture made Lafayette, IN feel like the mid 100s and Peoria, IL in the low 110s! 


Media Relations Director / Senior Meteorologist
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