Why is everybody Talking about “Wet Bulb Temperatures”?

Well, we’re halfway through 2021, so the Northern Hemisphere is melting, keeping up with the growing horror tradition. The Arctic Circle has reached temperatures of about 50 degrees Celsius (120 degrees F) and an unprecedented heatwave has caused hundreds of deaths in Canada and the Pacific Northwestern United States, with people asking: “What is the temperature of wet bulbs?” Have you ever been outside New York on a 30 degree centigrade (86 degree F) day, immediately regretting and wondering how anyone on earth could possibly endure a Phoenix 43 degree centigrade (110 degree Fahrenheit) summer?

Or maybe you saw the British annual practice of how unbearable their 35 degree centigrade (95 degree F) heatwave became and thought that “hot” was actually something different in England. The thing is, not all heat is created equally and the same temperature can be truly pleasant in one city and pure oppression in another. In conversational terms, you’ve heard people talk about “dry heat” being more pleasant than “wet heat”. What makes the difference is often the humidity: the concentration of water vapor in the air.

If we compare it with the concentration of water vapor, the air can potentially hold at the current temperature, but we get relative humidity. For example, one percent relative humidity compared to one day – the lowest recorded on Earth – contains one percent of water vapor in the air which it could potentially. However, with 100% relative humidity in one day, the air is fully saturated and cannot take anything else. But it can be a big problem, because people cool themselves with sweat: the surrounding heat evaporates sweat from our skin and it prevents us from getting too hot. If the relative humidity is already close to 100 percent, the air cannot take in anything more. Our sweat does not evaporate easily and we cannot cool down. This makes humid heat not only uncomfortable, but also dangerous.

“Physiologically, there is a point when heat and humidity not only become uncomfortable, but in reality it is impossible to be inspired,” Colin Raymond, lead author of the 2020 study on intense heat and humidity, told the American Association for the Advancement of Science (AAAS). “[Laughs] the situation is already going on and only getting worse.” And that’s where the temperature of the wet bulb comes in. Instead of looking at how hot the temperature is, a wet bulb temperature is taken by first wrapping the bulb of the thermometer in a wet cloth – so, “wet bulb”. The fabric acts as a kind of proxy for human skin: if water evaporates, the thermometer cools down and the temperature of the wet bulb will be lower than the air temperature.

However, at high humidity, when water cannot evaporate, this does not happen. Basically, the temperature of a wet bulb can be thought of not only as a measure of how hot it is, but also as a measure of how well humans can expect to cope with it. Theoretical human limit for wet-bulb temperature is 35 ° C (95 ° F) – close to human skin temperature. Even if it is heated more and the body is given limited water and shade, additional heat will start. Fortunately, temperatures on Earth are rare, but never heard of.