Wet-bulb globe temperature

WBGT is used to assess potential heat stress in workplaces, sports and other outdoor activity. It is not simply the air temperature and should not be confused with wet-bulb temperature alone. Depending on the instrument and setting, the index combines a natural wet-bulb measurement, a black-globe measurement of radiant heat, and a dry-bulb air-temperature measurement. These components help represent evaporative cooling, radiant heat from the Sun or nearby sources, and ambient temperature.

The body's heat burden depends on more than the temperature reported by a standard thermometer. Humidity can limit cooling through sweat evaporation; sunlight and hot equipment add radiant heat; and air movement affects cooling. Physical exertion also produces metabolic heat, while clothing and protective equipment can impede heat loss. The U.S. Occupational Safety and Health Administration therefore advises assessing both environmental heat and the workload rather than treating a weather reading as a complete measure of risk.

The heat index combines air temperature and relative humidity to describe how hot conditions feel under specified assumptions, including shade and light wind. WBGT is intended to account for additional environmental factors, including radiant heat and wind, and is particularly useful for strenuous work or activity in direct sunlight. The two measures are not interchangeable; the appropriate measure depends on the exposure and purpose. Forecast values may also differ from conditions at a particular worksite.

Occupational health and safety guidance uses WBGT to help plan work and activity in hot environments. Measurements should be taken near the actual exposure, following the instrument's setup and calibration instructions. A WBGT value is one part of a heat-risk assessment, not a guarantee of safety: workload, clothing, acclimatization and individual circumstances also matter. Safety programs may use the assessment to guide work-rest schedules, shade, hydration and other preventive steps.