At the end of In the 1970s, a physicist and textile engineer from Texas named Robert Steadman published an article called “The Assessment of Sultriness”. The title reflected a kind of nasty steam – how temperature and humidity combine to make life hard on the body. To do this, he relied on a long history of experimentation. In the 18th century, people climbed into ovens heated to 250 degrees Fahrenheit to see how long they could suffer, watching steaks cook next to them. In the 19th and early 20th centuries, researchers observed people sweating in Turkish baths and reported from mines where they measured ambient conditions as workers collapsed from heat exhaustion. Later, the military took over more testing, deriving equations for how blood flow, sweat, and respiration respond to atmospheric extremes.
What was unique to Steadman was his intimate knowledge of clothing; he was known for projects like a universal sizing system for clothing and motors capable of spinning fine cotton yarn. After all, he theorized, people are rarely naked in the heat, so our perception must be mediated by a combination of physiology and clothing. His formulas assumed precise percentages of how much skin was covered by fabric and how specific mixtures of air and fibers would transfer heat from the air.
What’s surprising is that, for a set of calculations developed by a textile researcher, Steadman’s measure of how hot it feels has proven useful for weather forecasters, especially in the United States. In 1990, a National Weather Service scientist adapted them with the main Steadman features more or less intact. Henceforth, the heat sensation index has become more (or perhaps less) known as the “heat index”, although it is also sometimes referred to as “apparent temperature” or “actual sensation”. If you’ve been caught in this summer’s heat waves, this is probably a number you’ve consulted to better understand the tortures outdoors. It’s the measurement that’s supposed to include an overlooked factor in the human experience with heat: humidity. This moisture in the air slows the evaporation of sweat from your skin, an essential way to stay cool.
What made the Steadman index so successful is that the numbers felt right, in the literal sense. The heat index reads like a temperature, but it’s more wonky than that, a perception rooted in physiological reality. When two different combinations of heat and humidity give the same heat index, say 96 degrees Fahrenheit/50% humidity and 86 degrees/95% humidity, both of which have a heat index of 108, that means that the body in each scenario is under a similar level of stress as it attempts to calm down. As the heat index rises, the miracle of internal thermoregulation that sets our bodies at 98.6 degrees begins to break down. Our core temperature rises, which starts out unpleasant and then becomes dangerous. There is a window of about 10 degrees before all the chemistry that sustains life begins to fail. It means death.
But there’s a problem with Steadman’s calculations: they weren’t designed to handle these kinds of extreme conditions. At a certain threshold, which includes a presumably scorching combination of 80% humidity and 88 degrees Fahrenheit, the heat index veers toward predicting what David Romps, a physicist and climatologist at the University of California at Berkeley, calls “non-physical conditions” that rarely occur in the lower parts of the atmosphere. This includes supersaturated air coming into contact with the skin, i.e. air that is more than 100% water saturated.
Temperature and humidity conditions above this threshold are quite rare and when they occur, it is possible to extrapolate from the Steadman model to obtain an estimated value of the heat index. But estimates are estimates, and these types of heat waves are becoming more common as temperatures rise. So Romps and his graduate student, Yi-Chuan Lu, started taking a look at the fundamentals of the model. They soon realized that for the long list of assumptions in the equations, some things were missing. For one thing, there is a natural solution to the oversaturation problem: when the air is too humid for human sweat to evaporate, it can still bead and drip onto the skin, providing some relief.
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