Following record cold, below-zero wind chills, and another nuisance snow threat, the atmosphere evidently is aiming to spring forward to April.

After single-digit temperatures caused serious delays for school buses and public transit — “We had our challenges this morning,” SEPTA assistant general manager Scott Sauer said Thursday — the region is poised for one more day of well-below-freezing temperatures and perhaps a half-inch of snow Friday afternoon.

But readings could make a run at 60 early next week, meteorologists said. That would be normal for the first week in April.

In short, in the mercurial winter of 2018-19, the operative advice is that if you don’t like the weather, wait a few days.

“Transiency seems to be the character of the winter,” said Judah Cohen, a scientist with the Atmospheric & Environmental Research firm in Massachusetts. And, he said, he believes it has everything to do with one of his specialties, conditions in the Arctic.

The official low of 7 at Philadelphia International Airport just before midnight set a record for a Jan. 30, beating the 8 recorded in 1948. Thursday’s high, 18, was the lowest daily maximum temperature for a Jan. 31, and the daily average of 11.5 the lowest for a Jan. 31 in records dating to 1874.

The Philadelphia School District reported that many school buses were delayed Thursday morning, at least two of them for up to two hours and about 40 others from 60 to 90 minutes.

Kristin Luebbert, a humanities teacher at the U School, a high school in North Philadelphia, said attendance was abysmal — just three students showed up for her first period class, which typically has 15.

“A lot of them opted out, and I can’t blame them,” Luebbert said.

Luebbert sent one of her students to the nurse because 15 minutes after entering the building, the girl’s fingers were still frigid, stiff, and sore. Luebbert feared frostbite.

“She has gloves, but they were thin — not polar vortex gloves,” Luebbert said. “My kids aren’t prepared to be outside in this cold.”

Camden earlier decided to close its schools for the day.

Sauer said the deep freeze caused SEPTA its share of headaches. The Airport Line was down for 40 minutes, and service to Wilmington was out for a little longer.

He said equipment issues caused delays on the Broad Street Line, and “trolley services had some issues.”

Freezing temperatures forced the nuclear plant in Salem County, N.J., to shut down a reactor Wednesday night as ice accumulated on screens that filter the water that goes into the plant, Public Service Enterprise Group officials said Thursday.

The cold is due to ease Friday, with highs forecast in the 20s and lighter winds, but throughout the region, counties are maintaining their cold-weather alerts, and stepping up efforts to protect the homeless and get them into shelters.

“Even if we ever get to maximum capacity, we will never turn people away,” said Josh Kruger, communications director for the Philadelphia Office of Homeless Services. “There may not be beds available. But you’ll have a roof, perhaps a comfy chair.”

“Philadelphia isn’t the worst city for homeless people,” said Joshua Brown, 40, a self-described homeless man who has struggled with physical problems and addiction. “There’s a lot of options for shelters and programs.”

But Brown says he prefers to stay outside until the cold winds become unbearable, then look for relief in public places.

Kruger said that is not a wise plan.

“I can’t stress enough that the cold weather is a matter of life and death, especially for people with mental illness and others trying to minimize the pain of the cold,” he said. “We want to make sure everyone knows that there’s help available and there’s no reason for people to be outside in this weather."

With the renegade piece of the polar vortex heading back whence it came, the cold weather alerts should be lifted Saturday as temperatures head toward 40, then to the 50s on Sunday and Monday, and perhaps 60 on Tuesday.

Cohen said he expects the bipolar behavior to continue for a while. His research has correlated the well-documented warming of the Arctic with more frequent frigid outbreaks in the East as the polar regions export their cold.

The Arctic warming over the last several months, however, hasn’t been as robust or as uniform as in recent years, Cohen said, and that could explain the irregular spillages of Arctic air.

“It’s episodic," he said. “You get a finger of it, it wanes, then it returns again.”

While Philadelphia has had just 6.5 inches of snow so far, that’s about triple what Boston has had.

Cohen said that computer models are suggesting that a more-prolonged chilly spell is possible in the middle of February.

But as he posted on his site this week: “Trying to understand and predict the weather is continuously humbling — just when you feel that you figured a new phenomenon out or take a certain outcome as a given, Mother Nature throws you a surprise.”

Staff writer Kristen A. Graham contributed to this article.