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'Corn sweat' and beyond: 5 things to know about the heat dome in Missouri this week

Flickr | Paul Downey

The “heat dome” has arrived in Missouri, in which high temperatures and high humidity have teamed up to unleash incredibly uncomfortable hot weather. According to the National Weather Service, the daytime heat index is expected to exceed 105 this week in the St. Louis area. City officials have warned the public how dangerous the heat can be, especially for children, the elderly and those without air conditioning.

Extreme weather events like this often spur discussion of what’s responsible for it. As University of Missouri atmospheric sciences professor Anthony Lupo explained to St. Louis Public Radio, there are multiple forces at work in the case of this week's heat dome.



1. The transition to La Niña brings in more heat.

As the warming of Pacific waters known as El Niño subsides this summer, forecasters predict that it will be followed by a strong La Niña, when those ocean waters become cooler. Both El Niño’s and La Niña's  impacts on winters have been well documented, with El Niño making winters warmer and La Niña making them colder. But until recently, not much has been said for how summers would be affected.


In May, Lupo and his colleagues published a paper in the journal Atmospheric and Climate Sciences that said that the transition to La Niña is linked to hotter summers in the Midwest.


“In the summertime, when we have this transition from El Niño to La Niña, the jet stream tends to be located further north,” Lupo said. “That’s allowing the sun to heat up the continent. Those kinds of summers are warmer than normal.”


2. The dry June weather followed by heavy rains brought more moisture into the atmosphere.

The month of June was very hot for the U.S., with temperatures 5 degrees above normal in Missouri. On top of that, weather conditions were also very dry. When heavy rains arrived earlier this month, that saturated the ground, Lupo said, leading to more moisture being released into the atmosphere.


3. Winds traveling through the Gulf of Mexico bring moisture to the Midwest.

When wind travels from the south up to Missouri, it often picks up moisture from the Gulf of Mexico. According to Lupo, this wind movement is what meteorologists on television refer to when they say that the “Gulf is open for business.” The moisture picked up from bodies of water, and even land, as the wind moves north contributes to the humidity in the Midwest.


4. “Corn sweat” can make some areas more humid. Yes, corn sweat.


The idea of corn perspiring sounds ridiculous, but corn and other crops do “sweat” as a part of a process called evapotranspiration. The plants absorb moisture from the soil, which emerges as water vapor through the leaves. Missouri state climatologist Pat Guinan said to think of the water moving through the plant “like a straw.”


When a breeze comes by, the plant releases those water molecules into the air, as demonstrated by this CNN infographic, which adds to the humidity in the area. Lupo said the dew point, a measure of humidity, in a cornfield tends to be significantly higher than the dew point of an urban sidewalk.

5. It's hard to say how much climate change is to blame.


“You don’t ascribe any one event to climate change,” Lupo said, as a reminder that climate change refers to weather patterns over a long period of time. The heat dome could be a part of a trend, but it’s too soon for scientists to say for sure.


Generally, Lupo said, there hasn’t been an increase in extreme heat events, but heat waves over time have become longer. “Instead of lasting a week, they last 10 to 12 days,” he said. Climate change has also increased the likelihood of heat waves to happen. A 2014 study published in Nature said that Europe is 10 times more likely to experience a deadly heat wave now compared to a decade ago due to global warming.


Here are a few resources for those of you looking to stay ahead of the heat:

In Illinois? To find a cooling center near you, call the IDHS hotline at (800) 843-6154 or visit keepcool.illinois.gov.



Eli is the science and environment reporter at St. Louis Public Radio.

Send questions and comments about this story to feedback@stlpublicradio.org.