What it really means when they say it will feel like 105 in Philly. It’s probably not what you think.

Whether the “heat index” reaches 104 or hits 105 in Philly on a given day represents more than a degree of difference to the National Weather Service.

But would your body know the difference between a “feels like” temperature of 104 vs. 105?

And would your mind care?

» READ MORE: Has it ever been muggier than this in Philly? Yes, way muggier, but the trend is disturbing

As it had on Thursday, the weather service warned of health-threatening conditions on Friday, when the actual air temperature may reach the century mark for the first time in 11 years, and again on Saturday.

And as it did on Thursday, the heat index again reached triple digits in the city the next two days — that’s in the shade, by the way.

» READ MORE: These are the neighborhoods that feel it the worst in the heat

The index, with its sometimes eye-popping numbers, does have value for alerting the public to heat hazards, said Sarah Johnson, warning coordination meteorologist for the Mount Holly weather service office.

But the warming of the planet — and the World Meteorological Organization said Thursday that after weeks of wildfires and record heat, July is on target to become the hottest month on record — has coincided with rising interest in alternative measures.

“There’s certainly research afloat,” said Johnson.

While far from perfect, the heat index numbers are the prime triggers for determining whether the weather service will issue warnings.

The agency had issued a heat “warning” for Friday for Philadelphia, but why only a heat “advisory” for Saturday? Friday had merited a warning because the index was due to reach and exceed the 105 threshold. (It didn’t quite get there.) It was forecasted to top out at 104 Saturday — one shy of the warning level.

» READ MORE: It's a paradox: Summers are warmer but heat-wave deaths are down

The distinction was blurry enough for the city to include Saturday in the heat “emergency,” for all three days, with the opening of cooling centers and Philadelphia Corporation for Aging operating its “heatline” — (215) 765-9040.

The human body rivals the atmosphere for complexity, and no index is going to capture the effects of the heat and humidity on all body types and age groups and health profiles.

The heat index, based on the temperature and the humidity, is derived from the work of Australian Robert G. Steadman, a former textile researcher at Texas Tech University in the 1970s. For now it is the most commonly used measure of potential discomfort.

Steadman based his calculations on 15 “parameters,” including a person’s size and clothing, and wind speed. For simplicity, he applied assumed values — for size, 5-foot-7, 147 pounds; clothing, long pants and short-sleeved shirt; wind speed, about 6 mph.

Over the years his computations have been refined, and the weather service uses a rather involved equation to solve for the heat index. We’ll save the details for a cooler day, and you’re way better off consulting this calculator.

The heat index assumes that you’re in the shade: In the sun, add about 15 degrees Fahrenheit, the weather service said.

Among the alternatives, one index favored by some government officials, sporting event organizers, and meteorologists measures heat stress in direct sunlight.

The so-called Wet Bulb Globe Temperature, or WBGT, considers temperature, humidity, wind, cloud cover, and the amount of solar radiation, which varies day-by-day, moment by moment as Earth makes its orbit around the sun.

It is a valuable measure for outdoor activities, said Johnson, and Occupational Safety and Health Administration and the military have used it for years.

It is publicly available. However, it uses a different scale and a set of numbers that aren’t especially attention-getting and wouldn’t be “interchangeable” with the heat index, said Johnson.

For example, the WGBT forecast for Friday in Philly during the peak heating period would be 89. That looks harmless enough but under the guidelines, that’s in the “red” zone, meaning anyone working or exercising outside should limit time in direct sunlight to no more than 20-minute intervals, with 40-minute breaks for every hour in the sun. You can find the Philly forecast here by plugging in the city’s approximate latitude (39.8).

The WGBT does not address nighttime warming, which is of a particular hazard for the vulnerable elderly who live alone.

» READ MORE: The nights in Philly have been getting more sultry

The weather service also is looking at a “HeatRisk” system that would consider factors that include nighttime warming, along with how unusual the heat would be given the time of year and how long it will last.

It would use a color-coded scale, from green (go ahead and go out and play) to magenta, heat that is potentially life-threatening.

The system is still in the development phase.

The heat and mugginess are forecast to persist through Saturday, and then forget about the heat indexes for awhile.

Strong showers are possible Saturday night, but it appears that on Sunday July will end with one of the most splendid days of the summer of 2023, followed by encores during the first days of August.