One of the more dreaded announcements for boarding airline passengers goes something like this: "Sorry folks, but we have an air traffic delay. We'll be held on the ground here for about an hour."

Some pilots will elaborate and explain why. But usually that's all the explanation the passenger gets. "Air traffic delay" becomes the villain of the day, especially when the weather outside the aircraft is clear.

Behind the phrase, however, is a large cluttered room on the sixth floor of Federal Aviation Administration headquarters here where a dozen professionals -- two supervisors, two meteorologists, a computer systems analyst and eight specialists -- make decisions every day that affect the travel plans of millions of people.

This is the office the FAA formed to control the flow of aircraft throughout the country when weather or other factors interrupt air traffic in any area of the country. It is now called the National Traffic Management Facility, although almost everyone still uses its original name, Flow Control.

The air traffic system is so near saturation today that delays in one part of the country have a ripple effect across the country. For instance, if Chicago O'Hare can accept fewer flights than normal because of bad weather, then planes taking off from hundreds of other airports will be delayed.

In past years, an aircraft might simply fly in a holding pattern until it could land. In the last decade, however, as fuel prices rose and the airways became crowded, the FAA decided it was safer and less costly to hold planes on the ground until they could be assured a landing time at the destination airport.

Flow Control does not "control" aircraft, but issues delay orders to air traffic control centers throughout the country. It also consults with airline officials and dispatchers. It essentially slows down the system across the country whenever air traffic controllers in one area cannot safely handle a normal amount of traffic.

A computer is programmed to handle each delay scenario. Once the delays are punched into the computer, aircraft across the country are assigned individual delay times, ranging from a few minutes to hours.

Most delays are weather-related, but there can be unusual delays, such as loose deer on runways. Runway construction and other similar factors also can result in slowdowns.

Here is an example of how the system works:

American Airlines Flight 1388 was one of the least likely candidates for a weather delay one day last September. Capt. Andy Bowe said he saw nothing but clear sky as his twin-jet MD-80 climbed east from Seattle bound for Chicago, so clear that he and copilot Bill Shelby could point out to the passengers almost every volcanic mountain in the Northwest, from Mount Baker to Mount St. Helens.

"We discussed the fact you could see forever, which is not typical of Seattle," Bowe said.

The flight was taking off late, however. Earlier that morning, it had been stuck on the ground for more than an hour because of weather. Not the beautiful Seattle weather, but thunderstorms that had struck Chicago before Bowe and Shelby even reached the Seattle airport.

In Washington, in the crowded room on the sixth floor of FAA headquarters, traffic management coordinator Gerald L. Shakley pointed to a computer screen showing a list of flights headed for Chicago. Flight 1388 had been assigned a delay of 42 minutes at that point, which would lengthen later.

Before the crew had eaten breakfast, a meteorologist in the large secured Flow Control room had spotted a developing line of thunderstorms headed for Chicago. O'Hare Airport can normally handle about 90 planes an hour on three runways, but when the weather turns bad, only two runways can be used and that reduces operations to about 60 an hour.

Specialists, armed with the adverse weather information, fed the information into the computer, which contains every airliner scheduled to fly that day, plus every general aviation aircraft that has filed a flight plan. Within minutes, the computer spits out the bad news. One by one, airline crews around the country are told how long they will be delayed. Just a few minutes at first, then longer periods as delays pile up.

As Bowe waited at his hotel for an airline van, he received a call from his airline. Flow Control had already assigned him a 60-minute delay. By then, the thunderstorms were already moving out of Chicago, and by the time Flight 1388 reached O'Hare the sun was shining through a few broken clouds. But the ripple effect of those thunderstorms on the country's aviation system lasted all day.

Flow Control bases its decisions on the best weather forecasts available, but Shakley acknowledged that he sometimes relies on experienced guesses from local airport officials as much as weather forecasts.

"Past experience adds a lot to it," Shakley said.

Shakley said the worst time of the year is thunderstorm season, but the system can be tied in tangles at any time of the year. For instance, on Dec. 28 there were snow delays in Boston, Detroit, Newark, New York and Philadelphia; fog delays in Atlanta, Dallas-Fort Worth, Charlotte, St. Louis, Cincinnati and Phoenix; low ceilings in Denver; and a disabled aircraft at Chicago O'Hare.

"That's about as bad as it gets," Shakley said. "It makes for a long day. Truthfully, those are the fun days. Time just seems like it's nothing."

As for the pilots, there is always the question of what to tell the passengers. "I usually try to make it detailed but somewhat light," Bowe said. "The weather is great here, but it isn't in Chicago. Ninety-nine percent of the time, the feedback from that is positive.

"You can't think that you put them in this big aluminum tube, you close the door and you got 'em."