Late In the 1970s, a Texas physicist and textile engineer named Robert Steadman published a paper entitled The Assessment of Sultrines. The title reflected an uncomfortable nature of vaping – how temperature and humidity combine to make life miserable for the body. In doing so, he drew 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 while watching steaks cook alongside them. In the 19th and early 20th centuries, researchers observed people sweating in Turkish baths and reported mines where they measured environmental conditions when workers collapsed from heat exhaustion. Later, the military took on more testing and derived equations for how blood flow, sweat, and respiration respond to atmospheric extremes.
What was unique to Steadman was his intimate knowledge of dress; He was known for projects such as a universal size system for clothing and motors that could spin fine cotton yarn. After all, he theorizes, people are rarely naked in the heat, so our perception of it must be mediated through a combination of physiology and clothing. His formulas assumed precise percentages of how much skin would be covered by cloth and how certain mixtures of air and fibers would transfer heat from the air.
Surprisingly, Steadman’s measurement of mugginess proved useful for weather forecasters, particularly in the United States, for a series of calculations developed by a textile researcher. In 1990, a National Weather Service scientist adjusted them, leaving Steadman’s key features more or less intact. Henceforth, the mugginess index has been more (or perhaps less) succinctly referred to as the “heat index,” though it’s also sometimes referred to as the “apparent temperature” or “true feeling.” If you’ve been caught out by this summer’s heatwaves, this is probably a number you’ve consulted to better understand the outdoor torment. It’s the metric meant to capture an overlooked factor in the human experience of heat: Humidity, wetness in the air slows the evaporation of sweat from your skin — an important way to stay cool.
What made Steadman’s Index successful were the numbers felt right, in the truest sense of the word. The heat index reads like a temperature, but it’s shakier than that, a perception rooted in physiological reality. When two different combinations of heat and humidity result in the same heat index — say, 96 degrees Fahrenheit/50 percent humidity and 86 degrees/95 percent humidity, both of which have a heat index of 108 — it’s meant to signal that the body is in either scenario is under a similar level of stress while trying to cool down. As the heat index rises, the miracle of internal thermoregulation that fixes our body at 98.6 degrees begins to crumble. Our core temperature rises, which becomes uncomfortable at first and then dangerous. There is about a 10 degree window before all the chemistry that sustains life begins to fail. That means death.
But there’s a problem with Steadman’s calculations: They weren’t actually built to deal with such extreme conditions. At a certain threshold — one that encompasses a plausibly muggy combination of 80 percent humidity and 88 degrees Fahrenheit — the heat index transitions into predicting what David Romps, a physicist and climate scientist at the University of California, Berkeley, calls “unphysical conditions ‘ are rarely found in the lower parts of the atmosphere. This includes supersaturated air that comes into contact with the skin—that is, air that is more than 100 percent saturated with water.
Temperature and humidity conditions beyond this threshold are fairly rare – and when they do occur it is possible to extrapolate from Steadman’s model to obtain an estimated Heat Index value. But estimates are estimates, and these types of heatwaves are becoming more common as temperatures rise. So Romps and his graduate student Yi-Chuan Lu started looking at the basics of the model. They quickly found that the long list of assumptions in the equations was missing a few things. For one, there’s a natural solution to the satiety problem: when the air is too humid for human sweat to evaporate, it can still bead and drip off the skin, providing some relief.