Corn adding humidity to Minnesota

Update 4:45pm: (Original post 8:29am)

Check out the 4pm dew point reading of 68 degrees at St. James Tuesday.

1 a a a st james.PNG

Notice how the 68 degree dew point reading is a good 10 degrees higher than many of the surrounding surface sites. It looks like the corn is doing a good job of evaporating soil moisture into the lower atmosphere in corn country today!


Anyone who’s driven through southern Minnesota knows corn is king in the summer landscape. Now we know it’s making our summers a bit more humid too.

According to Pete Boulay at the Minnesota State Climatology Office, Minnesota’s corn crops may be boosting dew points by as much as 1 to 5 degrees on summer days in and near cornfields.

Pete confirmed the readings last week by taking measurements with a little thing called a “pshychro-dyne” instrument which measures temperatures used to calculate the dew point. August 12th was a hot steamy day (high was 92 at MSP) with relatively light winds in the morning. So Pete headed out into the dense corn stand at the UM St. Paul Campus and began to take readings.

Here’s what he found: (These are excerpts from Pete’s email)

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.

Here’s a photo of the instrument used.

First value (T) is dry bulb, the second value (Tw) is wet bulb, the third value (Td) is dew point temperature. Dew point temperature was calculated at All are in degrees F.

August 12, 2010

11:18am field by parking lot 1/3rd mile south of station T85 Tw75 Td71

11:30am middle of corn next to station to south T87 Tw79 Td76 (light wind)

11:33am in instrument shelter T88 Tw76 Td71 (light wind)

4:40pm in instrument shelter T89 Tw78 Td74 (moderate south wind)

4:43pm in corn south of station T89 Tw79 Td75 (moderate south wind)

Campus station HMP35C reading

11:00am T86 Td76

Noon T89 Td76

4pm T90 Td78

5pm T90 Td79


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

State Climatology Office

DNR – Division of Ecological and Water Resources

The salient point here is that Pete found dew points between 1 and 5 degrees F higher in the corn on a hot summer day, than in nearby areas. Now, this is by no means a comprehensive scientific study, it’s just one really qualified climate expert taking readings in a corn field. But I’ll take Pete Boulay and his “psychro dyne” readings any day.

A little inside baseball here. Several of us in the local meteorological community have wondered if automated weather stations like St. James read higher dew points because they are located very near cornfields. Corn is an efficient evaporator of soil moisture, and releases that moisture into the surrounding air.

My take away from Pete’s little experiment is that corn does play a role by increasing summer dewpoints in densely planted areas. The effect is real, and millions of acres in the Midwest are planted with corn. This is not instrument failure, but rather success in picking up on the air mass modification by some row crops.

The next question is; are the higher moisture levels significant enough to cause additional low level moisture to fuel thunderstorms and enhance rainfall? In science, we call this process a “feedback loop.”

There are still many unanswered questions about corn and humidity. But on one day in August in the sweaty summer of 2010, a guy with a “psychro dyne” in a corn filed in St. Paul confirmed what many meteorologists have long observed. The corn is making things more humid in Minnesota….at least in the middle of the corn field.

Also unanswered is whether or not Pete saw Shoeless Joe Jackson dissapear into the corn at the UM St. Paul Campus. (Sorry, I couldn’t resist.)


  • Brooke

    That’s fascinating! I wonder how hard it would be to calculate the difference in rate of evaporation between corn fields and other landscapes. How much moisture does it take to increase dewpoint temps by several degrees?

  • Paul Huttner

    Excellent question Brooke:

    I don’t know the answer, but I’ll bet somebody who reads this blog does.

    Please jump in here if you have any info oh wise MPR listeners!


  • bsimon

    fascinating stuff. I’m left wondering how widespread the effect is; i.e. the airmass at 5′ in a corn field is one thing; what about at 100′ or 1000′? If that air is rising & carrying the cornfield moisture with it, its more plausible (to this layman) that cornfields could contribute to thunderstorm development. But if its only affecting dewpoints at or near ground level, its more an interesting factoid than a new variable in storm prediction. But, I’m just a news junkie, not a meteorologist.

  • Tony

    Not scientific by any means, but I have noticed this when I am out running. On my route I pass a good size corn field and the stalks come right up to the running path. It has always felt more humid and a bit more warm while running along this cornfield. Interesting post!!!

  • Terry Bovee

    OK Paul, time to start sorting out the variables – Where is the St. James site in relationship to adjacent crop fields (distance, direction, elevation, etc.)? How does the St. James site differ from the other reporting sites using these factors? Also, it is likely the crops planted adjacent to the recording station change from one year to the next so the ET rates would vary between corn or soybeans…so you need to compare from year to year to see how dewpoints may vary per site and from site to site. Sounds like a graduate student is needed!

    Or, you could contact the UM Extension stations in Waseca and Lamberton and ask them for ET data, etc.

  • Andy Kornkven

    A very fascinating and disturbing topic. Does corn emit humidity at a much higher rate than other crops like soybeans?

    I believe at least a quarter of the corn grown is a non-edible variety designed for ethanol production. Besides that, a significant portion is grown solely in response to federal government crop subsidies that encourage excess production of commodities like corn. We are growing way more corn today than we were even a quarter century ago. Only a tiny fraction of this corn is eaten by humans as whole corn. Some of it is processed into dubious byproducts like High Fructose Corn Syrup and Modified Corn Starch. Much of it is fed to animals, like cattle, that were not evolved to eat corn.

    We may be looking at man-made global warming right under our sweating noses– all caused by an expensive federal government program already under fire for numerous reasons. This topic needs to be examined and discussed more thoroughly!