The thermometer only
tells you half of it.
WetBulbTracker shows the real heat stress your body faces: the standard the pros rely on, made personal and hyper-local. Free, anywhere on Earth.
Your thermometer doesn’t tell you the whole story.
Air temperature ignores the one thing that decides whether heat is dangerous: whether your sweat can still cool you. When humidity climbs, evaporation stalls, and the same 95° can be a pleasant dry afternoon or a serious health risk.
That’s heat stress, and wet-bulb globe temperature (WBGT) measures it, capturing heat, humidity, wind, and the sun beating down in one honest number. It’s the standard the military, athletic associations, and occupational safety boards rely on to decide when it’s no longer safe to be out. It’s the number we put front and center.
Our name comes from heat science’s most extreme line: at a wet-bulb temperature near 35°C (95°F), even a healthy person at rest in the shade can no longer cool down. You’ll rarely be near that limit, but the same physics drives everyday heat stress, and that’s the danger we put in your hand, wherever you are.
Everything you need to read the heat.
Built for accuracy, open for scrutiny.
Best available data
Leading numerical weather models (ECMWF and national blends, via Open-Meteo) for live conditions, and ERA5 reanalysis for 35 years of history.
Rigorous physics
The full Liljegren (2008) iterative energy balance: solving for the black-globe and natural wet-bulb temperatures, not a rough approximation.
Checked in public
Cross-checked against the U.S. National Weather Service's own WBGT (agreement within about ±1.5 °C, or 2.7°F). Every formula and constant is published.
If heat can hurt you, this is for you.
Athletes & coaches
Plan practices and races on the standard governing bodies actually use — your state's own zones — then set intensity to match.
Outdoor workers
Know when to add water breaks, shade, and rest: before someone goes down.
Parents & caregivers
Judge the playground, the game, the walk: for the people who feel heat hardest.
Dog owners
Check the pavement before you walk: asphalt burns paws long before the air feels dangerous.
Event organizers
Plan around the day's real danger window, not just the forecast high.
Travelers
Understand a climate you've never felt before, anywhere on Earth.
Researchers & media
A transparent, citable heat-stress reference with an open methodology and a published error.
Common questions.
What is wet-bulb globe temperature (WBGT)?
WBGT combines temperature, humidity, wind, and direct sun into a single measure of heat stress on the body. It's the standard the military, athletic associations, and occupational-safety bodies use to decide when it's no longer safe to be active outdoors.
How is WBGT different from the heat index?
Heat index estimates how hot it feels in the shade, from temperature and humidity only. WBGT also accounts for wind and direct sunlight, so it reflects real outdoor heat stress, which is why safety organizations rely on it instead.
How accurate is WetBulbTracker?
We use the validated Liljegren (2008) WBGT method and continuously compare our results in public against the U.S. National Weather Service's own published WBGT, which is itself a model: typically within about ±1.5 °C (2.7°F). The live comparison and our full methodology are open for anyone to inspect.
Is WetBulbTracker free?
Yes, and generously. Live readings, plain-language guidance and safety warnings, a written outlook for the day, six ways to read the heat, the map animated through the next day, seven days of daily highs, 35 years of this-day history, the paw check, the state policy tables and PNG and PDF export are all free forever, anywhere on Earth — the core needs no account at all. A free account adds three saved places and up to three heat alerts, with overnight quiet hours and your own risk profile applied to them. Optional paid plans add planning depth — the week ahead, past date ranges, CSV export and multi-site documents — and never gate a reading or a warning.
What does the “safe for me” personalization do?
You can add optional personal factors: age, heat acclimatization, activity level, and health sensitivities, to get a personalized risk read shown alongside the objective number. It's general guidance, not medical advice.
Why does sun vs. shade matter?
Direct sun can add several degrees of heat stress compared with full shade. WetBulbTracker shows both so you can see how much cooler, and safer: shade really is right where you are.
Heat is the weather hazard we measure least well.
It kills more people in most years than floods, hurricanes, or cold. Yet the number that actually measures it has stayed in safety manuals, costly hardware, and research papers. We’re making it legible to everyone, everywhere: personal, hyper-local, checked in public, and free.