Wet-bulb temperature (TW) is the temperature recorded by a thermometer whose bulb is wrapped in a wet wick and exposed to an air current. It represents the lowest temperature achievable by evaporative cooling: the drier the air, the greater the difference between the dry-bulb (conventional) temperature and the wet-bulb temperature.
Human survival limit
TW has acquired enormous relevance in the context of climate change because it defines the absolute limit of human thermoregulation. At a TW of 35 °C, the human body cannot dissipate metabolic heat through sweating (the air is so warm and saturated that sweat does not evaporate), and death from hyperthermia is inevitable within hours, even at rest, in shade, with water available, and unclothed.
Until recently, TW values of 35 °C were considered theoretical, but they have now been briefly recorded in the Persian Gulf, Pakistan, and the Indo-Gangetic Plain. Recent studies suggest the functional threshold for healthy humans is even lower: 31-33 °C TW can already be fatal under physical exertion.
In Spain, heat waves with high humidity (Mediterranean coasts, Guadalquivir valley) can reach TW values of 25-28 °C, dangerous for vulnerable populations. TW is a far more reliable indicator of heat stress than conventional temperature. See also: wind chill, heatstroke.