ST. LOUIS — St. Louis is expecting its hottest temperatures of the summer this week, but not all 100-degree days are created equal.
Things like humidity, cloud cover and wind can all make the weather seem a bit more comfortable. All those factors can affect how long workers should be out in the sun or whether people should exercise outdoors.
One tool used by the U.S. Military and the Occupational Safety and Health Administration to determine a safe amount of time to work outside is the wet bulb globe temperature (WBGT).
While not as common as temperature or heat index, WGBT can provide additional information that can keep you safe when temperatures get dangerously hot.
What is wet bulb globe temperature?
According to the National Weather Service, WBGT is an experimental measure of heat stress in direct sunlight that accounts for temperature, humidity, wind speed, sun angle and cloud cover.
The goal of wet bulb globe temperature is to determine the potential heat stress on people.
All those factors affect how well the body can cool itself with evaporative cooling. Humans and most other mammals cool themselves by sweating. Body heat converts sweat into water vapor, and that evaporation process cools the body.
If WBGT gets too high, the weather can make it hard or impossible for the body to cool itself while outside.
How it's measured
A specialized tool is used to measure WBGT. According to the National Weather Service, the tool includes:
- A wet bulb, which measures the temperature read by a thermometer covered in a wet cloth. As water evaporates from the cloth, evaporation cools the thermometer. This mirrors how the human body cools itself with sweat.
- A black globe, which measures solar radiation. Solar radiation heats the globe and wind blowing across it cools the globe.
- A dry bulb, which calculates the air temperature measured in the shade. It is the temperature you would see on your thermometer outside.
How it's used
All those factors are combined to come up with a single number. In the U.S., that number is on the Fahrenheit scale. So numbers in the 80s and 90s indicate weather that could be dangerous for people outside.
The National Weather Service advises the following precautions for high WBGT.
- 80-85 degrees – Take at least 15 minutes of breaks each hour if working or exercising in direct sunlight.
- 85-88 degrees – Take at least 30 minutes of breaks each hour if working or exercising in direct sunlight.
- 88-90 degrees – Take at least 40 minutes of breaks each hour if working or exercising in direct sunlight.
- Higher than 90 degrees – Take at least 45 minutes of breaks each hour if working or exercising in direct sunlight.
Where to find WBGT
If you want to find WBGT but don't want to invest in a specialized tool, NWS has an interactive map to see the expected WBGT highs for the day at your location.
The map also allows you to change data points like cloud cover percentages to adjust for real-time conditions.
Click here to try it out for yourself.