Local Climate Change
December 2004
Frank Watson, Meteorologist

Last summer (2003) I was having coffee with my father, who was also a Meteorologist, and I asked him if he saw the report last night on television on how the corn farmers are planting today is transpiring more water into the air than corn of old? The report centered on a way to explain why dew points where higher in the upper Midwest as compared to dew points along the Gulf, which has been observed at times. The report also mentions the increase in irrigation as an additional source of water for the corn to transpire.

My dad thought about the details of the story for a short time before casting some doubt on the story. While I see the logic behind the story I also had my doubts. I clearly see the argument of how one could change the climate in a micro sense. We experience it all the time when we water our own yards on a hot day and a gust of wind will blow a spray of water on you cooling you down. The lake effect is well documented for increasing snows in winter several miles in and cooling in the summer.

One of the many weather observations that we take is what type of air mass is over us. In the summer of 2003 we observed an above average number of days with tropical air masses and it was a humid summer. Tropical air masses are generally warm and humid. Warmer air is able to hold more water.

What controls what type of air mass we have over us? The upper winds. I think one could conclude that a high number of Tropical air masses aid in higher dew points. I'll concede that corn today expiries more water into the air and that irrigation allows for more water to be available for plants but does that explain higher dew points?

I recently ran into the State Climatologist from Iowa and was telling him about this report. He thought it interesting but added that farmers in Iowa do very little irrigating. In fact of the 24 million acres farmed only 140,000 acres are irrigated, that's .6 percent. What he did notice however that he is seeing less extremes high temperatures. He also noted that farming acreage has grown from 11 million in 1924 to the 24 million today.

The face of Iowa has changed from pasture and prairie grass to crops. Surfaces absorb and reflect heat differently and with the greening of the surface it has become more difficult to obtain higher extreme temperatures. Also soil moisture is not consumed until vegetation breaks out of dormancy in the spring but with crops being planted later no soil moisture is used during the spring but later when crops breakout allowing more moisture available later on in the growing season meaning more water to transpire.

This summer the upper Midwest was very cool, especially August. Not much was mentioned of high dew points. What happen? For one thing the number of Tropical air masses over the Midwest were well below average, you can't have high dew points if air can't support it.

I think one can affect their local microclimate by changing the surfaces and practices they employ on the land but in my opinion it's the upper winds that control surroundings as a whole. It's the upper winds that bring us heat, cold, and moisture that in turn affect us on the ground.

Frank Watson is a White Bear Lake, Minnesota meteorologist and can be found on the web at WeathermanWatson.com