Breaking down the heat index

August has started off on a mild note compared to the scorcher that was late July across much of America’s Heartland. Last month featured a handful of days with highs in the mid to upper 90s in St. Joseph and surrounding areas. The hottest stretch so far was July 26th and 27th, when temperatures reached 97 degrees at Rosecrans Airport. 

Mercury thermometers only tell part of the story when it comes to extreme heat, and how it’s sensed by the human body. The heat index is a commonly used measure of apparent temperature that accounts for humidity when the mercury reaches 80 degrees or higher. Also called 'feels like temperature', the heat index was created in the 1970s to measure the impact of high humidity on the human body’s natural cooling system - sweat. 

The purpose of perspiration is to bring down our body temperature through evaporative cooling. As sweat evaporates, air near our skin cools by several degrees. When humidity is high, less evaporation takes place, therefore less cooling. Essentially, hot humid air makes sweat less efficient compared to a dry heat, which supercharges the evaporation process. 

Looking back at this year’s late July heat wave, temperatures peaked in the upper 90s. Meanwhile, heat index values reached well into the triple digits, ranging from 112 degrees to 118 degrees for several afternoons in a row across the Mid-Missouri River Valley. Tropical moisture left the air saturated with oppressive humidity on those days, allowing the heat index to spike.

The highest heat index ever recorded in St. Joseph didn’t take place in July, but August of 2023, reaching a record high of 120 degrees at Rosecrans Airport. 

Aside from high humidity due to fluxes of Gulf moisture, summer corn crops also tend to sweat in this part of the country in a process called transpiration. This has been known to increase humidity near the surface, and spike heat indices several degrees higher.