If you are a college-bound student headed to Pittsburgh, or a fan taking in a Pirates or Steelers game, you will have one less choice for getting there in January.

Scrappy and unconventional Southwest Airlines Co., Philadelphia's second-busiest carrier, will stop flying to Pittsburgh and three other cities from Philadelphia after Jan. 8.

Southwest, whose entry here in 2004 invigorated air travel with friendly service and reduced fares on routes where it competed, particularly with US Airways Group, is downsizing at Philadelphia International Airport.

The nation's largest low-fare carrier plans to eliminate four daily flights to Pittsburgh; four to Manchester, N.H.; four to Providence, R.I.; and one to Jacksonville, Fla. from Philadelphia.

Only US Airways, Philadelphia's dominant carrier, will fly to those cities. If history holds true, passengers can expect fares, now as low as $59 one-way to Pittsburgh, to creep higher.

Southwest now has 55 daily nonstop departures to 18 cities from Philadelphia. Come January, four of those cities will disappear.

US Airways operates 440 daily flights here, including nine a day to Pittsburgh, 16 to Boston, eight to both Providence and Manchester, and three to Jacksonville.

"We still have some pretty substantial service left" to those cities, said Mark Gale, chief executive officer of Philadelphia International Airport. "But we all know that competition on routes typically yields the best opportunity for the consumer, not only the best fares but also a choice in product offering."

The airport wants Southwest to continue to grow in Philadelphia. "They provide competition on some of our most popular routes," Gale said.

Airport executives are talking to other airlines, including Delta, Continental, United, American, and JetBlue, about the possibility of flying to the cities Southwest is leaving.

In January, Southwest will also trim its Philadelphia-Boston flights from eight a day to six, and then to five in February.

Southwest - which recently acquired smaller AirTran Airways - is dropping routes that don't make money. And in Philadelphia, US Airways has proven to be a tough competitor, with fares that often beat Southwest ticket prices.

"This was a matter of rightsizing the market for us," Southwest spokesman Chris Mainz said. "We felt we could reallocate those aircraft to be more productive. It's a matter of finding a sweet spot of where we match up supply with demand. In Philly, making the changes would bring us more to that rightsizing place."

In plain English: Planes weren't full enough, or they were full, but passengers were paying such low fares that the airline did not make money.

"With fuel being over $100 a barrel, we have to figure out where our planes can be the most profitable," Mainz said. "That encompasses load factors, which is filling the planes, what the demand is, and the mix of business and leisure" travelers.

"We are at, or around, 55 daily flights out of Philadelphia," Mainz said. "We didn't feel those specific markets were performing to our satisfaction."

The airline will also reduce from four to three its daily flights from Philadelphia to Raleigh-Durham, N.C., after the first of the year.

Southwest will add flights during the winter on its existing routes from Philadelphia to Orlando, Tampa, and West Palm Beach.

In all, Southwest will shrink flights and trips 3 percent across its network in January and 1.5 percent in February.

Southwest will cut 12 routes, affecting Birmingham, Louisville, Boise, Reno, Salt Lake City, Seattle, Kansas City, Las Vegas, Spokane, Phoenix, Philadelphia, Manchester, Providence, Pittsburgh, and Jacksonville.

"I am seeing Southwest shrink by a similar amount at a lot of different cities," said airline analyst Daniel McKenzie of Hudson Securities. "It's not just Philadelphia."

Southwest will increase flights at Newark, N.J.; Chicago Midway; and Charleston, S.C.

"Those are the three big markets Southwest seems to be focusing on," McKenzie said.

In Philadelphia, US Airways is a "very formidable competitor" with a "nice blend of assets - international flying, long-haul, and short-haul flying using regional jets," McKenzie said. "US Airways has a few arrows in its quiver that Southwest doesn't have, and Southwest finds it hard to compete."

US Airways' frequent-flier loyalty program draws travelers who, as they earn more miles, can "redeem them for more opportunities with a bigger airline," McKenzie said.

"Southwest, because of the volatile fuel environment, is looking at where it can compete most effectively, and right now it's concluding that is not Philadelphia," he said. "But it's also concluding that at a lot of other cities as well."

In May 2004, Southwest saw big opportunity in Philadelphia. US Airways had twice filed for Chapter 11 bankruptcy and was plagued by poor customer service, high costs, and high fares.

Southwest swooshed in with 14 daily flights to six cities and expanded rapidly to 67 flights to 20 cities.

The recession, and US Airways' improved health, stalled Southwest's growth by 2009.

For consumers, the advantage of competition is lower fares.

US Airways for years had a monopoly between Philadelphia and Pittsburgh, and fares were sky high. The same was true for Boston. Before Southwest began flying to Boston Logan airport in June 2010, US Airways' website advertised $550 one-way coach fares, or $1,100 round-trip, on weekday nonstops to Boston.

US Airways matched Southwest's introductory fares, and ticket prices dropped as low as $59 and $89 one-way.

What are the chances Southwest will add new routes and destinations in Philadelphia any time soon?

"We don't have any plans to come back and grow aggressively in Philadelphia like we have in some other markets," Mainz said. "That's not to say it will never happen, but it's not in the cards right now."