Terry’s Take: Evapotranspiration makes the muggies muggier!

Posted on: 10:44 pm, July 9, 2013, by

Terry Swails Weather Blog

When the corn starts to tassel and the calendar says July, there is a level of humidity that surpasses uncomfortable and soars into the unbearable category.  That type of humidity can be found each summer right here in the Corn Belt of the United States.

Why the Corn Belt you say?  One word. One 7-syllable word. Evapotranspiration.

Evapotranspiration is the total amount of water that is transferred from the earth’s surface to the atmosphere. It is made up of the evaporation of free water surfaces (lakes, rivers, soil moisture, etc.) plus the transpiration from plants.

EVAPORATION AND TRANSPIRATION MAKE UP EVAPOTRANSPIRATION

EVAPORATION AND TRANSPIRATION MAKE UP EVAPOTRANSPIRATION

We are all familiar with evaporation but transpiration…not so much.  It’s actually fairly similar to the act of sweating but instead of humans, we are dealing with plants and vegetation.  Water within a plant changes to water vapor and in turn is released into the atmosphere. In fact, the USGS says 10 percent of the moisture found in the atmosphere is released by plants through transpiration.

CORN HARVESTED FOR GRAIN 1 DOT EQUALS 10,000 ACRES

CORN HARVESTED FOR GRAIN 1 DOT EQUALS 10,000 ACRES

The United States Corn Belt is a region where corn is the chief harvested crop.  The USDA map shows that the belt includes the states of South Dakota, North Dakota, Wisconsin, Nebraska, Iowa, Minnesota, Illinois, Indiana and Ohio.

Evaporation of water from soil with high moisture content (due to passing thunderstorm complexes) and the transpiration of water from tens of thousands of acres of a mature corn crop contribute to an atmosphere that at times can get sopping wet.

When high pressure and consequently high heat builds over the Corn Belt region during the summer months, humidity levels simply go through the roof thanks to evapotranspiration. Dew point temperatures can occasionally reach the 80s. Suffice to say, a dew point value of 80 or higher is darn right oppressive.

TRANSPIRATION FROM PLANTS, EVAPORATION FROM SOILS EQUALS EVAPOTRANSPIRATION

TRANSPIRATION FROM PLANTS, EVAPORATION FROM SOILS EQUALS EVAPOTRANSPIRATION

Let’s put it this way.  An air temperature of 90 degrees and a dew point of 65 degrees makes it feel like it is 92 degrees.  Not bad actually.  But an air temperature of 90 degrees combined with a dew point of 82 degrees makes it feel like it is 111 degrees.  That’s a “feels like” temperature difference of nearly 20 degrees!

HEAR STRESSED CORN, TRANSPIRATION GREATER THAN INCOMING MOISTURE

HEAR STRESSED CORN,
TRANSPIRATION GREATER THAN INCOMING MOISTURE

Moreover, evapotranspiration is the gift that keeps on giving too.  The high dew point values that evapotranspiration helps to produce are also one of the ingredients that fuels the development of thunderstorms.  Those thunderstorms then go on to produce very heavy rainfall which consequently creates high soil moisture content and lush vegetation.  The cycle repeats. It’s all a big part of the reason that the best corn in the world is growing in a field near you!