With a persistently steamy air mass and triple-digit heat indexes forecast to linger, the National Weather Service has extended its heat warning, and the city’s first heat emergency of the season remains in effect until 11:59 p.m. Wednesday.

In addition, the Delaware Valley Regional Planning Commission issued an air-quality alert for Wednesday, warning of high concentrations of pollutants.

Tuesday’s official high at Philadelphia International Airport, 94, was 10 degrees shy of the record for the date, 104 — set back in 1934 — but at mid-afternoon it felt like 103 thanks to all that water vapor in the air, which likely kept the temperature from going higher. When it’s humid, some of the sun’s energy is diverted toward evaporation.

An encore is due Wednesday, with highs again in the mid-90s and triple-figure heat indexes.

The city has opened seven cooling centers, and SEPTA has made a fleet of cooling buses available. In addition, the Philadelphia Corporation for Aging is operating its heatline, 215-765-9040, until midnight Tuesday and from 8:30 a.m. to midnight Wednesday.

Languid winds allowed heat indexes to creep past 100 across the region Monday, and late in the afternoon the National Weather Service issued a heat warning, in effect until 8 p.m. Tuesday for the city, adjacent areas, and northern Delaware.

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Temperatures might not get below 75on Wednesday morning, and warm nights are particularly perilous to the elderly who live alone in rowhouses without air-conditioning. Without overnight cooling, those homes heat up in a hurry when the sun comes up.

As so often happens as the region approaches what is climatologically the hottest period of the year, winds from the south on the western flank of high pressure over the Atlantic are swamping the air with water vapor, making the heat all the more enervating.

Although the heat wave isn’t expected to last the workweek, it likely will be the longest stretch to date of 90-degree days in what so far has been a generally benign start to summer. June temperatures have been only about a degree above normal, and Philadelphia has yet to record a heat-related death.

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The heat could break as early as Thursday, with a potentially robust dousing from heavy showers. The forecast becomes a muddle for the end of the week, meteorologists said, but nature might provide some fireworks of its own during the July Fourth weekend, forecasters say.

In the meantime, with the heat Wednesday, officials are offering a standard regimen of advice: Keep hydrated, limit outdoor exercise, take shade breaks if working outside, and look in on elderly relatives and acquaintances.

Officials also warn against leaving children and pets in locked cars. “A car is like a greenhouse,” said Janette Fennell, of Bala Cynwyd, founder and president of Kidsandcars.org, which has recorded more than 1,000 deaths of children left in hot cars in the United States since 1970. She added that countless other children have been traumatized or seriously sickened while stuck in ultrahot vehicle interiors.

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On Monday, Mike Silva, a lead meteorologist at the National Weather Service Office in Mount Holly, measured a temperature of 158 Fahrenheit inside his car at the agency’s parking lot. (A stuffed llama he had strapped into the passenger seat showed no sign of stress.) On Tuesday, the temperature approached 190.

Silva said the point of the exercise is to draw attention to the hazards.

“You wouldn’t put a kid in the oven,” he said. “Don’t put a kid in a hot car.”

Emergency measures

  • PCA Heatline: 215-765-9040

  • Library cooling centers (masks recommended):

    • Until 7 p.m. Tuesday and Wednesday: Frankford Library at 4634 Frankford Ave.; Lillian Marrero Library at 601 West Lehigh Ave.; Widener Library at 2808 West Lehigh Ave.

    • Until 8 p.m.: Blanche A. Nixon Cobbs Creek Library at 5800 Cobbs Creek Parkway; Fox Chase Library at 501 Rhawn St.; Lucien E. Blackwell Regional Library at 125 S. 52nd St.; Whitman Library at 200 Snyder Ave.

  • SEPTA cooling buses: Germantown and Allegheny Avenues; Wyoming and Rising Sun Avenues; 52nd Street and Larchwood Avenue; Broad and Snyder Streets; 22nd and Moore Streets; North 29th and York Streets

  • Spraygrounds and pools: Residents are also encouraged to visit any of the Parks and Recreation Department’s 92 spraygrounds. Pools start to open Wednesday.

  • Residents can find all of the identified cooling centers, as well as pools and spraygrounds, on this map or by calling 311.