In covering this heat wave, CNN wrote that there’s a dangerous heat dome forming over the United States, and “corn sweat could be to blame.”
Let’s just do this. Is corn sweat the cause of the coming heat wave? Definitely not. At most, it could make it more uncomfortable for some places.
“Corn sweat” is an extremely simple way of referring to evapotranspiration, the process by which moisture in plant leaves evaporates into the air. Plants draw water out of the ground through their roots for photosynthesis, and the water in the plant cells is exposed to the air once it gets above the ground. It evaporates off the leaves just as sweat evaporates off our skin — although it doesn’t take place to keep the plant cool, like it does for us.
So evapotranspiration is not making things hotter. But it is making things more humid — which can certainly be just as bad.
In regions where there are vast swaths of any plant, corn in particular, humidity can be notably higher than it otherwise would be. And the Midwest is covered in corn. Over 94 million acres of corn was planted in 2016, the USDA reported last month, up 7 percent from 2015 and the third-highest corn acreage in the United States since 1944.
These regions are where the humidity is more likely to be higher because of evapotranspiration. During big heat waves, this translates into a higher heat index — or what the temperature feels like to our living bodies. When it’s 90 degrees out but the relative humidity is 40 percent, it pretty much feels like 90 degrees. When it’s 90 degrees and the relative humidity is 95 percent, it feels like an oppressive 117 degrees.
We use the dew point to determine how much moisture is in the air. Put simply, it’s the temperature the air must cool to in order for dew to form, or the temperature at which the air is “saturated” with moisture — when it can’t get any more humid. Any dew point below 60 is pretty comfortable, but once you start getting near the 70-degree mark, things can get oppressive, fast.
In the corn belt, the dew point has been known to exceed 85 degrees during extreme heat waves, leading to a heat index of 134 degrees. These high-humidity events have been linked to evapotranspiration, along with wet springs and flooding in days prior.
The other interesting thing we have to remember about higher humidity is that it actually limits how hot the temperature can get. When the sun beats down on the ground, two things can happen: All of the sun’s energy can be used to heat up the ground, or some of it can be used to evaporate the moisture in the ground. Evaporation takes heat to accomplish, which is why it’s a cooling process. It also means that not as much of the sun’s energy is spent heating up the ground.
This is why the hottest temperatures on Earth are recorded in deserts, like the Middle East, the Sahara and the Mojave— there’s very, very little moisture to be evaporated. The world record for hottest temperature, 134 degrees, was set in Death Valley in 1913.
You’ll never see any place that is even remotely humid hit a temperature that high.