'Corn sweat' adding to heat misery in Midwest

[7/8]

I heard the term "corn sweat" used on the local news (CTV Kitchener) a few weeks ago, so I had to look it up. I found this article from July 22 explaining the concept:
http://www.usatoday.com/story/weather/2016/07/22/corn-sweat-iowa-midwest-heat-wave-evapotranspiration/87442376/

From muck fires to drunken forests, everyone loves weird science. Here's another one to add to the list: "corn sweat," which is contributing to the fierce heat wave scorching the central U.S.

No, the nation's 94 million acres of corn don't actually sweat, said Mark Licht, an Iowa State University extension agronomist.

Corn sweat is just another way to describe evapotranspiration, the natural process of water evaporating from plants to the air.

Like a giant wick, a growing corn plant pulls moisture out of the soil, according to the National Center for Atmospheric Research (NCAR). Some of that moisture escapes through the plant's leaves and enters the atmosphere, a process known as transpiration. Water vapor also enters the air from lakes and other surfaces through evaporation. Together, the processes are known as evapotranspiration.

During the growing season, an acre of corn sweats off about 3,000 to 4,000 gallons of water each day, according to the U.S. Geological Survey.

In Iowa, corn pumps out "a staggering 49 to 56 billion gallons of water into the atmosphere each day" throughout the state, the National Weather Service said. That can add 5 to 10 degrees to the dew point, a measure of the humidity in the air, on a hot summer day.

Dew points this week skyrocketed to the upper 70s and even low 80s in parts of the Upper Midwest, which is about as humid as it gets in the USA. Dew points above 70 degrees are considered oppressive.

"We'd be humid, anyway, but we're more so with the row crops that we have," said Iowa state climatologist Harry Hillaker.

So while the corn sweat isn't causing the temperature to soar, it is adding to the humidity, which makes the outside weather feel even more miserable to people.

"The warmer the air is, the higher the capacity it has to hold moisture," Hillaker said. "That can increase very dramatically as it gets warmer."

Iowa farmers this year planted the fifth-largest corn crop since the 1920s, the U.S. Department of Agriculture estimates. Nationally, it's the third-largest number of corn acres planted since 1944.

I can tell you that Southern Ontario has also been having a hot and humid summer. What stood out to me, though, is the huge amount of water transpired by crops!

[noelbotevera]

I actually live near a farm that's about maybe two miles from my house, and it grows corn. No wonder it feels like I'm walking across the sun whenever I try to go outside.

[GaryV]

And of course no other plant in the world transpires.  It's just agriculture that's bad.  Forests are good.

You ever walked through a pine forest in hot weather?  It's miserable.

[7/8]

And of course no other plant in the world transpires.  It's just agriculture that's bad.  Forests are good.

You ever walked through a pine forest in hot weather?  It's miserable.

Good point; some of the other articles I read do mention that it's not just corn, but nevertheless, they all stuck with the name "corn sweat". "Plant sweat" would be a better name (or they could just use the proper term "transpiration")

[kphoger]

A week ago, I was tent camping in the Chihuahuan Desert with no running water or air conditioning.

Just a little bit ago, I finished mowing the lawn here in Wichita, with a 77° dew point. You know what? I'll take the desert! Holy cow, that was oppressive! I think I'm going to blame my discomfort on ethanol subsidies.

... he typed from the comfort of his air-conditioned house, cold water in hand fresh from the tap, having just taken a shower ...

[english si]

Dew points above 70 is basically the short bit of British summer where it's actually hot (like above 30C). People wonder why we always complain those few days, normally people going "90?, that's weak - it's 110 here in Dubai/California/some other dry-ish place". Such comments also seem to forget that the UK doesn't do widespread aircon, buildings that are designed for hot weather or more than a couple of unbearable days each year.

[kphoger]

Exactly. The air temp was only 93° while I was mowing, but I wasn't 20% done yet and drenched in sweat. In the Mexican desert, it was the same temperature, but the dew point was at least 20° lower.

[jwolfer]

I live in Jacksonville Florida.  It's hot and humid from April til October

[ET21]

That's nothing new, occurs every year across the Cornbelt. I think the recent dew point temps of 80 or above being noted on TV stations has startled everyone. It's quite common with the peak between late June and early September

[Duke87]

In Iowa, corn pumps out "a staggering 49 to 56 billion gallons of water into the atmosphere each day" throughout the state, the National Weather Service said. That can add 5 to 10 degrees to the dew point, a measure of the humidity in the air, on a hot summer day.

5 to 10 degrees compared to what? Bare dirt? Natural prairie grass?

[ET21]

In Iowa, corn pumps out "a staggering 49 to 56 billion gallons of water into the atmosphere each day" throughout the state, the National Weather Service said. That can add 5 to 10 degrees to the dew point, a measure of the humidity in the air, on a hot summer day.

5 to 10 degrees compared to what? Bare dirt? Natural prairie grass?

Prairie grass doesn't sweat as much since their water needs are much lower than corn stalks. Want a difference?

Same airmass, Des Moines, Iowa and Pierre. South Dakota can have the same 90 degree air temperatures. However Pierre's dew point would be less than Des Moines because there's not much corn out in South Dakota compared to Iowa.

An even closer difference, Rockford vs Chicago. Chicago's dew point is a couple degrees less than Rockford because of corn vs urban environment in most cases. It's a fascinating trend to watch on what small scale impacts can do to the overall airmass

[Duke87]

Oh I don't doubt that there's a difference. My question is what baseline the figure of "5 to 10 degrees" is comparing to.

i.e., is that 5 to 10 degrees over no vegetation which is only, say, 2 to 5 degrees over natural vegetation? Or 5 to 10 degrees over natural vegetation which might be 10 to 15 degrees over none?

I suspect it is the former, and they have deliberately not stated this in order to mask their exaggeration of the effect. Either that, or the folks writing this article are a bunch of numbskulls who don't realize that they're throwing out numbers without providing proper context. Either way it's shoddy journalism.

[ET21]

It's most likely your latter, where the data isn't really thoroughly investigated. One of my friends in college did her final essay on evapo-transpiration in Illinois and how it affects the dew point drastically, so I'm sure her numbers are much better than the articles