Washington ? Fliers beware: This summer vacation season could rival 2000, the worst ever for flight delays, cancellations and cranky travelers crammed into airport terminals.

Takeoffs are now returning to pre-Sept. 11, 2001, levels, and millions of tickets already have been sold. Security-related delays are a concern, and so is one thing that no one controls: Mother Nature.

“In bad weather, that’s when it shuts down,” said Jack Evans, spokesman for the Air Transport Assn., the industry group for major airlines.

Hot, humid air produces summer thunderstorms, typically in a zone that stretches from Texas to Michigan. They’re too high to fly over, too dangerous to fly through and often too wide to fly around.

At hub airports – especially Chicago’s O’Hare International – bad weather can combine with huge numbers of passengers to cause delays that cascade through the entire system. That’s what happened in 2000, forcing thousands of passengers to sleep on terminal floors or sit fuming while their planes lingered on runways for hours.

Much has been done to improve things since then. Airports have added runways and terminals and improved efficiency, and the Federal Aviation Administration has adopted new strategies.

But the system has yet to be tested during a bad thunderstorm season, and the signs so far aren’t encouraging.

The Memorial Day rush at Washington Dulles International Airport began early Thursday afternoon as security lines suddenly ballooned to thousands of travelers.

Travelers meander through a line to clear security Thursday at Dulles International Airport in Chantilly, Va. The Memorial Day rush at Dulles International Airport began early Thursday afternoon as security lines suddenly ballooned to thousands of travelers.

Many were taking advantage of cheap ticket prices. The average airfare for a 1,000-mile flight is now $118, 20 percent lower than it was in 2000, according to Transportation Department Inspector General Kenneth Mead.

Bargain airfares allowed Nancy Cross, a physician’s assistant from Ashburn, Va., to take her husband and two stepdaughters with her to Orlando, Fla., where she was attending a medical conference.

Cross couldn’t believe the security line. “I can’t even see where it begins,” she said, standing on tiptoes.

At least the weather was good.

For the first 10 weeks of this year, slightly more flights were delayed, and delayed longer, than they were during the same period in 2000. Most delays were caused by weather.

At the 55 airports tracked by the FAA, more than 25 percent of flights were delayed for an average of 50 minutes from Jan. 1 to March 15, according to Mead. That’s up slightly from 24 percent of flights delayed an average of 48 minutes five years ago.

Mead said Thursday that six airports are likely to experience delays this summer: Philadelphia, New York-LaGuardia, Newark, N.J., Dulles, Atlanta and Fort Lauderdale, Fla.

To avoid massive weather disruptions throughout the whole system, the FAA has planned alternate routes based on where storms are likely to develop.

“If we have a certain weather pattern, we have a different way of getting them out,” FAA chief Marion Blakey said.

The agency also has installed faster and more accurate weather prediction equipment, which allows air traffic controllers and the airlines to respond more quickly to changes in weather systems.

The FAA, though, can’t control the volume of passengers or the number of people who work for the airlines.

In 2000, there were 698.9 million passenger boardings of commercial planes in the United States, according to the FAA. In 2004, there were almost as many – 698.7 million – and that number is expected to rise in 2005.

Cash-strapped air carriers slashed their work forces in the past few years to save money. U.S. airlines cut 81,717 jobs between July 2000 and July 2004, and another 16,990 by February, leaving a total of 400,253 full-time equivalent workers, according to the Bureau of Transportation Statistics.

Understaffing was responsible for more than 500,000 US Airways passengers being delayed during the December holidays, according to a report issued by Mead. The report criticized the way US Airways Group Inc. handled the shortages of fleet service employees and flight attendants, particularly in Philadelphia.

Managers tried to mitigate the problem by paying overtime, but their “plans ultimately did not work,” the report said.