Thursday, July 15, 2010

A recipe for dangerous heat

On a scale of 1-10, with 1 being very comfortable and 10 being unbearable, I think most midwesterners would have given yesterday a 27. So what made it so darn hot and miserable? Well, yesterday was a textbook example of how moist ground, fully transpiring corn, and a warm air mass can combine to create deadly heat and simultaneously provide fuel for intense thunderstorms.

The left graph comes from Kanawha in northern Iowa and the right graph comes from Mead in eastern Nebraska. Both AWDN stations are located in large corn producing areas and both sites have received copious amounts of rain this summer. So here's how corn + moist ground + warm air mass = miserable...

At this time of year corn is starting to enter the reproductive phase (when the grain is produced) and it wants to maximize the time during a day when it can undergo photosynthesis. During the process of photosynthesis, stomata on the leaves of healthy corn are open and ready for business. When corn is healthy, there is usually ample soil moisture and essentially the corn will transpire (release) water vapor into the atmosphere until the vapor pressure deficit (VPD) is high enough to force the corn to close its stomata for the day.

Quick lesson:

Vapor pressure deficit (VPD) is essentially the difference between the vapor pressure of the air at a particular temperature and what the vapor pressure would be at saturation at that particular temperature. In other words, if the VPD is low, the air is quite moist and if the VPD is high, the air is quite dry. When the VPD gets high, plants close the stomata to preserve soil water and their turgor.

Back to the main show...

During drier times, the closing of stomata on the corn happens earlier in the day, and we are deprived of its water vapor making the air mass even steamier. However, the opposite is true now. The soils are very moist in almost all of eastern Nebraska and Iowa right now and the combination of soil evaporation and a warm, moist air mass being advected from the south led to very humid conditions before the corn had a chance to get to work yesterday. And yesterday the corn was getting overtime benefits once it got started..

The red dots on the graph represent the temperature and the blue dots represent the dew point temperature (i.e., temperature that air would have to cool to before dew would form). At both locations it was already miserable by 8 AM and things would only get worse. As alluded to earlier, the stomata on the corn were open for business and the corn began to release water vapor into an already very humid air mass. At both locations the temperature and dew point increased throughout the day, although the dew point increased more noticeably at Kanawha in northern Iowa.

By late afternoon residents of the Midwest were no doubt uttering expletives and wondering if they had been transplanted to the Persian Gulf as temps in the 90's and dewpoints in the 80's pushed the heat index close to 120F. Relief was on the way though...

Cold Front to the Rescue!

By late afternoon, a cold front was situated from Minnesota down into eastern Nebraska and Kansas. All of the heat and moisture that made it feel so miserable helped to form powerful thunderstorms over many parts of the north central U.S. once it interacted with the cold front. (See Storm reports from the SPC: In addition to high winds, many places received yet another soaking. Mead picked up almost 3 inches of rain and Kanawha came in with close to an inch.

Don't let guard down...

The cold front has brought relief to the region in the form of cooler and much drier air but the reprieve looks to be short lived. Heat and humidity will return by the weekend in eastern Nebraska, to most of the Midwest by early next week, and unfortunately it looks like it could be hot and miserable for a while afterward. But just think, in 5 months we can look forward to blowing snow and subzero temperatures!

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