How hot is it in South Florida? Let us count the ways
“WBGT” may sound like a classic rock radio station playing cool songs, but it’s really yet another way for experts to tell us how dreadfully hot it is in South Florida, with a little more precision.
The National Weather Service routinely informs us what the heat index, or “feels-like” temperature is, but the agency has recently been emphasizing what’s called the Wet Bulb Globe Temperature, or WBGT.
Like the heat index, the WBGT measured air temperature and relative humidity, but also adds in key factors such as wind speed, cloud cover and even sun angle to estimate the combined effect on the human body.
“WBGT is a measure of the heat stress in direct sunlight,” said Robert Molleda, head meteorologist at the National Weather Service’s Miami office.
He said it’s an additional tool in keeping the public informed of how the weather may affect them.
“It’s designed for people who are working or exercising in direct sunlight, such as the military — they’ve been using it for many years. Universities use it for athletic training.”
According to the Journal of Science and Medicine in Sports, the U.S. military developed the WBGT in the 1950s to limit serious heat illness during training sessions. They wanted something more nuanced, and they found that by using the WBGT to judge how brutal heat would be on soldiers, they were able to reduce heat illness, but also lose fewer training hours than if they went by “feels-like” temperatures.
The agency releases color-coded maps forecasting levels of heat risk based on WBGT.
Yellow “elevated” risk means working or exercising in direct sunlight for 45 minutes will stress your body, so a 15-minute break every hour is advised.
Orange indicates “moderate” risk, with stress occurring after 30 minutes. The agency advises 30 minutes of breaktime every hour.
Red indicates “high” risk, with stress occurring after 20 minutes. 40 minutes of breaks per hour is suggested.
And gray indicates “extreme” risk, with stress occurring after just 15 minutes. The agency suggests 45 minutes of breaks per hour.
This week has given us a taste of “high” risk WBGT readings, but this weekend will reach “extreme” in some areas.
Saturday at 2 p.m. will likely be peak heat. The actual temperature is forecast to be 90 degrees F, the heat index will be 105 and the WBGT heat risk will be “high,” meaning heat stress will occur after 20 minutes of work in direct sunlight.
But parts of Broward and Palm Beach counties, such as Coral Springs and Boca Raton, will hit “extreme” WBGT readings at around 2 p.m., as will wilderness areas farther west such as Big Cypress National Preserve.
Sun angle and time of day
The WBGT can change pretty drastically throughout the day because of sun angle, especially in Florida, where in summer the sun is close to directly overhead mid-day, delivering more direct solar radiation. “The WBGT will account for that,” Molleda said.
Sun angle also has vastly different impacts geographically.
We get vertically blasted here in South Florida, whereas in Boston, Massachusetts or Iceland, the sun stays at a much lower — and more forgiving — angle in summer, which is pretty cool.
Bill Kearney covers the environment, the outdoors and tropical weather. He can be reached at bkearney@sunsentinel.com. Follow him on Instagram @billkearney or on X @billkearney6.