As temperatures rise in July and August, transpiration from plants might add to the humidity.
One plant that gets more blame than others is corn, said Dan Quinn, assistant professor of agronomy and extension corn specialist at Purdue University.
“I try to reiterate that it’s every plant, and humans, that are transpiring, too,” Quinn said. “But in a state like Indiana, we have five and a half million acres of corn, so it’s contributing to the humidity.”
In July and August, corn crops are at a growth stage and transpiring most, Quinn said. The crops use water and release it back into the atmosphere as water.
During the later summer months, transpiration is highest, Quinn said, because the crops are at pollination. When there is healthier and more corn, the crop is likely to transpire.
“The flip side of this is that, in many cases, it’s often well overstated how much corn actually contributes to the humidity,” Quinn said. “Corn is not the only plant that is transpiring. Your lawn is transpiring, the trees are transpiring, and the soybeans are transpiring.”
It’s difficult to tell how much corn transpires and to quantify how much it impacts the area’s humidity, Quinn said.
“If you look at it on a per area basis, in many cases we look at, it can be about anywhere from 3,500 to 6,000 gallons per acre per day from that crop,” he said. “That’s quite a bit of moisture, but you can look at an inch of rainfall, which isn’t uncommon; that’s 27,000 gallons of water per acre per area basis.”
Gino Izzi, meteorologist with the National Weather Service, said transpiration is “basically when corn sweats.”
Sometimes humidity from corn can lead the temperature to be one to two degrees cooler, Izzi said, but added humidity likely results in a higher heat index.
“If there was no corn, it might be 90 degrees with a heat index of 94,” Izzi said. “If you go out to a corn field and take the temperature, it could be about 88 degrees with a heat index of 94.”
Although temperatures will most likely stay high through the end of July, Izzi said it’s typical weather for this time of year. He recommends people stay hydrated, spend time in air conditioning and take breaks if they have to be outside during the heat of the day.
“(The weather) certainly puts a lot of stress on the body, especially the more vulnerable populations,” Izzi said. “It increases the risk of heart-related illnesses.”
mwilkins@chicagotribune.com
