Heat Index and Wet-Bulb Globe Temperature (WBGT) are both tools for assessing environmental heat stress, but they measure different things and serve different purposes. Heat Index estimates how hot it feels by combining air temperature and humidity—it's the number you typically see in weather forecasts and public health warnings. However, Heat Index assumes you're in the shade and doesn't account for direct sunlight, wind speed, or reflective surfaces. WBGT is a more comprehensive measurement that factors in air temperature, humidity, wind speed, sun angle, and solar radiation. It provides a more complete picture of the actual heat stress environment, especially for outdoor activities. This is why athletic organizations like the NCAA and state high school associations use WBGT rather than Heat Index for making practice and competition decisions—it better reflects the real-world conditions athletes experience on fields, courts, and tracks.

Sports organizations prefer WBGT because it accounts for the specific environmental factors that affect athletes during outdoor training and competition. Unlike Heat Index, which assumes shaded conditions, WBGT incorporates solar radiation and sun angle—critical factors when athletes are exposed to direct sunlight on playing fields. WBGT also factors in wind speed, which affects how effectively sweat can evaporate for cooling, and uses globe temperature measurement to capture radiant heat from all sources, including hot artificial turf or other reflective surfaces. These additional variables make WBGT significantly more accurate for assessing the actual heat stress environment athletes face. A day might have a moderate Heat Index but a much higher WBGT reading due to intense sun exposure and minimal wind—conditions that create real heat stress for athletes even though the weather forecast seems manageable. This precision is why organizations use WBGT-based thresholds to guide decisions about practice modifications, rest breaks, and activity restrictions.

WBGT uses specialized measurement devices that capture three key temperature readings, each reflecting different aspects of environmental heat stress. Dry-bulb temperature is standard air temperature measured in the shade—the basic temperature reading you're familiar with. Wet-bulb temperature reflects the cooling effect of evaporation and is influenced by both humidity and airflow—when humidity is high, wet-bulb temperature is closer to dry-bulb temperature because sweat can't evaporate as effectively. Globe temperature captures radiant heat from sunlight and surrounding surfaces using a thermometer inside a black sphere that absorbs heat the way objects (including athletes) do in the environment. For outdoor conditions with sunlight, WBGT weighs wet-bulb temperature most heavily, recognizing that humidity and evaporative cooling capacity are the dominant factors in how athletes experience heat stress. This multi-component approach makes WBGT a more sophisticated and accurate assessment tool than simple temperature readings alone.

Most standard weather apps and forecasts report Heat Index rather than WBGT, so you typically cannot rely on your phone's weather app for WBGT readings. Measuring true WBGT requires specialized equipment that can capture wet-bulb temperature, globe temperature, and dry-bulb temperature simultaneously at your specific location. However, many athletic programs and schools now have access to WBGT monitoring devices specifically designed for sports settings, and some weather services provide WBGT estimates for specific locations. There are also WBGT calculators that can estimate readings based on available weather data, though these are less accurate than on-site measurement. For athletic programs making decisions about practice modifications or activity restrictions, on-site WBGT measurement is ideal because conditions can vary significantly between the weather station location and your actual practice field, especially when considering factors like artificial turf, stadium structures, or local wind patterns.