Wet Bulb Globe Temperature, or WBGT, is a measurement used to assess heat stress on the human body in direct sunlight. It provides a more comprehensive value than a standard temperature reading by integrating multiple environmental factors. The primary purpose of WBGT is to offer a single, actionable number that helps prevent heat-related illnesses for people active outdoors.
The Components of WBGT
The WBGT value is derived from three separate measurements. The first, dry-bulb temperature, is the air temperature as measured by a standard thermometer shielded from direct sunlight and moisture. This is the temperature typically reported in weather forecasts and provides a baseline for the ambient heat.
A more specialized measurement is the natural wet-bulb temperature. This is taken with a thermometer whose bulb is covered by a wet cotton wick and exposed to natural air movement and humidity. This component simulates the cooling effect of sweat evaporating from the skin. When humidity is high, less moisture evaporates from the wick, resulting in a higher wet-bulb temperature and indicating the body’s cooling mechanism is less efficient.
The final element is the black globe temperature, which measures the impact of solar radiation. It uses a thermometer placed inside a flat-black copper sphere that absorbs heat from direct sunlight. This reading accounts for the intense heat felt when exposed to the sun, which can be significantly higher than the air temperature in the shade.
Interpreting WBGT Readings
The three components of WBGT are combined using a weighted formula that gives the most significance to the wet-bulb temperature, as humidity is a dominant factor in heat stress. The formula for outdoor environments is WBGT = 0.7Tw + 0.2Tg + 0.1Td, where Tw is the wet-bulb temperature, Tg is the globe temperature, and Td is the dry-bulb temperature.
Safety guidelines, often represented by colored flags, translate WBGT values into risk levels and recommended actions. These systems are used by organizations like the U.S. military and the American College of Sports Medicine to modify activity based on heat stress. For instance, a WBGT reading below 80°F is considered a low-risk “green flag” condition with no restrictions on activity.
As the WBGT rises, the risk increases. A reading between 85°F and 88°F may trigger a “red flag,” limiting strenuous work to 30 minutes per hour. A WBGT value of 90°F or higher often corresponds to a “black flag” condition, where all non-essential outdoor physical activity is halted. These thresholds can vary by region to account for local climate acclimatization.
WBGT Versus Heat Index
A common point of confusion is the difference between WBGT and the heat index. The heat index, often called the “feels like” temperature, combines air temperature and relative humidity to provide a reading for conditions in the shade. It is a useful metric for the general public during daily activities in shaded areas.
The WBGT is a more comprehensive tool for assessing heat stress for individuals active in direct sunlight. Unlike the heat index, WBGT incorporates the heating effect of direct solar radiation through the globe temperature reading. It also accounts for the cooling effect of wind, captured in the natural wet-bulb measurement.
This makes WBGT a superior indicator for protecting athletes, outdoor laborers, and military personnel. The heat index can underestimate the true heat stress on a person exercising or working on a sunny day because it omits the significant heat load from the sun. Including solar radiation and wind allows WBGT to provide a more accurate assessment.
Practical Applications and Guidelines
The development and implementation of WBGT originated within the U.S. military in the 1950s as a response to a high number of heat-related illnesses among soldiers during training. The military continues to use WBGT to establish work-rest cycles, water intake requirements, and activity levels based on the measured heat stress, significantly reducing heat casualties.
In the civilian sector, the Occupational Safety and Health Administration (OSHA) recommends WBGT to protect workers in industries with high heat exposure, such as construction and agriculture. OSHA’s guidelines help employers implement protective measures, such as providing more frequent breaks, ensuring access to shade, and adjusting work schedules to avoid the hottest parts of the day.
Athletics has also widely adopted WBGT to ensure player safety. Governing bodies, from high school associations to professional leagues, use WBGT to guide decisions on practices and competitions. For example, guidelines may dictate modifying practice length, removing protective equipment to enhance cooling, or canceling an event when WBGT readings reach dangerous levels.