The dew point temperature is a measure of the amount of water vapor in the atmosphere and is also a good way to tell how uncomfortable you might feel on a hot day. Hot days and high water vapor contents are a serious health risk for some. Also, high water vapor content is a key fuel for severe thunderstorms.
There are three primary sources of water vapor that address this question: advection of water vapor largely from the Gulf of Mexico, evaporation over the Great Lakes, and transpiration from corn fields. The contribution from the Great Lakes in summer is minimal in comparison to the other factors, except maybe near the shorelines.
Southerly winds help bring warmer temperatures to the Upper Midwest but they also transport moisture from the Gulf of Mexico. This leads to higher dew points over Wisconsin and the Midwest. Studies have demonstrated that summertime precipitation in the Midwest is strongly correlated with the strength of this Great Plains low-level jet stream, which transports relatively warm, moist air northward from the Gulf of Mexico.
Transpiration is the release of water vapor from plant leaves. Transpiration has three main steps: the plant’s roots uptake water from the soil, the water moves through plant tissues, and the leaves release water vapor into the air through their stomata. During summer months, maturing cornfields become a huge source of water vapor to the atmosphere. Studies suggest a single acre of corn can transpire 3,000 to 4,000 gallons of water per day. On calm days, an analysis of dew points over the Midwest can identify the location of the Corn Belt. This transpiration by corn is sometimes referred to as “corn sweat.”
Together, these factors can result in summer dew points in the low to middle 70Fs for the Upper Midwest.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
