Summer may be winding down, but oppressive heat and humidity will smother much of the Eastern U.S. this weekend. By Sunday, August 17, 38 million Americans will face “major” risk of heat stress, with another 7 million at “extreme” risk, according to the National Weather Service.
The forecast calls for the highest temperature anomalies to spread from the Central Plains to the Midwest this weekend, with temperatures in the mid-to-upper 90s and low 100s Fahrenheit (mid-to-upper 30s Celsius), the NWS reports. The combination of this baking heat and high humidity will produce dangerous wet-bulb temperatures—pushing the body’s ability to cool itself to its limit. As such, the agency has issued heat advisories in parts of 15 states from Nebraska to Florida on Friday, August 15. Some of these advisories will remain in place through Monday.
On Sunday, forecasters predict an extreme risk of heat stress for most of northern Illinois. Locally extreme risk may also impact portions of central Iowa and Missouri—as well as areas along the Illinois border with these two states—plus northwestern Tennessee and parts of west-central Florida. By Monday, extreme risk could spread into parts of central Tennessee, western Kentucky, and southwestern Indiana.
“Drink plenty of fluids, stay in an air-conditioned room, stay out of the Sun, and check up on relatives and neighbors,” the NWS advises.
Wet-bulb temperature is a measure of heat stress that accounts for the combined impact of high temperatures and humidity on the human body. Humans lower their body temperature by sweating, and when sweat evaporates, it cools the surface of the skin. Humidity slows that process down, increasing the risk of heat-related illness. High wet-bulb temperatures are most dangerous for heat-sensitive groups, including children, older adults, people with chronic health conditions, and outdoor workers or athletes.
For an even more comprehensive measure of heat stress, meteorologists look at the wet-bulb globe temperature. WBGT takes the wet-bulb temperature into account with air temperature, wind speed, cloud cover, and the angle of the Sun to provide a measure of heat stress in direct sunlight.
For Saturday and Sunday, the NWS predicts afternoon WBGT values in the upper 80s and low 90s Fahrenheit (low 30s C) across most of the Midwest and Southeast. WBGT values over 90 degrees Fahrenheit (32 degrees C) can induce heat stress in just 15 minutes when working or exercising in direct sunlight, according to the NWS.
This isn’t the first time the Eastern U.S. has suffered oppressive wet-bulb temperatures this summer. At the end of July, a heat dome prompted the NWS to issue warnings for nearly 170 million Americans as WBGT values soared into the high 80s and low 90s Fahrenheit (low 30s C). It won’t be the last time either. Global warming is increasing both the intensity and duration of extreme wet-bulb conditions, making WBGT an important metric for understanding human survivability in a changing climate.