Can’t handle the heat? Blame Iowa’s corn

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, he said, 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.

Hillaker provided an 'educated guess” at the amount of water vapor corn could be contributing to the atmosphere this week.

On Wednesday afternoon, for example, the dew point in Dallas was 69 degrees - a reflection of Gulf of Mexico moisture in the air unaffected by vast fields of corn.

In Ames, which is influenced by Gulf moisture and surrounded by corn, the dew point was 80 degrees.

With a dew point of 69 degrees, he said, a kilogram of air would contain 17 grams of water, while that same kilogram would contain 23.5 grams of water at an 80 degree dew point. Acknowledging that factors other than the presence of corn may have contributed to the disparity, Hillaker said the air in Ames contained 38 percent more water than air in Dallas on Wednesday afternoon.

Hillaker said the state's highest credible dew point temperature, 85 degrees, occurred in Cedar Rapids during the July 1995 heat wave. Combined with an actual temperature of 100 degrees, it yielded a heat index reading of 133.

During that same heat wave, on July 13, 1995, a 90-degree dew point and a 101 degree temperature were recorded in Appleton, Wisconsin, yielding a heat index reading of 148 degrees, perhaps the highest ever recorded in the United States.

While corn contributes to high humidity, the humidity helps hold down actual temperatures. Heat required to evaporate water is diverted from heating the soil, moderating actual air temperatures, Hillaker said.

Both Hillaker and ISU's Licht said widespread beneficial rains immediately preceding the heat wave and the relative briefness of the hot spell will minimize stress on corn and other crops.

Rain and overcast skies earlier this week delayed the onset of the heat wave and shortened it, according to Hillaker.

'Overall, we will come through this pretty well,” Licht said.

The degree of yield-reducing crop stress depends on how much rain fell before the heat wave in a given area and how high the temperatures actually get, he said.

Notwithstanding the amount of moisture available, corn is likely to suffer stress when daytime temperatures exceed 95 degrees and nighttime temperatures exceed 70 degrees, Licht said.

'We need nighttime temperatures below 70 to help plants recover from the heat of the day,” he said.