Late In the 1970s, a Texas physicist and textile engineer named Robert Steadman published a paper called Evaluating Contempt. The title reflects an unpleasant kind of vapor – how temperature and humidity combine to make life difficult for the body. To do this, he relied on a long history of experiments. In the 18th century, people climbed into ovens that heated up 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 due to heat exhaustion. Later, the military picked up more tests, deriving equations for how blood, sweat, and breathing flow in response to extreme atmospheric conditions.
What was unique to Steadman was his intimate knowledge of clothes. He was known for projects such as the Universal Garment Size System, and engines that could spin fine cotton threads. After all, according to his theory, people are rarely naked in heat, so our perception of it must be mediated by a combination of physiology and clothing. His formula assumed exact percentages of how much leather is covered in fabric, and how a particular mixture of air and fibers can transfer heat from the air.
What is surprising is that for a range of calculations developed by a textile researcher, the Steadman Rudeness Scale has proven useful for weather forecasters, especially in the United States. In 1990 a scientist at the National Weather Service adapted it to fairly intact key Steadman features. Henceforth, the severity index became more (or perhaps less) known as the “heat index”, although it is sometimes called the “apparent temperature” or “real feeling”. If you’ve been caught in heat waves this summer, this is probably the number you consulted for a better understanding of the agony of the outdoors. It’s a procedure that supposedly includes a factor that’s been overlooked in the human experience with heat: humidity. This moisture in the air slows down the evaporation of sweat from your skin – an essential way to stay cool.
What made the Steadman indicator successful are those numbers Poetry Right in the literal sense. The heat index reads like temperature, but is more oscillating than that, a perception rooted in physiological reality. When two different combinations of heat and humidity produce 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—then the body in each scenario is exposed to a similar level of stress as He tries to calm down. As the heat index rises, the miracle of internal thermoregulation that holds our bodies at 98.6 degrees begins to unravel. Our core temperature rises, which starts out as annoying and then becomes dangerous. There is a window of about 10 degrees before life-sustaining chemistry begins to fail. This means death.
But there’s a problem with Steadman’s calculations: They weren’t actually built to handle those kinds of extreme conditions. At a certain threshold — one that includes a reasonably steamy mixture of 80 percent humidity and 88 degrees Fahrenheit — the heat index deviates to predict what David Romps, a physicist and climatologist at the University of California, Berkeley, calls “non-physical conditions” that rarely occur in lower parts of the atmosphere. This includes excessively saturated air in contact with the skin – that is, air that is 100% saturated with water.
Temperature and humidity conditions exceeding this threshold are fairly rare – and when they do occur, it is possible to extrapolate from the Steadman model to arrive at an estimated heat index value. But estimates are estimates, and these types of heat waves are becoming more common as temperatures rise. So Romps and his graduate student, Ye Chuan Lu, began taking a look at the basics of form. They soon realized that for the long list of assumptions in the equations, certain things were missing. For one thing, there’s a natural solution to the problem of oversaturation: When the air is too humid for human sweat to evaporate, it can still drip and drip off the skin, providing some relief.
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