When it comes to summer heat, it’s all about the index
By Jared Shelton News-Press NOW Chief Meteorologist
After a tame start to June in terms of summer heat across the mid-Missouri River valley, the stifle has returned this week. With the first large-scale heat wave of the season building west to east across the country also comes the return of some metrological jargon. Opposite to that of wind chills in the winter, soaring summer temperatures bring a reemergence of the heat index.
Most have heard the phrase, “It’s not the heat, it’s the humidity;” the heat index makes a decent case for this assumption. Also known as “feels-like temperature,” heat indices were created to consider variables that exacerbate the effects of heat on the body. Humidity is the primary variable taken into account in heat index calculation, as high-moisture content in the air places strain on the human body’s natural cooling system.
Sweat is the body’s primary method of maintaining a healthy core body temperature in hot environments. When skin secretes beads of sweat, the extra moisture starts to evaporate into the surrounding air. Interestingly enough, evaporation is a cooling process, which acts to rapidly transport heat away from the body. This is why we may get a shiver before drying off with a towel at the swimming pool, as evaporation starts to take place in the open air. But some air masses are better at promoting evaporation than others.
When relative humidity is low, and air is fairly dry, evaporation happens readily. However, when humidity is high, evaporation takes place at a much slower rate. Because less evaporation equals less cooling, sweat is far less efficient at cooling the human body on hot muggy days rather than hot dry ones. High temperatures and high humidity tend to make us sweat more, but cool less- resulting in a “feels-like temperature” or heat index that’s several degrees higher than the ambient air temperature. For example, an air temperature of 90 degrees can “feel” like 97 degrees at 55% relative humidity. Meanwhile, the same temperature at 70% humidity can yield a heat index (“feels-like temperature”) of 105 degrees.
Yes, heat indices are legitimate when it comes to the oppressive combination of high temperatures and high humidity, but there is a loophole that can make heat indices less impactful when temperatures soar — the cooling power of a breeze. Because circulating air speeds up evaporation rates, breezy or windy conditions can help lessen the intensity of high heat indices. A notable positive more days of the year than not among the rolling plains of the heartland, but not a complete fail-safe from the effects of searing heat and oppressive humidity.
