How Iowa's cash crop impacts heat and humidity

With heat and humidity on the rise as we head into Wednesday, the National Weather Service is forecasting dangerous heat indices well over triple digits across the state.

Also known as the "feels like" temperature, heat index is a measure of how hot our body perceives the surrounding air. Our body’s natural defense to the heat is to sweat. When sweat evaporates, it cools us down. But as the outdoor humidity increases, that process becomes less efficient.

"The body’s ability to cool down in hot and humid environments is much slower than it would be in a hot and dry atmosphere," explains Chad Hahn, a meteorologist with NWS Des Moines.

In the spring and summer months, a southerly wind dominates the central United States, bringing a hot and humid airmass northward into the upper Midwest.

But Iowa's cornfields also play a major role. Late July and early August, the corn crop is undergoing its most mature stage of growth. As it does so, it releases a lot of water into the atmosphere. A single acre of corn can add 3,000-4,000 gallons of water into the atmosphere in a single day.

"In doing the math, Iowa alone can add as much as 56 billion gallons of water on a daily basis during the peak of the summer months," says Hahn.

It's a process called evapotranspiration: rainfall is stored in the soil, and when temperatures heat up, the water in the soil not only evaporates into the air, but the corn plant itself transpires, or quite literally sweats, even more water into the air.

Typically, this can raise the dewpoint temperature by 1-4 degrees Fahrenheit, but in extreme cases, it can raise it by more than five degrees, dramatically raising the humidity.

While several factors can contribute to this, the increase in corn acreage over the past several decades may actually be keeping temperatures a few degrees lower in Iowa, or at least preventing more extremes.

The last time Cedar Rapids hit 100F was all the way back on August 30, 2013. Between 1920 and 1950, on average, you could expect three days to reach the triple digits each year. The increase in corn production is one hypothesis as to why this number has gone down dramatically, particularly in the last 30 years.

Another hypothesis: climate change.

As counterintuitive as it may sound, a warmer atmosphere, on average, will be capable of taking in more moisture from lakes, oceans, and plants. In an already humid environment in Iowa in the summer, it's likely we see this downward trend in afternoon high temperatures continue.

"As we heat the globe at a quicker rate, the amount of moisture that we're able to store in the atmosphere is higher, and what that's going to cause is lower afternoon high temperatures and higher morning low temperatures, and I think that's what you're seeing in the data," explains Hahn.

While temperatures, on average, cool a degree or two in the coming years, it's quite possible that heat indices stay the same.

For now, with heat and humidity on the rise for one more day here in Iowa, remember to take frequent breaks in the shade, or limit outdoor activity to the late morning or early evening. Also be sure to stay hydrated if you plan on being outside, and of course, never leave anyone or anything alone in a car during this dangerous heat.