What Is WBGT? A Practical Guide to Wet Bulb Globe Temperature

March 2, 2026 Monica Devlin Comments

Wet Bulb Globe Temperature (WBGT) is a heat-stress index used to estimate how hot conditions feel to the human body during work, sports, or physical activity-especially in direct sun. Unlike plain air temperature, WBGT accounts for multiple factors that drive heat strain, including humidity (your ability to sweat and cool), radiant heat (sun or hot surfaces), and air movement (wind/ventilation).

WBGT vs. Heat Index: why professionals use WBGT

You've probably seen the “heat index” in weather apps. Heat index is helpful for general public guidance, but WBGT is widely used in occupational settings, athletics, and military training because it better represents real-world heat load particularly when sunlight and radiant heat matter.

What does WBGT measure?

A traditional WBGT instrument uses three sensors to capture the main environmental drivers of heat stress:

Dry-bulb temperature (Ta): the ambient air temperature
Natural wet-bulb temperature (Tnwb): how effectively sweat can evaporate (influenced by humidity and air movement)
Black globe temperature (Tg): radiant heat load from sun or hot surroundings

These three readings are combined into a single WBGT number.

The WBGT equations (indoor vs. outdoor)

WBGT is calculated slightly differently depending on whether you're in shade/indoors or outdoors in the sun:

Indoors / shaded (no solar load):

WBGT = 0.7⋅Tnwb + 0.3⋅Tg

Outdoors in sunlight (solar load):

WBGT = 0.7⋅Tnwb + 0.2⋅Tg + 0.1⋅Ta

Notice that the natural wet-bulb term (0.7) has the largest weight because humidity and evaporation strongly control how well the body can cool itself.

How WBGT is used in real life

WBGT isn't just “a number.” It's typically used as an input to heat-safety decisions such as:

Adjusting work/rest cycles (more breaks as WBGT rises)
Changing workload intensity (heavy work becomes unsafe sooner)
Increasing hydration and cooling strategies
Adding controls (shade, fans/ventilation, scheduling, acclimatization)

Many heat-stress guidance frameworks also consider non-environmental factors like clothing/PPE and work rate (metabolic load), because they can dramatically increase heat strain even when WBGT stays the same.

Quick FAQ about WBGT

What is a “dangerous” WBGT?

There isn't one universal cutoff. Risk depends on workload, acclimatization, clothing/PPE, and individual factors. WBGT is meant to be used with a guideline or standard decision chart.

Is WBGT only for outdoor heat?

No-WBGT applies indoors too. Radiant heat from equipment, limited airflow, and high humidity can make indoor WBGT hazardous even without sun.

Why not just use air temperature?

Because air temperature ignores humidity, radiant heat, and wind/ventilation-key drivers of how fast the body accumulates heat. WBGT intentionally includes those variables.

WBGT is one of the clearest, most actionable ways to translate complex heat conditions into safer decisions for workers and athletes especially when sun, humidity, and airflow are changing hour by hour.

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Monica Devlin