Corn sweat: the hidden source of moisture in the Midwest
Stepping outside across Mid-Missouri over the past few days has felt less like a typical summer afternoon and more like walking into a fully cranked steam room.
While we often point fingers at deep tropical moisture surging north from the Gulf, the culprit behind our latest round of suffocating humidity is actually homegrown. Right now, our own agricultural fields are actively making the air feel nearly 10 degrees hotter.
We are currently in the peak window for a meteorological phenomenon known as "corn sweat", or, in scientific terms, agricultural evapotranspiration.
Just like humans sweat to regulate body temperature, crops do something very similar. Corn plants pull moisture up from deep within the soil through their root systems to transport nutrients. Once that water reaches the leaves, the plant cools itself by releasing excess moisture into the atmosphere as water vapor through microscopic pores called stomata.
While a single plant's contribution is tiny, the sheer scale of agriculture in the Midwest turns our fields into massive, natural humidifiers.
Mid-Missouri corn is fully mature and demanding peak water intake. Looking at the agricultural and atmospheric data alone, a single acre of mature corn can transpire between 3,000 and 4,000 gallons of water vapor into the air every single day.
Across Mid-Missouri's fertile agricultural counties, millions of gallons of water are pumped directly into the lower atmosphere every 24 hours. That is the equivalent of emptying several backyard swimming pools directly into the sky above every square mile of farmland.
Under a sprawling summer heat dome, high atmospheric pressure creates sinking air and light surface winds. Instead of allowing that moisture to mix upward or blow away, the atmosphere traps it within the lowest few thousand feet of the ground.
The direct consequence of trapped evapotranspiration is an explosion in surface dewpoints, that being the metric used to measure the amount of moisture in the atmosphere.
While a dew point in the upper 60s feels moderately humid, corn sweat routinely drives local dew points along the I-70 corridor and the Missouri River bottomlands into the upper 70s, potentially even nearing 80 in select instances.
That added moisture dramatically slows the rate at which human sweat can evaporate, preventing your body from cooling itself. For example, an air temperature of 93 degrees and a dew point of 68 degrees would lead to a heat index of 98 degrees, but bump that dew point up to 78 degrees, and the heat index jumps to 108 degrees.
That 10-degree jump is the exact threshold that pushes an ordinary hot afternoon into the more dangerous territories of heat advisories and extreme heat warnings.
Until an approaching cold front can sweep across the state to flush out this trapped moisture, expect the tropical air mass to stick around. If you are working or exercising outdoors, take frequent breaks in the shade, hydrate with electrolytes, and remember to check on pets and neighbors.
