Wet-Bulb Temperature
The lowest temperature evaporative cooling can achieve under current humidity, measured by a thermometer wrapped in a wet cloth.
Wet-bulb temperature is the temperature a thermometer reads when wrapped in a wet cloth and exposed to airflow. It represents the lowest temperature evaporative cooling alone can reach. At 100% humidity, wet-bulb equals air temperature (no evaporation possible). At lower humidity, wet-bulb is lower than air temperature.
The 95 F wet-bulb is widely cited as the physiological survival threshold. Above wet-bulb 95 F, a healthy adult cannot maintain core body temperature even with shade, water, and rest, because the body cannot shed metabolic heat fast enough.
Wet-bulb is the preferred metric for athletes and outdoor workers because it accounts for the actual cooling potential of the environment. Some areas of the world (Persian Gulf, parts of South Asia) have started briefly exceeding wet-bulb 95 F in extreme heat events.
The wet-bulb globe temperature (WBGT) extends the concept by also factoring in direct solar radiation and wind. WBGT is what athletic governing bodies and the US military use for activity decisions.