The measure mimics the process that cools the human body in a heatwave – and explains why high humidity is harder for us to weather
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.
