In recent days, you may have seen or heard many mentions of a metric of discomfort called the heat index, which purports to measure how hot the air actually feels, based on the temperature and humidity.
You also may have noticed that the heat index is a higher number than the temperature — sometimes impressively, oppressively higher. For example, as I type these words, the temperature in Central Park is 98, but the heat index is 105. Basically, it feels hotter when it’s more humid because more moisture in the air means that less perspiration evaporates from your skin.
But what, exactly, is the heat index, and how did “they” figure it out?
It turns out that the heat index is simply a more computationally intensive variant on the maxim “it’s not the heat, it’s the humidity,” in which, if T is temperature and H is relative humidity, the maxim becomes this:
“It’s not just the heat, it’s (T × 2.049) + (H × 10.143) – (T × H × 0.225) – (T² × 0.007) – (H² × 0.055) + (T² × H × 0.001) + (H² × T × 0.001) – (T² × H² × 0.000002) – 42.379.”
And how, you may wonder, did they come up with that? We asked the author of the equation. His name is Lans Rothfusz, and he’s a retired National Weather Service meteorologist and recipient of the National Weather Association’s 2014 operational achievement award.
Mr. Rothfusz, 66, said he developed the heat index equation in 1990 when he was “a punk intern” at the Weather Service’s Southern Region Headquarters in Fort Worth.
He said that he was merely standing on the shoulders of a heat-measuring giant: an Australian textile engineer named Robert G. Steadman, who in 1979 published a paper called “The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science.”
Dr. Steadman died in 2022, but his daughter, Jennifer Steadman, said Thursday that he developed an apparent temperature scale because he was a competitive racewalker who needed to figure out how to dress while training and found existing temperature models to be “wholly inadequate.” He came up with a chart that took into account many variables. All Mr. Rothfusz did was use a math technique called multiple regression analysis to develop an equation that yields, approximately, the Steadman chart.
In his 1990 technical paper for the Weather Service, Mr. Rothfusz listed some of the Steadman sultriness parameters that are folded into the equation, including convection from the surface of the skin (“influenced by kinematic viscosity of air”) and vapor pressures of skin and clothing.
He also noted that the Steadman calculation was based on a set of assumptions: A person is assumed to be 5-foot-7 and 147 pounds. Clothing cover is assumed to be long trousers and a short-sleeve shirt, covering 84 percent of the skin. Activity level is assumed to be walking at 3.1 miles an hour.
“It is far more complicated to understand the heat index for individuals,” Mr. Rothfusz said on Thursday, “because everybody’s different — different weight, height, that sort of thing.” The Weather Service’s heat index, in other words, may not be yours.
The Weather Service also does not account for variables like sunshine in its heat index.
But another meteorological outfit does. The private forecasting juggernaut AccuWeather has a product called RealFeel that factors in, among other things, sunshine, wind speed, precipitation and cloud cover. But exactly how it weights these things is a trade secret, much like Colonel Sanders’s blend of 11 herbs and spices.
“The RealFeel Temperature is protected by two patents, which ensure that no other index can include temperature and more than one other factor,” AccuWeather warns.
Mr. Rothfusz, whose work is public because it was produced for the government, remains impressed by the reach of his calculation.
“I published this little paper that was just intended to communicate it within the National Weather Service,” he said, “and suddenly it just became the thing.”
We asked if he felt a rush of pride when he heard people talking about the heat index.
“I’m just glad that the Weather Service is still communicating that information,” he said.
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