WBGT vs. heat index: what's the difference?

Both wet-bulb globe temperature (WBGT) and the heat index try to answer "how dangerous is this heat?" — but they measure different things, and the gap between them matters most exactly when conditions are worst.

The short version

• Heat index = how hot it feels in the shade, from temperature and humidity only.
• WBGT = full heat stress outdoors, adding wind and direct sun.

If you are active outside in the sun, WBGT is the more honest number — which is why safety organizations use it to set activity limits.

What heat index captures (and misses)

The heat index (the familiar "feels like" figure on US forecasts) combines air temperature and humidity. It's useful and easy, but it was derived for shady, light-wind conditions. Step into direct sunlight and the real heat load can be up to ~15 °F higher than the heat index suggests. It also ignores wind, which can cool you, and the radiant heat from pavement and equipment.

What WBGT adds

WBGT folds in two things heat index can't:

• Direct sun and radiant heat, via the black-globe temperature.
• Wind, which changes how fast you shed heat.

That makes WBGT the standard for athletics, the military, and outdoor work — anywhere people exert themselves in the sun. The trade-off is that it needs more inputs (solar radiation and wind), so it's harder to compute by hand. That's exactly what WetBulbTracker does for you.

Which should you use?

And "wet bulb" itself?

The plain wet-bulb temperature is a third, narrower number — the evaporative-cooling limit. Near 35 °C wet-bulb, the body can't cool itself even at rest. It's the famous survivability threshold, but for day-to-day decisions, WBGT (heat stress) is the number to watch.

Get the real number

• Open WetBulbTracker → for live, validated WBGT anywhere
• WBGT calculator →
• What is WBGT? →