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We're headed into the steamiest time of the year here in the Midwest. Statistically, the next two weeks yield some of the highest moisture levels as crops, especially corn, reach a point of maturity that allows them to sweat (just like people). The process, known as evapotranspiration adds additional moisture to what's already a muggy time of the year. We measure it in the form of dew points. Anything 70 or above is considered a tropical environment. 75 or higher is flat out oppressive!

The combination of summer heat and humidity can make for some wicked and potentially dangerous conditions. In fact heat index values annually reach 100-110 degrees at some point in July. As recently as July 21st, 2016 the dew point in Cedar Rapids (enhanced by evapotranspiration) reached a remarkable 83 degrees. With a 92 degree temperature the heat index soared to 116 degrees. Thanks for that corn.

If you think that's disgusting, the heat wave of July 13th, 1995 was one for the ages. In Cedar Rapids the high of 100 combined with a dew point of 86 produced a heat index of 131. That's never been seen before or since. Walk out the door and the feel of a sauna overwhelmed you. My glasses instantly fogged up to a point where I could literally not see. By the time I wiped them off sweat was pooled on my forehead. Crazy stuff. Here's a brief summary of the event from the NWS in the Quad Cities.

Intense heat overspread Iowa on July 12, 1995, and lasted well into the evening of July 14th. Dew point temperatures ranged from the upper 70s to the middle 80s through much of the time. The highest dew point readings were over the east half of the state. Winds remained light through the period and were generally less than 10 mph. High temperatures during the period were generally in the 98°F to 108°F range. The highest known temperature was 109°F in the Council Bluffs area. Most of the west half of the state broke the century mark on the 13th, and nearly every station by the 14th. Overnight low temperatures struggled to reach the middle 70s, with some areas remaining around 80°F. The highest heat indices were in the east half of Iowa, where the higher dew point temperatures were. The highest reading came from Cedar Rapids, IA on the 13th, with a heat index of 131°F by late afternoon.

Three people died from the heat in Iowa, one in Des Moines, one in Marshalltown, and a third in Burlington. A 95 year old woman died in her home when the temperature in the house climbed above the 110°F mark. She had no air conditioning or fans and the windows were closed. In Marshalltown, a 71-year-old man died in his un-airconditioned home. In a similar way, a 37-year-old man died in his un-airconditioned apartment in downtown Burlington on the 13th.

A significant loss also occurred in livestock during the heat wave. Statewide figures indicate the losses approaching the $5-$6 million range. Losses were placed at 4,000 head of cattle, 370 hogs, 1,250,000 chickens, and 250,000 turkeys. On one Webster County farm alone 250,000 laying hens perished on the 2nd day of the heat. Another egg producer had 1.5 million laying hens on two farms, one in Winterset, the other in Guthrie Center. They reported a loss of at least 500,000 hens. Disposal became a serious problem as rendering plants were overwhelmed.

In addition to problems caused to humans and livestock, there were numerous heat buckles reported on streets and highways around the state. Early indications were there was little in the way of crop damage. The combination of light winds and extremely high dew point temperatures helped keep the crops from stressing too much. Heavy dew would form overnight that would last well into the early afternoon hours.

During the month of July, approximately 70 daily maximum temperature records were set at locations from the central and northern Great Plains to the Atlantic coast.

By the way, I came across a nice article on evapotranspiration by Orlan Love of the Cedar Rapids Gazette written during the 2016 heat wave. Here's an excerpt.

In Iowa, we love our corn.

But go ahead and blame the maize for contributing to your discomfort during this week’s heat wave.

“Corn is always the one getting picked on, but it does increase the humidity,” a key factor in heat index readings topping 100 this week, said State Climatologist Harry Hillaker.

With corn fully grown and engaged in pollination, “this is prime time for corn’s contribution to high humidity,” he said.

Any large, deep-rooted plant puts moisture into the air through evapotranspiration — a process by which moisture in plant leaves evaporates into the air. Corn, which covers about 36 percent of the state, is Iowa’s predominant large, deep-rooted plant.

It’s probably not a coincidence that the portion of the Midwest under an excessive heat warning through Saturday closely approximates the area also known as the Corn Belt — a stretch roughly covering western Indiana, Illinois, Iowa, Missouri, eastern Nebraska and eastern Kansas.

When it’s fully grown and pollinating, as it is now, corn uses as much as 0.45 inches of water on a hot day, according to Mark Licht, Iowa State University Extension corn specialist. Some of that moisture helps the plant fill kernels, which are largely water at this stage of development, but “a good amount gets transpired,” he said.

Because corn plants actively draw water from the soil, more water vapor escapes a cornfield than would evaporate from a lake of the same size, Hillaker said.

Water vapor emitted by corn plants can increase the dew point, the standard measure of moisture in the air. The dew point — the temperature at which the air is saturated with moisture, the temperature at which dew forms — is used in combination with the actual temperature to calculate the heat index.

On a day with a high temperature of 95 degrees and a dew point of 80 degrees — a day much like many Iowans have been experiencing this week — the heat index, often defined as what the temperature feels like to the human body, is 115 degrees, according to the National Weather Service calculator.


The heat usually finds us at some point in July and Wednesday (the 4th) will be one of those days. Advisories are in effect for heat index values of at least 105. Hotter than a firecracker!

Hope you enjoyed this corny post. Have a great holiday and as always, roll weather...TS

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