It will feel like 109 in Philly Thursday, maybe 125 in the sun, forecasters say. But how do they know?

Philadelphia is about to experience perhaps its hottest day of the summer, forecasters are saying, with a high of 97 degrees, near the record for the date, 99. By 1 p.m., the official temperature already had cracked 90.

But more significant, they say, it is going to “feel” several degrees hotter with heat index values ascending to 109 during the afternoon in the city — and that’s in the shade. In the sun, it might feel more like 125 at Citizens Bank Park, where the Phillies are hosting the Dodgers, and elsewhere.

“That’s really, really nasty,” said Gregory Carbin, chief of the National Weather Service’s forecast-operations branch. With an encore expected, an “excessive heat warning” remains in effect through 8 p.m. Friday.

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The heat index is a function of the temperature and humidity, which is the amount of moisture in the air. Sweat gives off a cooling effect when it evaporates; high humidity inhibits sweating and that will be the case Thursday.

But is it really going to feel like 109?

For complexity, the human body can go toe-to-toe with the atmosphere. That’s one reason that Laurence Kalkstein, a climatologist who helped Philadelphia develop its heralded heat-emergency system, doesn’t like the “one-size-fits-all” heat index.

The outdoors are going to feel differently to a linebacker type who just consumed a Wawa hoagie and is jack-hammering a sidewalk, compared with the person who is cooling off with coconut water after climbing off a yoga mat.

A healthy teenager isn’t going to experience heat the same way as an elderly person with a respiratory illness, for whom it could be life-threatening.

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The weather service warning calls for heat-index values up to 109 from Doylestown to Glassboro, areas that can have quite diverse conditions on any given day. For that matter, so can South Philly and Center City.

The 109 figure is more about message than precision. Said Nicholas Carr, a weather service meteorologist in Mount Holly, “It sounds scary.”

But it does have strong mathematical and scientific lineage.

The calculations are based largely on the work of Robert G. Steadman, an Australian who was a textile researcher at Texas Tech University.

He published a two-part technical treatise on the subject in 1979, and it is not necessarily beach reading. Steadman’s work was “groundbreaking,” said former colleague Richard Peterson, a Texas Tech emeritus professor of atmospheric sciences.

While the temperature-humidity relationship might seem simple enough, its origins were quite complex.

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Steadman had worked up an equation for the heat index using several factors. He assumed his subject was 5-foot-7 and weighed 147 pounds (we’ve seen bigger and smaller); was wearing long trousers and short-sleeve shirt, with material that was 80% air; and was walking 3.1 mph.

Here is his actual equation, if you want to try this at home; the “t” and “r” variables represent temperature and humidity.

Heat Index = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783 x 10⁻³T² - 5.481717 x 10⁻²R² + 1.22874 x 10⁻³T²R + 8.5282 x 10⁻⁴TR² - 1.99 x 10⁻⁶T²R²

On the other hand, this calculator is a whole lot easier to use.

Steadman, by the way, eventually returned to Australia after he was denied tenure at Texas Tech, Richardson said.

Other heat indexes are out there, including the “Heat Risk” scale being used on an experimental basis in the West and South. It rates heat hazards on a 0-to-5 scale from “very low” to “very high,” relative to other conditions, such as whether people in a given area are acclimated to heat.

Carbin points out that while the sultry days and ultra-warm nights present a hazard to those especially vulnerable to heat, at this point in the summer most people in the Philadelphia region should be reasonably accustomed to temperatures in the 90s. That might not be the case, say, in Portland, Maine.

The “Heat Risk” scale is not being used in the East. “Some people think it’s over-aggressive at times,” said Carr, who transferred to Mount Holly from Utah, and has some experience with the scale.

Carr likes the “WetBulb Global Temperature” concept, which takes into account sunlight and requires a more complex calculation than the heat index. The weather service issues daily forecasts.

The index is the basis for the American College of Sports Medicine’s guidelines for athletic practices.

However, the scale likely would confuse the general public, said Carr, since it uses a different numerical scale ranging from 80 to 90-plus, which probably wouldn’t sound particularly dangerous to most folks, like, say, 109.

One could use “a lot of different approaches” to characterize the discomforts of the next few days, said Carbin.

But, “at the end of the day, it’s going to be hot.”

This story contains corrections to the heat-index formula.