What's causing the incredible 90-degree dew points in Iowa this week?

Dew points have hit 90 degrees at several Iowa weather observation sites this week.

It’s part of a record heat dome that’s been shattering records and producing eye-opening temperature and heat index values across the central United States in recent days.

Example?

The heat index in Lawrence, Kan., hit 134 degrees Monday afternoon. It hit 130 degrees in Ottawa, Kan.

Many meteorologists and weather observers have commented on multiple dew point values at and above 90 degrees in western Iowa this week. Some believe sensor calibration issues could be producing inaccurately high dew point readings.

That may be true in some cases. But there are also two main atmospheric and land use factors that may be causing dew points to spike to near if not above 90 degrees.

‘Corn sweat’

The first is what we call “corn sweat.” It occurs as corn plants in the massive sea of cornfields in the Upper Midwest transmit moisture from soils into the atmosphere above the plants. The U.S. Geological Survey estimates an acre of corn can pump out 3 to 4 thousand gallons of water into the air in a day!

I’ve written multiple times about how Pete Boulay with the Minnesota Department of Natural Resources State Climatology Office did an informal experiment above the dense corn plot that recorded dew points as much as 1 to 5 degrees higher over the cornfield than surrounding areas at the same time.

Here’s a clip from my 2016 post.

Measurements by Pete Boulay at the Minnesota DNR Climate Working Group have found dew point levels from 1 to 5 degrees higher inside a relatively small corn plot at the University of Minnesota's St. Paul Campus.

Pete's brief experiment in 2010 confirms what other sources say about extensive corn crops adding to dew low-level dew points. Dew point spikes of 1 to 5 degrees are quite likely in the Upper Midwest in summer depending on wind velocity and trajectory.

Hello all,

There's been discussion about certain AWOS sites in Minnesota and their proximity to row crops, especially St. James. The dew point temperatures at sites like St. James are consistently higher than other locations during the high dew point season of July and August. Could the close proximity of actively transpiring crops be the explanation?

I wasn't quite hot enough on Thursday, so I did a little dew point experiment on August 12 using a "pshychro-dyne" instrument. I measured the wet and dry bulb temperature at the St. Paul Campus Weather Station and the small, but dense corn plot in front of the station. It was a sunny day with very few clouds. Winds were light before noon, but became fairly breezy from the south by afternoon. Readings were measured at 5ft above the ground and were conducted in either shade or in the instrument shelter.

Observations...

The dew point temperature was higher in the corn by 1-5 degrees F, wind may play a role. It feels very hot and muggy in the middle of a corn field in August.

Pete Boulay - Minnesota DNR Climate Working Group

The National Agricultural Statistics Service (NASS) reports U.S. farmers planted 91.7 million acres of corn in 2019. That’s about 69 million football fields.

So it’s almost certain that corn sweat is boosting dew point values from western Iowa into southern Minnesota by several degrees this week.

Record hot Gulf of Mexico water temperatures

The second factor that’s likely contributing to the astoundingly high dew point readings in the Upper Midwest this summer is record heat in the Gulf of Mexico. Water temperatures in the Gulf have hit 90 degrees in some areas from Texas all the way to Florida.

Warm water in the Gulf of Mexico has been feeding extraordinary heat and moisture into air masses in the southern and central U.S. this summer. Southerly wind flow has pumped those tropical dew point levels into the Upper Midwest in the past week.

Dew points reached the 80s at multiple locations in Minnesota Tuesday. The highest dew point ever recorded in Minnesota is 88 degrees in Moorhead on July 19, 2011.

It may take some time to see how much sensor calibration issues factored into the incredible 90-degree dew points recorded in Iowa this week. But corn sweat and a superheated Gulf of Mexico air mass are also very likely significant factors.

Stay tuned.