The “Dog Days” of summer — so named for the appearance of Sirius, the “dog star,” which was so bright that the ancient Romans believed it contributed to summer heat — technically end Wednesday.

But for the Philly region, a “perfect example” of the sultry conditions popularly associated with the Dog Day concept will be persisting through the workweek, said AccuWeather Inc. senior meteorologist Tom Kines, with triple-digit heat indexes Wednesday, Thursday, and Friday.

Painting the local weather map with a rather foreboding crimson, the National Weather Service has issued an “excessive heat warning” for the entire region, save for southeastern New Jersey, through Thursday, when the heat index might go as high as 110.

And with the relief-promising taking its time getting here, the warning might be extended through Friday, said Jonathan O’Brien, meteorologist at the weather service office in Mount Holly. Philadelphia has declared a heat emergency in effect from noon Wednesday through 8 p.m. Friday. it will open cooling centers, and the Philadelphia Corporation for Aging will operate its “heatline.”

With high pressure over the Atlantic supplying the heat and moisture, the air is well-cured with energy-depleting water vapor. That also means random downpours are possible each day, with a better shot at more-organized, stronger storms Friday and perhaps into Saturday, O’Brien said.

So far five people have died from heat-related causes this summer in Philadelphia, said health department spokesperson Jim Garrow. No details were released.

The summer of 2021

O’Brien pointed out that the heat had been on something of a summer break, with temperatures for the first eight days of August averaging nearly 4 degrees below normal. July’s readings at Philadelphia International Airport finished just a shade above normal, 0.2 degrees.

While the city has experienced four heat waves — defined as three or more consecutive days of official high temperatures of 90 or better — they have been short-lived.

In July, the average daytime high, 87.3, was 0.5 degrees below the 30-year normals. But the low, 70.4, was 0.8 above the normal.

Warmer nights have been a trend, O’Brien noted, and that probably is related to increased levels of water vapor as the planet has been warming; warmer air can hold more moisture.

» READ MORE: Philly’s summer temps have risen 3 degrees since 1970 — and nights have gotten even warmer

At night, that water vapor can retard the escape of daytime heating, especially in the city with its sun-soaking buildings and paved surfaces, presenting a special hazard to some of the vulnerable elderly population.

Conversely the humidity likely has trimmed a few degrees from daytime highs.

Temperatures have not reached 100 in Philadelphia since 2012, and that streak should stay intact this week. In addition, Philadelphia hasn’t experienced a heat wave of 10 days or more since 2011.

Frequent afternoon showers would make those streaks hard to sustain, Kines said.

One positive trend, here and elsewhere around the country, is a decline in heat-related deaths. In the 10-year period that began in 1993, 399 deaths were blamed on heat. In the last decade, the city has averaged fewer than 10 annually

About the Dog Days

While it would be reasonable to conclude that “Dog Days” have to do with panting, overheated, and stressed canines, the concept actually is astronomical.

» READ MORE: Perseid meteor shower approaching peak, and the moon is getting out of the way

Between July 3 and Aug. 11, Sirius, the brightest star in the Canis Major constellation, rises around dawn, more or less in conjunction with the sun. Our ancestors put one and one together, and figured that two rising stars would make it hotter.

We all make mistakes.

Even though the Dog Days are over astronomically on Wednesday, the heat has been known to persist.

The infamous bank holdup that was the impetus for the movie Dog Day Afternoon occurred on Aug. 22, 1972.

Staying cool

  • The PCA Heatline (215-765-9040) will operate from noon until midnight Wednesday, 8:30 a.m. until midnight Thursday, and 8:30 a.m. to 8 p.m. Friday.

  • Library cooling centers, masks required, open until 7 p.m. Wednesday through Friday: Frankford, 4634 Frankford Ave.; Haddington, 445 N. 65th St.; Lillian Marrero, 601 W. Lehigh Ave.; Paschalville, 6942 Woodland Ave.; and Widener, 2808 W. Lehigh Ave.

  • Library cooling centers, masks required, open until 8 p.m. Wednesday through Friday: Blanche A. Nixon Cobbs Creek, 5800 Cobbs Creek Parkway; Fox Chase, 501 Rhawn St.; Joseph E. Coleman Regional, 68 W. Chelten Ave.; Lucien E. Blackwell Regional, 125 S. 52nd St.; West Oak Lane, 2000 E. Washington Lane; and Whitman, 200 Snyder Ave.

  • Cooling buses, 11 a.m. to 7 p.m. Thursday and Friday, masks required: Germantown and West Hunting Park Avenue, West Wyoming and Rising Sun Avenues, and South Broad Street and Snyder Avenue.

  • Also residents are encouraged to visit any of the city’s 92 “spraygrounds.”