CHICAGO -- The weather was bad, which meant business was good in the stand-up bar on Concourse K. It was Thursday, the heaviest travel day of the week, and the usual afternoon rush, with the usual afternoon delays, was just starting to build when the thunderstorm hit.

In the bar, passengers lined up shoulder-to-shoulder. The woman flying to Hartford stood next to the man flying to Los Angeles who stood next to the man flying to Dallas who announced he was switching to rum.

By early evening, what had begun as a routine day at O'Hare International Airport had disintegrated into one of those chaotic scenes featured in "Airport" movies. The terminal was jammed. Runways were shut down, then reopened, then closed again. Almost all the flights were delayed.

At the American Airlines "Connection Counter," where flights are rebooked after connections are missed, the line was 23 passengers long and eight passengers deep. Over at United Air Lines, a similar scene assembled, complete with mothers holding squalling children, teen-agers on skateboards, lost tourists and sad-eyed businessmen.

"I used to get on a plane at 8 o'clock to get to a 1 o'clock meeting. But you can't do that anymore," said Mark Whittaker, the man in the bar waiting for his flight to Dallas. "It costs the company a day extra of my time, coming and going back. I always could spend the weekend with my wife. Now, sometimes I can't."

A team of Washington Post reporters spent July 9 at Chicago's O'Hare International Airport for a detailed examination of the problem of airline flight delays. What follows is a report based on more than 80 interviews with passengers there.

That weather causes most flight delays was implicitly understood. That Chicago's storms that day were spectacular, everyone agreed. But the passengers whiling away the hours at the busiest airport in the world took issue with the notion that airline travel in this country should be so finely tuned, so delicately balanced, so fragile that it can so easily all come apart.

"How much weather can there be?" said James Lutz, a librarian in a New Jersey law firm, as he watched his Continental Airlines flight to Newark be delayed and then canceled. "I've had to change my life style. Now, I travel a day ahead of time."

Lutz's problem is hardly unique. Flight delays, according to the Federal Aviation Administration, are up 7 percent over last year, and in 1986 they leaped 25 percent over 1985. They have become so routine that business travelers say they schedule delays into their travel plans -- at a cost of $4 billion a year, according to Air Transport Association estimates.

Public complaints about air travel have also soared. The Transportation Department has announced it received almost as many complaints in June as it did for the first half of last year. In June, the department received 5,759 complaints about flight delays, cancellations and lost baggage, compared with 6,393 complaints received between January and June last year.

"I fly at least twice a month, and I have not had a plane on time -- not one -- in nine to 12 months," said Robert McCarthy, a Chicago businessman waiting for his American Airlines flight to Boston.

Because of the airlines' "hub-and-spoke" method of operation, cities thousands of miles apart have been strung together in airline route systems so intricate that thunderstorms over Cincinnati rain on passengers in Albuquerque. Adding to the problem, jetways are so crowded that pilots complain that a storm over Newark can delay a flight between Washington and Chicago because of rerouting of the flights headed to Newark.

Airline schedules, based on elaborate mathematical formulas for takeoff and landing rates, baggage loading rates and taxi time, are so precisely timed that any little thing can throw them off.

At O'Hare, the day of July 9 begins busy, but routine. The airlines exude an appearance of efficiency and precision. United's schedule monitors flicker overhead, listing 114 departures, seven delays. American's monitors display 58, one delay. In all, there are 2,175 flights scheduled, including 435 commuter flights.

American and United, the two main carriers here, will board 59,977 passengers. Thursdays have become the heaviest travel day, the airlines say, because hundreds of thousands of business travelers have adjusted their travel habits away from Fridays.

"You read a lot about this place," said Kelly Kunsch, from the queue at Eastern Air Lines, where he is waiting to catch one of Eastern's 15 departing flights. "But if it's the most heavily used, it doesn't look too bad."

By 6:45 a.m., the crowds at the check-in counters at United and American begin to build with the morning business travelers headed east. Later, around 10 a.m., the lines will swell again when the East Coast business crowd passes through on its way west. United's mathematical formulations begin in the line: 4.3 minutes to process a ticket, 1.2 minutes to check a bag, 1.5 minutes to print a ticket. Or so the theory goes.

Just off Concourse F, in United's operations center, United's master gate controller plots the progress of the day's 748 flights by moving small, colored-coded magnetic airplanes around on a giant board. United, as the biggest player at O'Hare, controls 42 gates, and its flights are timed to come in and go out in waves, 14 times a day. (The formulas again come into play: a Boeing 737 gets 30 minutes of gate time to unload and reload.)

Early on, the gate master has a trouble spot: a gash in the rear tire of the plane being used on Flight 870 to Newark was discovered after the passengers boarded; the departure is pushed back from 6:30 until 6:45, then to 7, and finally 7:30.

Out at Gate E8, the passengers get back off the plane. "It's blown my day," gripes a businessman as he rushes to catch a Newark flight at another gate. Flight 870 finally departs at 7:46, an hour and 16 minutes behind schedule.

By 7:30, all the airlines tap into the telephone conference call with the wizards at the FAA headquarters' "Flow Control," which prescribes the daily traffic flows in and out of the country's 34 major airports. Flow Control's weather wizard issues the first warning: thunderstorms building over Nebraska and heading east.

But for the morning rush hour, Chicago's weather is clear and O'Hare receives no restrictions. All six runways will be used, and between 6 and 9 a.m., 344 planes will land or take off.

United, which regards the sheer volume of O'Hare's traffic -- 50 million passengers and 800,000 takeoffs and landings annually -- as a measurement of monumental efficiency, is pleased. The mathematical formulas work -- 90 percent of its early morning departures leave the gates within five minutes of the scheduled departure time. Twenty-four morning arrivals come in early.

Down at Gate H2, things are not going so well. The gate crew at American Eagle, American's commuter airline, faces a surly crowd. The 7 a.m. flight to Moline in northwest Illinois is delayed, the passengers are told, because the flight crew has not spent enough time on the ground to satisify an FAA flight rule. The 8:50 flight to Peoria is delayed until the tardy crew shows up, and the 8:44 flight to Madison is pushed back until 11:20.

Until last month, 9:15 a.m. was known among regulars as the worst time to arrive at O'Hare because 39 arrivals were scheduled for that minute. Last month, the airlines shuffled their schedules and the number of 9:15 arrivals dropped to 12.

Passengers criticize the airlines for scheduling so many flights at the same time. Last spring, in scheduling discussions held by the FAA, the airlines operating at O'Hare adjusted their summer schedules. But the changes were subtle, some from one minute to the next, others from one 15-minute time block to the next, and they didn't eliminate all the delays. American says it is experimenting with 370 additional schedule changes to its national schedule in an effort to further reduce its delays.

About 9 a.m., the formulas start to unravel, the schedules start to slip, mostly because of delays of flights coming in from other airports. United's arrivals are coming in about 10 minutes late, and by 9:45, its departures to the West Coast fall behind. At American, flights to Denver, Seattle, Milwaukee, Kansas City, Albuquerque and Salt Lake City get off late. The delays, like the thunderheads, are starting to build.

By early afternoon, Northwest Airlines has canceled its flights to Minneapolis and Detroit. Continental's Flight 636 to Newark is delayed indefinitely because storms over Newark are so severe that Newark temporarily has shut down. In all, Continental operates 22 daily departures from O'Hare, of which half will be delayed today -- eight of them because of thunderstorms.

In the course of the day, Burton A. Schmarak, a Chicago insurance specialist, will miss a critical business meeting in Peoria. Sandy O'Brien will miss a wedding rehearsal in Boston. Patricia Heale, of Hayward, Calif., will arrive too late to yell "surprise" at her grandmother's 90th birthday party and Sherwood Zimmerman, of Sherman, Ill., will not make the family gathering the night before his uncle's funeral in Washington.

"I really don't expect to be on time," said Sue Martin of Chicago. "You become accepting of the fact that you're going to be late because you're always late. It's become a way of life when you travel."

The delays build on each other like a pyramid, each delayed flight produces more delays. American's Flight 549 from Washington is delayed 48 minutes at National Airport because of bad weather, which causes nine other American flights to be delayed because Flight 549 was carrying 82 passengers with connections.

"We will board on time today," predicted John Dye of Wichita, Kan., as he waited for a United flight to Syracuse, N.Y. "We will not leave on time. I've come to expect that of Chicago."

Similar scenes played out in Denver and Boston, although Denver was not as severely affected by the storms as Boston. "I always give myself time for delays -- always," said Leonard Stark of Naperville, Ill., after his United Flight 684 arrived in Boston from Chicago 30 minutes late.

In June, the FAA warned travelers to expect even more delays this summer, in part because the agency has slowed air traffic traversing overcrowded jetways and summer weather has been worse than predicted. Traffic has also increased by nearly 5 percent. Between Oct. 1 and June 30, the FAA counted 298,522 delays around the country, compared with 278,765 during the same period last year -- an increase of 7 percent.

At Chicago, American's J.F. Frawley acknowledges that American's performance has not been as good as last summer. "So far this summer, Chicago is more than 800 degree-days above normal," he said. "Chicago has had more 90- to 95-degree-days, and those are generators of thunderstorms."

July 9 brings 2,448 delays around the country, by the FAA's count. Among them: 19 at Denver, 97 at Boston and 286 at O'Hare.

Of Chicago's delays, 205 are departure delays and 81 are arrival delays, all because of weather. In addition, there are 148 delays at other airports of flights bound for Chicago.

But counting delays is much like a Catch-22. To the FAA, a flight delayed for an hour at the gate is not a delay if it gets off the ground 15 minutes after it leaves the gate. In other words, United's Flight 870, with the gashed rear tire, is not a delay, even though the man who said he was missing a three-hour business meeting considered it one.

The airlines say a plane that sits on the runway for an hour before taking off does not count as a departure delay because it left the gate on time.

Consequently, delay statistics disclosed today are often meaningless. The way delays are counted differ not only between the FAA and the airlines, but even among the airlines themselves. The Senate is fine-tuning a "Truth in Scheduling" bill that would, among other things, attempt to define the word "delay."

The Senate Commerce Committee cleared the bill last week, and if approved, it would also require the airlines to file monthly reports with the Transportation Department disclosing on-time performance figures, number of cancellations and number of passengers bumped from flights.

Thirteen airlines, including United, oppose the bill and have suggested an alternative, but voluntary, system for disclosing on-time performance.

Back at O'Hare, the storm hits suddenly at 4:30 p.m. It comes off Joliet, southwest of O'Hare, 80 miles wide and fierce. At O'Hare's Control Tower, the controllers stop all landings. By 5 p.m., 53 planes are backed up along the runways. Jetliners coming from the East have to be shifted to another arrival route. Landings stop again for 20 minutes just after 6, when another storm blows in.

By 9 p.m., the average wait along the runway is an hour. Downstairs, in the control tower's radar room, Robert Mischke, the evening supervisor, works four phones. The controllers at Chicago Air Route Center, 35 miles away, want to know if they should hold the arrivals.

"No, let 'em come," Mischke says. "If we do some holding, we do some holding. We'll be going till 1 o'clock tonight." The departures back up again: 15 on the north side, 25 on the south.

Flow Control's wizards telephone next to report storms at Nashville, Boston, John F. Kennedy, LaGuardia, Newark and Long Island. They are in the process of stopping traffic on the entire East Coast -- which means no arrivals at O'Hare, meaning no planes, meaning no departures.

Inside the terminal, the Connection Counter closes. The neatly listed flight schedules disappear from the television monitors, replaced with a new, slightly understated message: "DUE TO WEATHER CONDITIONS MOST FLIGHTS ARE SUBJECT TO DELAY."

American has had to cancel 10 of its 255 flights. United has canceled 14 arrivals and 19 departures, and ends the day with an "on-time" departure rate of 46.7 percent. American's is only slightly lower: 45.7 percent.

The Transportation Department has announced plans to hire 955 more air traffic controllers and other support personnel next year. The department has also been investigating airlines suspected of using deceptive scheduling practices. But few are optimistic that much is going to change in the age of delays, given the overwhelming logistics involved in running the air travel system.

"I think it's just because of the massive quantity of people traveling," said Gary Arnold of Prospect Heights, Ill. "I've gotten used to it. It doesn't bother me like it used to. I know there's nothing I can do, except complain. And all they're going to say is, 'Sorry, sir.' "