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Heat Waves are feeling hotter, but scientists are measuring them wrong, too

Our bodies sense heat differently when compared to the temperature of the air we’re breathing.

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While it is common knowledge that the Earth is becoming hotter, there are other factors that can affect how hot we really feel. According to new research, heatwaves can feel up to 10°C (18°F) hotter than typical estimates suggest because of shifting environmental factors and peculiarities in our physiology. The apparent temperature, also known as the heat index, is a measurement tool used by the US National Weather Service (NWS) to determine how various environmental circumstances physically feel to us. However, the system’s edges have been stretched to their limit by the new weather extremes we are currently seeing. Our brain’s calculations of the relative temperature would be influenced by the physiological reactions of sweating and chilling. Robert Steadman, a physicist, developed the heat index scale in 1979 by observing how various temperatures affect our skin’s blood flow under various humidity levels. Therefore, a typical human body in the shade would perceive 20°C as 20°C with an average humidity of about 70 percent. The body would progressively rely on its evaporating sweat to cool off at higher temperatures, making it feel hotter than it actually was. For instance, a temperature of about 30°C could feel more like 34.5°C. At high humidity levels, this difference simply gets worse. Even if temperatures in the UK only reached levels that other parts of the world would consider to be reasonably typical for July, the humidity factor is one of the reasons the country suffered with recent heatwaves. The harder it is for our bodies to use sweat to cool us off through evaporative cooling, the higher the humidity. In order to release heat, our bodies also circulate blood through veins that are closer to the surface of our skin. According to a study published earlier this year, our tolerance for excessive heat and humidity is significantly worse than previously believed, with a maximum temperature tolerance of only 31°C at full humidity. Unfortunately, water vapor also rises by about 7% for every degree Celsius that our atmosphere warms. Researchers use the heat index to predict the physiological effects of future warming, and the NSW regularly uses it to provide public warnings. In most cases, this measurement is a good representation of how various environmental factors affect us. However, the heat index was never intended for the heat and humidity extremes that we are currently experiencing—extremes that are getting worse and happening more frequently. For instance, a relative humidity of 80% was only physiologically mapped for temperatures between 15 to 31 °C, but today, temperatures in various parts of the US climb beyond 32 °C for extended periods of time. Unfortunately, expanding the computations by using the same method under more extreme circumstances does not correspond to what actually occurs physically to our body. “The heat index that the National Weather Service provides you with is usually accurate. Only in these rare instances do they receive the incorrect figure, “explains University of California, Berkeley climate physicist David Romps. “When you start to relate the heat index to physiological conditions, you discover that these folks are under a lot of stress and that their skin blood flow is quite increased, meaning that their body is almost at its limit in terms of how to deal with this level of heat and humidity. Consequently, we are now closer to that edge than we previously believed.” We are currently observing this measure being up to 10°C off on the hottest days. Romps and Yi-Chuan Lu of the University of California, Berkeley, extended the heat index earlier this year by taking into account human physiology as well. The blank area where Steadman said the human model failed was on the original table, which had a very narrow range of temperature and humidity, claims Lu. “Steadman used the appropriate physics. In order to develop a more precise formula, our goal was to expand it to include all temperatures.” When we are unable to perspire further due to a skin surface humidity of 100 percent, Steadman’s model fails. The calculations Steadman developed might be pushed to new boundaries of temperature and humidity by realizing that we continue to replace the perspiration that drips free as well. Romps claims, “I’m no physiologist, but when it gets really hot, a lot of things happen to the body.” “In order to try to raise the skin’s temperature, you pull blood that would normally go to internal organs and send it instead to the skin, which stresses the body. The health dangers of extreme heat waves are unintentionally minimized by the rough computation that the NWS uses and that is commonly accepted.” Id=”attachment 73794″ align=”alignnone” width=”642″] Comparison graphs between the old and new heat index. (Yi-Chuan Lu and David Romps/UC Berkeley) The team’s modified heat index was applied to the top 100 heat waves between 1984 and 2020 in their most recent paper. They determined that, contrary to earlier reports, the Midwest has the nation’s most physically dangerous heat. The Midwest was notorious for having moist soils during its most severe heatwaves, including one in particular in July 1995, which was symptomatic of the high humidity that contributed to 465 deaths. The new index indicates that the increase in cutaneous blood flow was more like 170 percent, as opposed to the old index’s prediction that persons would have seen a 90 percent rise. This was for those who were in the shade. The heat index is a crucial parameter to get right because heat waves are already the leading weather-related cause of death in the US, having a particular negative impact on older people and people who must work outside. A 200°F [93°C] heat index is the most that may be tolerated, according to Romps. “What does that actually mean for the future habitability of the United States and the globe as a whole, though, now that we have a model of human thermoregulation that functions well under these circumstances? Things we are looking at are quite awful.” Environmental Research Letters reported the results of this study.