It's already the worst year for thunderstorm-related flight delays and cancellations since at least 1999, and summer has barely started.

In May, about 50,000 of the almost 2 million scheduled flights were delayed -- and more than 78 percent of those delays were due to bad weather, according to the latest statistics from the Federal Aviation Administration. Delays in May for all reasons were almost double those in May 2003 and the highest for the month since at least 1999, the earliest date for which numbers were available. Normally, 65 to 70 percent of delays in May are attributed to weather, said FAA spokesman William Schumann.

The problem this year was that the summer heat started early. Temperatures that normally don't hit the country until July or August began stirring up storms in May.

The good news is that the unusual weather pattern has subsided, said Fred Johnson, chief of domestic operations for the Aviation Weather Center, which is part of the National Weather Service. But getting back to normal, for this time of year, means more thunderstorms.

Making weather delays and cancellations more challenging for airline workers this summer is the fact that the carriers are experiencing their busiest period in four years. More flights and more passengers have many airports bursting at their seams, worsening the ripple effect that inevitably results when thunderstorms keep planes on the ground. The airlines have to absorb the costs to pay employees overtime to rebook the displaced travelers, provide meal vouchers and hotel arrangements, and reroute and deliver misplaced bags.

American Airlines has been one of the hardest-hit carriers. Its hub is in the Dallas/Fort Worth area, where June rainfall was the highest recorded since 1928. Not coincidentally, June cancellations due to thunderstorms were 73 percent higher than in the same period a year ago and the highest ever for the month, spokesman John Hotard said.

Anyone who has ever flown into Atlanta's Hartsfield-Jackson International Airport, where Delta Air Lines has its primary hub, knows how often that airport is affected by bad weather. John Kennedy, a Delta spokesman, said the airline has had a record number of delayed and canceled flights during the past two months, thanks to storms in the Southeast and Northeast.

It's already been the worst summer for United Airlines in five years, says spokesman Jeff Green. Only 60 percent of United's flights out of its main hub at Chicago's O'Hare International Airport departed on time in June, compared with 82 percent normally, Green said.

Thunderstorms, more so than winter storms, are difficult and dangerous for the airlines to deal with. Unlike with winter storms, planes often can't fly above them: Thunderstorm cells can be as high as 50,000 feet up. Thunderstorms often cause violent winds. And unlike snowstorms, which are often concentrated in an area, thunderstorms can be spread out across routes.

"It's much easier to fly in the winter than in the summer," said Russ Chew, chief operating officer for the FAA's air traffic unit.

Chew said travelers who want to avoid summer storms should take early flights. That's because the later it gets in the day, the temperature rises and storms build up as the sun begins to heat the air. "Storms start to build after 11 a.m. Your best bet to avoid storms is to travel early in the morning," he said.

But there's really no avoiding storms on the typical return flights in late afternoon or evening. Even if the weather has cleared up, flights at those hours may be caught in the backup of flights that were delayed or canceled earlier in the day.

Many travelers still question how their flight can be canceled or delayed when there doesn't seem to be a cloud in the sky. That's because often there are high winds or storms along the route.

"Storms are not predictable. You can be in an area where there is no sign of a storm, and 30 minutes later, one pops up," Chew said.

Last month, frequent flier Andy Martin's flight from West Palm Beach, Fla., to New York's LaGuardia Airport was delayed four hours due to weather. Martin was flying Delta's low-fare carrier, Song. And just weeks later, Martin's AirTran Airways flight from LaGuardia to Akron, Ohio, was canceled because of weather. He and several other passengers rented a van and drove to Akron instead.

"What I find frustrating is that bad weather is predictable. The fact that there is a storm coming is not the second coming of Christ; it doesn't make any sense why no one is prepared," said Martin, a writer who lives in West Palm Beach.

According to Transportation Department guidelines, the airlines aren't required to offer travelers compensation if their flight is canceled or delayed because of weather. But some airlines, such as Air Tran, JetBlue and the new Dulles-based upstart Independence Air, have been handing out meal vouchers or ordering meals for passengers who are stranded for two or more hours.

"Clearly we are not responsible for the weather, but we realize that the ways these delays are handled can impact how a customer feels about an airline," said Robert L. Fornaro, president and chief operating officer of AirTran.