Sick of the heat, crowds and high prices, more U.S. travelers are discovering the offseason of international travel — and airlines and hotels are fighting for a windfall.

Flights to once-seasonal European vacation destinations now start when there’s still snow on the ground in the U.S. and wrap up when leaves are falling off the trees, if they end at all, instead of following traditional late-spring to late-summer travel seasons.

For example, American Airlines‘ flight to Edinburgh, Scotland, from New York began in March. United Airlines‘ nonstop route to Palermo, Sicily from Newark, New Jersey, will end in December and Delta Air Lines‘ service to Rome from Minneapolis, Minnesota, will run into January, months later than they have in past years.

With this year’s surge in jet fuel expected to take a $100 billion bite out of airline profits this year, according to the International Air Transport Association, it’s crucial for the industry to maximize on travel trends that attract high-spending customers.

Investors are upbeat that airlines can take the fuel hit from earlier this year after they trimmed unprofitable or less profitable flights and airline executives have said strong demand has helped them pass some — but not all of those expenses along.

Shares of Delta and United, the two most profitable U.S. airlines, each hit records in recent weeks, and American’s shares touched an 18-month high. Airlines start reporting second-quarter results and providing third-quarter updates this month, with Delta kicking the season off on Friday.

“We’ve seen this massive, what I would call, the creep of the seasons — the shoulder season is blending into the full season,” Patrick Quayle, United Airlines‘ senior vice president who designs the carrier’s network, said in an interview last month.

Shoulder season refers to the period between a destination’s peak tourist season and its offseason.

Airlines are trying to extend the season as much as possible to grow profits.

International flights to Europe generally carry more premium seats like lie-flat pods than smaller jets that are used for domestic travel — and airlines are planning to expand those options further. Business-class fares on some of those routes can cost $10,000 for a round-trip instead of less than half that on a domestic route.

Airfare overall is up this year compared with last as airlines try to pass along as much of their rising costs to customers as possible, but there are signs that prices are moderating, particularly as the industry braces for the peak summer travel period in July to pass.

For example, flights between the U.S. and Athens, Greece, on June 22 were going for $988 round-trip, up from $810 last year but down from $1,350 two months earlier, according to flight-tracking site Kayak.

The increase in shoulder season and off-peak travel is forcing Delta to rethink its maintenance and crew schedules, said Jeff Arinder, Delta vice president of international network planning.

“We would never give airplanes to the maintenance hangers, if we could avoid it, in the summertime … because that’s when we made all the money,” he told CNBC. “We are now doing more maintenance in the summertime because we want to save those planes for the fall.”

He said Delta is trying to “really flatten out our seasonality as much as possible.”

The latest challenge to usual summer European travel was the most recent, deadly heat wave.

In late June, locals and tourists alike faced dangerous record temperatures throughout Europe, where air conditioning isn’t widespread. Misting stations were set up from Warsaw, Poland, to Rome. The Paris LGBTQ+ Pride march was postponed, among other events, and public alcohol consumption was briefly banned in the city.

Residents of many European cities, like Barcelona, Spain, and Venice, Italy have also been raising concerns about overcrowding during peak summer months and beyond. Countries throughout Europe have been bringing in record numbers of visitors.

But it’s not just an aversion to heat and crowds that’s leading to changing travel patterns.

For younger generations, more flexible work policies are helping some consumers, even those with children, take trips outside of late spring and summer. Baby Boomers, meanwhile, are armed with piles of cash and plenty of time, giving them more flexibility for travel.

“Delta’s target demographic tends to be a little bit older and a little bit wealthier,” Arinder said.

Seattle-based Alaska Airlines, which recently debuted its first service across the Atlantic this year to London, Rome and Reykjavík, Iceland, is keeping this in mind. President and Chief Financial Officer Shane Tackett told CNBC that travelers are becoming more flexible.

“A lot of people want to go see the same destinations … [and that] makes it like very logical that those seasons would start to spread,” he said. “Maybe when I was growing up, my parents wouldn’t have even thought of taking me out of school in September, and I think maybe parents are a little more like, ‘Yeah, let’s go somewhere fun, and you’ll catch up on school when you get back.’”

American Airlines, for its part, is stretching some of the seasons of U.S. trans-Atlantic travel.

October “is not as strong as June or July to Europe, but it’s becoming a peak month for us,” said Brian Znotins, the carrier’s senior vice president of network planning.

But American doesn’t want to push planes too far off their proven track record for ski-and-sun-seeker vacationers in winter, he said.

“I’m not going to mince words: January and February are still very off-beat months. I would hate to have anyone come away and say that they’re good months, they’re just less off-peak than they used to be,” he said.

Some travelers split the difference.

Atul Mehta, a finance executive based in Chicago, said he is taking his family to Portugal this summer shortly before school resumes, but said when he visits family in Bahrain in the winter “we have taken them out of school.”