What Is Wet Bulb Globe Temperature (WBGT)?
The Wet Bulb Globe Temperature (WBGT) is a composite heat-stress index that estimates how hot conditions actually feel to the human body when exposed to direct sunlight. Unlike a simple air-temperature reading, WBGT accounts for humidity, air temperature, radiant heat (sun), and indirectly for wind. It is widely used by the military, athletic trainers, occupational safety officials, and event organizers to set work/rest cycles and decide when activity should be reduced or stopped.
How to Use This Calculator
Enter three measurements taken on site: the natural wet-bulb temperature (\(\text{Tw}\)), the globe temperature (\(\text{Tg}\)) measured inside a black globe thermometer, and the dry-bulb air temperature (\(\text{Ta}\)). All values are in degrees Celsius. The calculator returns the outdoor WBGT in both °C and °F.
The Formula Explained
For outdoor conditions with solar load, WBGT is a weighted average:
$$\text{WBGT} = 0.7 \times \text{Tw} + 0.2 \times \text{Tg} + 0.1 \times \text{Ta}$$
The natural wet-bulb temperature carries the most weight (70%) because evaporative cooling — driven by humidity — is the dominant factor in heat stress. The globe temperature (20%) captures radiant heat from the sun, and the dry-bulb air temperature contributes the remaining 10%. Indoors or in shade, a different formula (\(0.7 \cdot \text{Tw} + 0.3 \cdot \text{Tg}\)) is used instead because there is no significant solar radiant load.
Worked Example
Suppose \(\text{Tw} = 25\,°\text{C}\), \(\text{Tg} = 40\,°\text{C}\), and \(\text{Ta} = 32\,°\text{C}\). Then:
$$\text{WBGT} = 0.7 \times 25 + 0.2 \times 40 + 0.1 \times 32 = 17.5 + 8 + 3.2 = \mathbf{28.7\,°\text{C}}$$
(≈ 83.66 °F). A WBGT near 28–30 °C signals high heat stress, where strenuous outdoor activity should be limited.
FAQ
Is WBGT the same as the heat index? No. The heat index uses only air temperature and humidity in the shade, while WBGT also includes radiant heat from the sun and a wind effect, making it better for outdoor work and sport.
What is a dangerous WBGT level? Thresholds vary by acclimatization and clothing, but WBGT above roughly 31–32 °C is generally considered extreme, warranting suspension of strenuous activity.
Why is the wet-bulb temperature weighted so heavily? Because the body's primary cooling mechanism is sweat evaporation, which fails when humidity is high — making the wet-bulb reading the strongest predictor of heat-related illness.
