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The wet-bulb temperature is the temperature read by a thermometer covered in water-soaked cloth over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature ; at lower humidity the wet-bulb temperature is lower than dry-bulb temperature because of evaporative cooling.
The wet-bulb temperature is defined as the temperature of a parcel of air cooled to saturation by the evaporation of water into it, with the latent heat supplied by the parcel. A wet-bulb thermometer indicates a temperature close to the true wet-bulb temperature. The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.
Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C , equivalent to a heat index of 55 °C. The theoretical limit to human survival for more than a few hours in the shade, even with unlimited water, is a wet-bulb temperature of 35 °C – equivalent to a heat index of 70 °C.