How do you measure wet bulb temperature?

Wet bulb temperature is measured with a standard thermometer whose bulb is wrapped in a wet muslin wick. As water evaporates from the wick, it cools the thermometer — the drier the air, the more cooling occurs. In practice, WBGT measurement kits include a wet bulb thermometer, a black globe thermometer (a 6-inch copper sphere painted matte black), and a standard dry bulb thermometer, all exposed to the environment simultaneously.

WBGT Heat Stress Calculator

Calculate the Wet Bulb Globe Temperature index used by the military, OSHA, and sports organizations.

Wet Bulb Temperature

°C

Temperature from a thermometer wrapped in a wet cloth — measures evaporative cooling potential.

Globe Temperature

°C

Temperature inside a 6-inch black copper sphere — measures radiant heat from the sun.

Dry Bulb Temperature (Air Temperature)

°C

WBGT Index

28.8°C

83.8°F

Risk Category

RED FLAG

Limit strenuous exercise. Mandatory rest cycles.

Military Heat Flag System

Flag	WBGT Range	Water Intake	Activity Guidance
Green	< 25.6°C (78°F)	0.5 qt/hr	Unrestricted activity
Yellow	25.6–27.7°C	0.75 qt/hr	Caution — increase rest periods
Red	27.8–31.0°C	0.75 qt/hr	Limit strenuous exercise
Black	> 31.1°C (88°F)	1+ qt/hr	Suspend non-essential activity

How to Use This WBGT Heat Stress Calculator

Enter wet bulb temperature — this is the reading from a thermometer with a wet wick, representing how effectively sweat can cool you.
Enter globe temperature — measured inside a matte black copper sphere exposed to sunlight. This captures radiant heat load.
Enter dry bulb temperature — the standard air temperature reading from a shaded thermometer.
Read the result — the WBGT index and corresponding military flag color tell you how to adjust activity levels and hydration.

Understanding WBGT Heat Stress

The Wet Bulb Globe Temperature (WBGT) is widely considered the most accurate single-number metric for heat stress risk. Unlike the common Heat Index (which only uses air temperature and humidity in shade), WBGT incorporates three distinct environmental factors that determine how the human body gains and loses heat.

The WBGT Formula

The outdoor WBGT formula weights each temperature component according to its impact on human thermoregulation:

WBGT = 0.7 × T_{wet bulb} + 0.2 × T_globe + 0.1 × T_{dry bulb}

The heavy 70% weighting on wet bulb temperature reflects the critical importance of evaporative cooling (sweating) for human survival. When humidity is high, sweat cannot evaporate effectively, and the body overheats regardless of the air temperature. The 20% globe temperature component captures radiant heat from the sun and surroundings. The 10% dry bulb contribution accounts for convective heat exchange with the air.

WBGT vs Heat Index: Why It Matters

The standard Heat Index assumes you are in the shade with a light breeze. This makes it dangerously misleading for outdoor workers, athletes, and soldiers. Two environments with the same Heat Index can produce vastly different WBGT readings depending on sun exposure and wind. A construction site in full sun at 35°C with 50% humidity is far more dangerous than the same temperature and humidity in the shade — but the Heat Index cannot distinguish between them. WBGT can.

The Military Flag System

The U.S. military developed the heat flag warning system based on WBGT thresholds. Each color represents an increasing level of heat stress risk and triggers specific mandatory precautions including rest-to-work ratios, water intake requirements, and activity restrictions. This system has been adopted by athletic organizations (NCAA, FIFA), OSHA, and many countries' military forces worldwide.

OSHA Guidelines for Workers

OSHA uses WBGT to set workplace heat exposure limits. For workers acclimatized to heat performing moderate work, the recommended limit is a WBGT of 28°C. For heavy work, the limit drops to 25°C. Unacclimatized workers face even stricter limits. These thresholds account for typical work clothing; specialized protective equipment (like hazmat suits) requires further reductions.

Preventing Heat Injuries

Heat injuries progress from heat cramps (muscle spasms) to heat exhaustion (heavy sweating, weakness, nausea) to heat stroke (confusion, cessation of sweating, organ failure). Heat stroke is a medical emergency with a mortality rate exceeding 50% if not treated within 30 minutes. Prevention requires monitoring WBGT, enforcing work-rest cycles, ensuring adequate hydration (water plus electrolytes), and acclimatizing gradually over 7–14 days when beginning work in hot environments.

Frequently Asked Questions

WBGT (Wet Bulb Globe Temperature) is a composite heat stress index that combines three temperature measurements: wet bulb temperature (humidity effect on cooling), globe temperature (radiant heat from the sun), and dry bulb temperature (air temperature). It is the gold standard for occupational and military heat stress assessment because it captures how the human body actually experiences heat.

The standard Heat Index only uses air temperature and humidity, and assumes shade and light wind. WBGT also accounts for solar radiation (globe temperature) and wind effects (via the wet bulb), making it far more accurate for outdoor work, sports, and military operations. Two locations with the same Heat Index can have very different WBGT values based on sun exposure and wind.

The U.S. military uses a color-coded flag system: Green (WBGT below 25.6°C / 78°F) allows unrestricted activity; Yellow (25.6–27.7°C) requires increased caution; Red (27.8–31.0°C) limits strenuous exercise; Black (above 31.1°C / 88°F) suspends all non-essential physical activity. Water intake requirements increase at each level.

Wet bulb temperature is measured with a standard thermometer whose bulb is wrapped in a wet muslin wick. As water evaporates from the wick, it cools the thermometer — the drier the air, the more cooling occurs. In practice, WBGT kits include a wet bulb thermometer, a 6-inch matte black globe thermometer, and a standard dry bulb thermometer, all exposed simultaneously.

OSHA recommends work-rest cycles when WBGT exceeds 25°C for heavy work or 28°C for moderate work. Above 31.1°C WBGT (Black Flag), the military suspends all non-essential outdoor activity. Signs requiring immediate work stoppage include confusion, cessation of sweating, rapid pulse, or body temperature above 40°C — these indicate heat stroke, a life-threatening emergency.