Update 4:45pm: (Original post 8:29am)
Check out the 4pm dew point reading of 68 degrees at St. James Tuesday.
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)
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 http://www.csgnetwork.com/dewptrelhumcalc.html 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.
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.)