What is this?
A thermometer measures the air. Your body is also dealing with the humidity, the wind and the sun overhead, which is why two places can both read 90 degrees and be very different places to work, train or walk a dog. Wet-bulb globe temperature, or WBGT, folds all four into one number, and it is the measure militaries, athletic associations and occupational-safety standards use to decide when activity has to change.
It is not the heat index. Heat index estimates how hot it feels in the shade, from temperature and humidity alone. WBGT accounts for wind and direct sun, so it describes the load on someone actually outside in it. That is why the two so often disagree.
Sun and shade are different places
Direct sun adds several degrees of heat stress over full shade at the same moment, in the same spot, so both are computed and shown together. After dark the ground keeps giving back what it absorbed all day, which is why an evening reading can still be serious.
How accurate it is
Computed with the Liljegren (2008) energy-balance method from gridded weather data, and compared continuously against the U.S. National Weather Service's own published WBGT on a live accuracy page anyone can open, typically agreeing within about ±1.5 °C (2.7°F). Both sides of that comparison are modeled rather than measured, and the full method is published. Where a standard or policy calls for an on-site measured reading, that instrument is the one that governs.
Guidance for how you use it
Outdoor work · Athletics · Tactical · Paw safety
Heat stress by city
Phoenix · Houston · Dallas · Miami · Las Vegas · Atlanta · all cities
The readings, the guidance and the personal read are free forever, anywhere on Earth. There is a fuller tour on the about page.