The wet-bulb temperature, which is used by meteorologists to assess dangerous heat stress levels, is measured by wrapping a moist muslin wick around the bottom of a thermometer. As the water evaporates from the cloth it cools the thermometer bulb, causing the temperature reading to drop. When this process stops the wet-bulb temperature is reached.
It is exactly the same process that cools the human body during a heatwave. As sweat evaporates from the skin it takes away the heat. As anyone who has been to the tropics knows, a dry heat means humans can stand a much higher temperature than when the relative humidity or wet-bulb temperature is high. In a dry heat, drinking water and evaporation from the skin will keep you comfortable, but in higher humidity people can literally drip with sweat and suffer heat stress.
These last few days in southern England have been an example of the wet-bulb temperature indicators in action. Temperatures were in the low 30s centigrade during the day, and they remained as high as 20C at night with relatively high humidity levels, which meant that evaporation of moisture from the skin was slower. This led to discomfort for many, but possible danger to older people and very young children, who have more difficulty regulating body temperature.
