Tuesday was Baltimore's coldest day in 18 years, with temperatures cold enough to shatter two records, strain the region's electricity supply, fill homeless shelters and even render fire hydrants near a South Baltimore blaze useless.

The air temperature plummeted to 3 degrees with wind chills of 16 degrees below zero in the early morning at Baltimore-Washington International Thurgood Marshall Airport, five degrees below a record set in 1988. The airport reached only 16 degrees in the afternoon, six degrees lower than a record dating to 1996, and with wind chills only as high as the single digits.

The chill was forecast to linger into Wednesday, with more single-digit temperatures expected overnight and below-normal cold until a rebound to the 50s this weekend.

Schools in Harford County were set to open two hours late Wednesday because of the continued cold. Joppatowne High School and Fallston Middle School will be closed because of mechanical failures in the buildings.

The day's average temperature of about 10 degrees was unmatched since Feb. 5, 1996, according to the National Weather Service, enough to jar many around the region, including Kat Hyland, a public-interest attorney from Hampden.

"My phone froze as I waited for the bus that didn't end up coming," said Hyland, who decided to drive instead, only to find that the button on her automatic key had gotten too cold to open the door. She had to put it inside her glove to thaw it out, then warmed up her phone in the car once she got it open.

"I was very surprised because as a New Yorker, I thought I could handle the cold," she said.

The cold prompted record winter levels of electricity use, a problem that is more common amid intense summer heat. PJM Interconnection, the region's electrical grid operator, broke its winter peak usage record Tuesday morning, leading officials to ask residents to conserve power as much as possible.

The cold also caused a number of generator units to either shut down or potentially develop problems, said Michael J. Kormos, executive vice president of operations at PJM. The grid operator said that going into Tuesday evening, there were about 36,600 megawatts of forced generation outages, or about 20 percent of installed capacity, but could not say where. Energy giant Exelon Corp. said Tuesday morning that all of its Baltimore-area units were operational.

Temperatures were low enough to cause headaches for homeowners and motorists just trying to go about their days.

"We've just been bombarded with no-heat calls and people that are only getting water to certain parts of their home," said Ashley Bisaha, a customer service representative at A.J. Michaels, a plumbing and heating company in North Baltimore. The company advises leaving faucets dripping or turning up the heat if pipes show signs of partially or completely freezing.

AAA Mid-Atlantic had received nearly 7,300 requests for assistance as of about 3:20 p.m., 1,800 of the calls and online requests coming from drivers in Maryland. Most were for towing, battery problems and flat tires, with the cold temperatures reducing tire pressure and straining old or low batteries, spokeswoman Ragina Cooper-Averella said.

Efforts to bring homeless and vulnerable populations in from the cold were ramped up around the region. By 8:30 Tuesday morning, city officials had made contact 92 times with homeless people and had taken 59 of them to shelters to escape the cold, said Connor Scott, a spokesman for the city Office of Emergency Management. In some cases, people initially refused to go to shelters and were checked on twice by authorities, so the number includes some double-counting, Scott said.

The 275-bed homeless shelter at 620 The Fallsway was expanded by 60 beds and filled to capacity Monday night. Another cold-weather shelter the city opened had about 65 people in it Monday night, Scott said.

The Mayor's Office of Human Services said that its two regular overflow shelters were also at capacity, with about 200 people seeking beds. Numerous other private homeless shelters also were at capacity Monday night.

A "Code Blue" advisory remained in effect in the city through 10 a.m. Wednesday, maintaining the expanded shelter space and other extra resources for the homeless.

City firefighters responding to a rowhouse blaze in the 1400 block of Cooksie St. in Locust Point found two fire hydrants frozen when they went to connect their hoses, Fire Department spokesman Ian Brennan said. The firefighters had to run down another block to find a working hydrant, he said.

Firefighters are trained to adapt when they come across non-functioning hydrants, Brennan said, though that it's usually because of water main breaks or ongoing repairs rather than intense cold.

Three firefighters and two civilians were hurt in the two-alarm blaze, which was brought under control within about 45 minutes. The injuries were not life-threatening.

A burst water main in the 1900 block of W. North Ave. sheeted the side of a car and the entire block in ice, one of 20 water main breaks in the city Tuesday, Public Works Department spokesman Kurt Kocher said.