Sun adds to heat and humidity hazard

Published 11:07 am Saturday, June 18, 2022

By Skip Rigney

Hot and humid is a pretty good summary of our weather over the past several days. We haven’t been alone. Heat advisories and excessive heat warnings were issued by the National Weather Service (NWS) across a large portion of the South and Midwest.

The upcoming week will be even hotter with afternoon highs in the middle to upper 90s. It’s even possible, especially Wednesday through Friday, that Pearl River County will see triple digit temperatures for the first time since 2019. Some locations haven’t hit 100 since 2015.

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Most people have become familiar with the heat index, which combines the effects of temperature and humidity. If the actual temperature in the shade is 97 degrees and the relative humidity is 30 percent, then the heat index says it feels like the 97 degrees that it actually is.  But, keep the actual temperature in the shade at 97 degrees and raise the humidity to 45 percent, a likely situation this weekend, then the heat index says it feels like 106 degrees.

Obviously, the risk of heat-related illnesses, including life-threatening heat stroke, is higher when the heat index is 106 than 97. However, notice that the heat index assumes you are in the shade.

It doesn’t take a Ph.D. physiologist to know that our bodies get a lot hotter in the sun than they do in the shade or on a cloudy day. Because some people must work, or choose to play, in the sun, an indicator that factors sun exposure into the heat-related stress on the human body at work (or play) would be more informative than the shade-assuming heat index.

Wet bulb globe temperature (WBGT) does exactly that. It takes into account not only temperature and humidity but also sun angle, wind, and if measured directly, radiant heat from the ground, which is obviously different between surfaces such as asphalt and grass. WBGT was developed by the U.S. military in the 1950s. Both the Occupational Safety and Health Administration and the American College of Sports Medicine have developed guidelines for hazardous work and sports activity intensity using WBGT thresholds.

WBGT isn’t an apparent temperature, so it’s not as intuitive to interpret as the heat index. That’s probably why WBGT doesn’t get as much publicity as the heat index.

But WBGT is really the better measure of heat hazard. Researchers have found that the risk of heat-related illness becomes significant, even for moderate activity by people acclimated to outdoor work or play in the humid Southeast, once WBGT reaches the middle 80s. Outdoor high school sports practices are prohibited in several southern states, including Florida, Georgia, and Tennessee, once WBGT hits 92 degrees.

With high temperatures, humidity, and bright sunshine, WBGTs are predicted to rise into the middle 80s by 10 AM every day this week in south Mississippi, and reach a maximum in the hazardous upper 80s or lower 90s each afternoon.

To check the latest NWS forecast of WBGT online, go to, choose Wet Bulb Globe Temperature as the parameter to display, and zoom into south Mississippi.

Be careful in the heat this week. Or, just hibernate in the air conditioning.