It smells like cold coffee, like sour beer. It smells like exhaustion.

The air smells as if it has been inhaled and exhaled by too many people for far too long and they are breathing it still, snoring and snuffling, sighing and murmuring as they sprawl about O'Hare International Airport like refugees from some invisible war.

Everywhere you look there are bodies. Stretched along tables and the conveyor belts of X-ray machines. Curled up on baggage carousels, slumped against walls and draped along benches. There are people slung out on the floor, their faces inches away from swinging feet, and people draped around one another like sculpture, trying to find comfort in the curve of a shoulder or bend of a back.

Some feign or force themselves into sleep, shutting out the fluorescent lights, the blare of "Monday Night Football" on television sets they can't turn off, the incessant beep of motorized carts. Others stare, glassy-eyed, at lightning flickering against the dark, rain-spattered windows, thinking about meetings unmet, vacations postponed and children who went to bed unkissed.

There are almost 6,000 people at O'Hare tonight. They are all supposed to be somewhere else.

They are stuck here instead, in an airport that once prided itself on being the world's busiest and now is notorious for making more of its passengers late than any other airport in the country.

In many ways, the transformation of O'Hare from sleek symbol of the jet age to the bus station of the skies parallels the changes in air transportation itself: from fine china and travel suits to foil-packed peanuts and cutoffs, dirty diapers jammed into seat pockets and security guards stationed behind the customer service desk.

Almost 700 million passengers now fly each year and the system, already outdated and inefficient, can barely handle the load. Some days, like this one, it breaks down altogether.

The people who have seen this hellish slumber party before, who have seen the rows of squeaking, city-issued green cots and the undersize blankets, the overflowing trash cans and stopped-up toilets, have a name for nights like tonight.

They call it Camp O'Hare.

Camp O'Hare usually opens when there is too much snow, too much lightning—just too much weather—for planes to fly in and out. When, like today, a pair of thunderstorms chase each other across the Midwest like children trampling through a flower bed.

These storms are the worst in a summer of bad weather, two furious squalls that, by some freak occurrence, slam into the airport with only breaths of calm in between.

There could hardly be an unluckier day to try to fly into or out of O'Hare than Monday, Sept. 11, but there is more than the weather to blame.

Like the leaks in the airport's roof, the problems in the air travel system may remain unnoticed until the rain hits, but once the storms arrive, these flaws make the gridlock and misery much worse than they have to be.

On Sept. 11, the airlines have scheduled almost 2,700 flights, more than O'Hare could comfortably handle even if it were, in aviation parlance, a perfect "blue-sky" day.

The way these airlines send their flights around the country, by routing large numbers of passengers through "hub" airports and then on to their destinations, can quickly collapse under the pressure of even small delays.

O'Hare itself is inefficient and overburdened, but any attempt to expand it or build a new airport to bear some of the load is stymied by sandbox politics. A hostile stalemate between Chicago's mayor, Richard M. Daley, who wants to keep his administration's grip on O'Hare's revenue, and suburban politicians, who are vehemently opposed to expansion, blocks serious debate on any solution.

Although most passengers know little, if anything, about these issues, they bear the brunt of their effects. They're the ones who miss their cousin's wedding or grandfather's funeral because of delays that worsen each year, who stand helpless as a gate agent walks off the job for lunch even though 100 stranded passengers wait in line.

They're the ones sleeping on the floor tonight.

Before the evening's uneasy sleep settles over the airport, there will be fury and tears and a cacophony of cellular phones transmitting, like some Greek chorus, lamentations and curses. There will be one huge miscalculation and countless small decisions that send hundreds of passengers astray. There will be acts of clout and acts of kindness and pilots who guide their planes, by instinct and skill, through roiling black skies.

It all begins well before dawn, at the very onset of the day, when the loudest noises in the terminals are the distant hum of a vacuum cleaner and the occasional click of a flight attendant's heels.

Already, the earliest flight out of O'Hare has been canceled because of mechanical problems and one of its passengers, a lawyer, learns he will be late for his first day of work. A gate agent stands in her stocking feet, manicured fingers flying over a computer keyboard, frantic at 5 a.m. because she knows she has just 15 minutes to punch in gate numbers before the day's first passengers arrive and demand her attention.

The radar screens in the airport's control tower show a clump of thunderstorms hovering over the northern tip of Lake Michigan. They look like nothing more than daubs of paint.

Outside, dawn warms the sky to a muted blue.

The room where Janice Collier works is big and dim and hushed. The walls are lined with giant screens, just like every movie version of a war room since "Dr. Strangelove," except that the battles depicted here are fought between planes and the weather.

If there is a center to the web of air travel that spans the United States every day, it is in this room in Herndon, Va., where the Federal Aviation Administration runs its air traffic control system command center.

Nicknamed, in typical FAA-speak, "Flow Control," this facility opened six years ago in an effort to improve the movement of planes throughout the country.

Every day, Flow Control's specialists track the nation's 23,000 or so commercial flights, watching for the first signs of delays or other problems. In response, they may send planes on different routes or limit how many can arrive at or depart from certain airports.

Sometimes, these decisions seem arbitrary and illogical to passengers and airlines alike. Today will be no exception.

But at 5 a.m., as Collier, an air traffic controller and weather specialist for 25 years, wraps up her stint on the night shift, there isn't much to do.

There are only a handful of planes in the air. She has spent most of the night watching the weather, tracking a modest cluster of showers just west of Cincinnati, wisps of feathery clouds forming over the Dakotas and rain drifting across Texas.

She points to the band of thunderstorms over Lake Michigan, still more than 300 miles north of O'Hare.

"It's supposed to be gone by midday," she says.

The other FAA weather specialists she talks to during a predawn conference call agree. So, when Collier types the weather briefing that will be distributed to the FAA control centers and major airlines, she writes that the storm is expected to move into eastern Canada during the morning.

Despite all the science—the Doppler radars, the water vapor sensing systems—the job of predicting how weather will affect air travel still comes down to a guessing game. Collier and her colleagues have made their guess. Almost everyone else will make the same call.

Two hours later, at 6:15 a.m. CDT, Flow Control convenes the first in a series of teleconferences it conducts each day with FAA centers and representatives of the major airlines around the country. More than 20 groups of people gather around speakerphones in rooms that, like Flow Control, consist mostly of computer monitors and giant television screens. All of them have copies of the day's weather briefing.

By now, problems have cropped up like small brush fires. In the northeast, low cloud ceilings and fog have slowed the morning rush. Detroit is limiting the arrival of planes because of fog. Houston has showers east of the field.

At O'Hare, low clouds prompt controllers to limit the number of planes arriving to 72 an hour from the ideal rate of 100.

Because the slowdown will create a backlog of planes waiting to land, the FAA calls what is known as a Level 1 ground stop, telling flights headed to Chicago from airports in an area stretching from Pittsburgh to Kansas City they can't take off.

Air traffic controllers also space planes farther apart in the air than usual, both for safety and to reduce the volume of traffic.

Throughout the morning, the ground stops and other limits will click on and off at O'Hare like traffic lights turning from green to red, slowly increasing backups.

As the conference call wraps up, United Airlines flight dispatch manager Ron Smith walks away from the telephone at the company's operations control center in Elk Grove Township, coffee mug in hand. He's shaking his head.

"You worry about our future in air travel," he says, "when a little weather system like this can affect us so bad."

Smith's complaint is part of a familiar tug-of-war between the airlines and the FAA, which not only manages how planes move from place to place but also bears responsibility for every aspect of air travel from the color of rip cords in hot-air balloons to how many bailing buckets planes should have in their life rafts.

The airlines complain the federal agency is too conservative in its assessment of the weather, creating delays by slowing down traffic when it isn't necessary, a decision that costs them money in wasted fuel and late flights. The FAA, which points to its strong safety record, thinks the airlines overestimate what the system can handle.

By 7:30 a.m., with the ground stop and other weather problems limiting arrivals and departures in the northeast, Smith knows that at least 100 of the 888 United flights scheduled into and out of O'Hare today will be delayed.

The thunderstorms still hover over Lake Michigan, but hardly anyone seems worried that they haven't started to move away yet.

In another room at United headquarters, weather operations manager Garry Hinds, a pair of reading glasses dangling from his neck, studies his computer monitor.

"Even if it does come our way," he says, "I don't see this storm hitting Chicago until 8 or 9 p.m. It's very slow-moving."

You can attend mass at O'Hare International Airport. You can get a massage, play a game of pool, rent a laptop, get your broken ankle X-rayed, buy a suitcase or a set of Michael Jordan-endorsed golf balls, or take your children to see a life-size model of a Brachiosaurus. You can do almost anything you can do in any small city in the United States.

But what most of the 180,000 people who pass through the airport's four terminals every day want to do is leave.

That is why, at 10 a.m., Erik Madsen, a stocky man who works as a supermarket manager, stands in his sandals and socks and points quizzically at the window behind United Gate B22.

The window shows blue sky. It does not show the plane that is supposed to take him, his wife, Tonya, and their 7-month-old son, Jacob, home to Ft. Wayne, Ind.

The first flight the Madsen family was supposed to take, as they made their way home from a family visit in Denver, was canceled because of mechanical problems. Now, this one, which the gate agent told Madsen just 15 minutes ago would leave on time, has been canceled, too, because of the weather.

Madsen gestures at the cloudless sky and planes taxiing down the runway.

Weather?

It's only small aircraft that can't take off, the gate agent tells him.

But the Madsens' plane is still in Ft. Wayne, officials at United headquarters will say that morning. It can't leave because the ground stop to O'Hare is back on.

This brief encounter at the gate is just one of thousands of similar exchanges that will take place this day, but it illustrates one of the most widespread and frustrating problems with air travel: misinformation.

Many passengers believe that airline employees routinely lie to them about the reasons for delays and cancellations, a charge the airlines deny. But airlines acknowledge that outdated technology and poor communication sometimes leave gate agents with incorrect or incomplete information.

On this day, people waiting for a plane from London will be assured it has landed when, in fact, it is sitting on a taxiway in Toronto.

Passengers waiting to board a flight to London will be told it is being canceled because of mechanical problems when there is nothing wrong with the plane except that it is needed for people whose flight to Stockholm was diverted to Cleveland because of bad weather.

The view of blue skies is deceptive, too. Bit by bit, everything is beginning to clog. The on-again, off-again ground stops and other flight restrictions have created delays that now top 60 minutes.

Flow Control has begun rerouting planes to avoid the bad weather and, like cars being detoured from a highway to side roads, these planes create traffic problems of their own, interfering with flights on the alternate routes and slowing down the whole system.

Even worse, the storms over Lake Michigan, instead of heading north to Canada, are beginning to slip back toward Chicago.

No one can control the weather, but the airlines have helped create the conditions that are about to make the skittering series of delays tumble into an avalanche.

Flight schedules generally operate according to the demands of coveted business travelers and are organized like a typical workday: bunched together at the beginning and end of a 9-to-5 shift, with another rush at lunchtime called the "noon balloon."

Between noon and 1 p.m., 103 planes are supposed to land at O'Hare. Forty-two are scheduled to take off. The next hour: 83 arrivals and 105 departures.

On each of these planes are passengers who must disembark and, within a matter of minutes, catch connecting flights. This is the crux of the hub-and-spoke system, which is how most major airlines organize their flights, and it all depends upon these orchestrated waves of arrivals and departures, with passengers moving from plane to plane in precisely timed increments. A few false steps send the entire dance into disarray.

Even under the best of circumstances, it would be tough for all of these flights to arrive and leave on time. One hundred landings an hour are about all the airport can currently handle. But, because of the mounting delays, the airlines already are considering canceling flights just to catch up on their schedules.

At the gate, Erik Madsen tries to decide whether he and his family should take their chances on the next flight to Ft. Wayne, which is supposed to leave at 1:55 p.m.

If they rent a car now, they can make the 3½-hour trip in time for Tonya to punch in for her 3 p.m. shift at Domino's Pizza.

"No thanks," he tells the gate agent, glancing at his watch. They'll try driving.

They gather up Jacob, the diaper bag and the assorted toys and books and walk more than half a mile back to Terminal 3 to pick up the voucher they'll need to be reimbursed for the flight.

Tonya wonders if they have enough money left on their credit card to rent a car.

"Let's find out," Erik says as he pushes the stroller to the lower level.

He patiently waits in line at the only rental car company that has available vehicles—Dollar Rent-a-Car—but when he reaches the counter, he discovers there is no drop-off point in Ft. Wayne.

As Tonya calls her boss, Jacob starts to chew on his bib and cry. Erik takes off his cap and runs his fingers through his hair, the only visible sign that his nerves are beginning to fray.

They have to wait for the plane.

At 11 a.m., the planes waiting to take off from O'Hare line up on the taxiways like a flock of geese preparing to fly south.

At this moment, however, no one is going anywhere fast. As the thunderstorms approach O'Hare, the routes west begin to jam. Already, one of the airport's westbound routes has been shut down and two others are using far greater separations between planes than usual.

Passengers who choose to while away the wait by listening to the control tower's chatter instead of the pop-music channel on their airplane-issued headphones could hear a curious exchange that suggests old-fashioned Chicago clout can roll out a red carpet, even on a runway.

The flight in question is United 43 to Honolulu, whose passengers include the occupants of seats 25A and B, newlyweds Michael and Tracie Cicero, about to embark on their two-week honeymoon to Hawaii.

The Ciceros, who are both 26 and live in Glendale Heights, have never been on a plane. They don't quite know what to expect and they certainly don't expect that, because of a conversation they will never hear, they may be the reason their plane gets to take off ahead of 37 others.

While those planes are arranged in neat lines near the two open departure runways, the United DC-10 waits by itself, next to a runway being used for arrivals because this particular runway is the only one long enough for it to use for takeoff. The plane is waiting for a gap that would allow it to leave.

Then, about 11:15, a message comes from the control tower for Flight 43. A controller wants to pass along congratulations to a newlywed couple on board. The message is from a supervisor who is not on duty. He is a friend of the groom's boss.

"Big Mike says to say hello there from the tower here," the controller says, suppressing a laugh.

The pilot responds, with great enthusiasm. "All right. Big Mike. All right."

Several minutes later, another controller broadcasts this message. "United 43. One arrival and you'll go. ... Evidently, you have, uh, important cargo, I just found out."

The plane is told to taxi into position, and after a short series of unrelated radio exchanges, the pilot radios that he's ready for takeoff. "You have 43 heavy," he says, using the term to describe a large plane. Then, he adds this: "It helps to know Big Mike."

Another controller turns his attention to the other planes waiting to leave O'Hare, telling them they won't be going anywhere soon. "You can go ahead and shut down," he instructs the pilots. "There's quite heavy delays going westbound."

Later, the FAA will say that an unexpected gap in arrivals, not preferential treatment, allowed United Flight 43 to take off first, even though to the other stalled pilots, it may have seemed otherwise.

Aboard the DC-10, the Ciceros remain unaware that anyone knows they are newlyweds until, halfway through the flight, a bottle of chardonnay and box of Eli's turtle cheesecake arrive at their seats.

Attached to the cake is an anonymous note. "Mike and Tracie," it reads, "Happy Honeymoon."

At least one other passenger gets special treatment Monday. This time, the "important cargo" is so important the FAA won't publicly release his name and all visitors are asked to leave the control tower during communication with the plane.

The blue-and-white Boeing 757 sits alone in the former military area of the airport, surrounded by beefy men wearing suits and earpieces. When it prepares to leave, no other planes are allowed to land or take off.

And Vice President Al Gore, fresh from an appearance on "The Oprah Winfrey Show," takes off in Air Force Two, precisely as scheduled, at 12:05 p.m.

At the American Airlines main operations center in Dallas one desk usually sits empty.

That seat is for the person who cancels flights. The person nicknamed the Cancelator. The Vacation Buster, some joke.

No one wants to be the Cancelator. The Cancelator's name goes into the computer where other employees can see it, so they know exactly who is responsible for the lines of angry passengers.

By 1 p.m., tension surrounds the chair. It's clear someone will be sitting there soon.

The room's radar screens are filled with planes holding around O'Hare, making small circles and squiggles around the same stretches of airspace, over and over.

Then, at 1:32 p.m., Flow Control in Herndon, Va., stops all flights for O'Hare bound from anywhere in the United States.

Rich Milligan, a slight man with glasses, is the least busy of the six operations coordinators and the logical choice to take the Cancelator's chair. So, at 1:38 p.m., six minutes after the first raindrops are reported in the Chicago area, he slides without a word in front of the computer and clicks open the "Hub Cancellation Planner" software.

Normally, he can pick and choose the flights to cancel. He looks for flights that aren't full, for those carrying passengers who can take later flights. He avoids international flights. The purpose is twofold: minimize the inconvenience to passengers and maximize the profits to the airline. International flights are the biggest moneymakers.

Today, he doesn't have a choice. He begins dumping entire chunks of flights: everything arriving at O'Hare from 3 to 5 p.m.

Not all of these planes, however, will be stopped. Some have left their gates and won't be asked to turn back for now, although most are stuck on taxiways because of the existing delays. But, with the click of a computer mouse, the rest will show cancel signs at airports across the country and thousands of passengers will watch their days turn upside-down without a clue how it happened.

Milligan knows that, because of what he is about to do, a salesman may not close the deal he needs to reach a quota or that grandparents will be absent for the first hours of their grandchild's life. But he remains calm and matter-of-fact.

"This is pretty much the way it's got to be," he says.

At this point, there aren't many choices. Flights scheduled to stop in Chicago on their way, say, from New York to Los Angeles, could skip Chicago altogether. Or the airlines could try flying planes around the storms or divert them to other destinations. But those alternatives, the airlines say, could create more disruption than they prevent.

"Are you sure you want to cancel all flights?" the computer asks, issuing an electronic prompt that echoes a passenger's plaintive protest.

Milligan clicks "Yes."

Almost 1,000 miles away, at United's headquarters in Elk Grove Township, workers prepare to take the same step. Shift manager Robert Choroszy stands and walks toward a desk of four controllers.

Begin canceling flights, he tells them.

"We are going to cancel 15 percent from 4 p.m. to 8 p.m.," he says calmly and walks back behind his desk.

A few minutes later, based on a computer analysis, he raises the figure to 17 percent: 62 flights.

At 2:10, the Madsens, standing near their gate, anxiously scan the monitor, and, almost on cue, United 5870 disappears from the screen.

"That's it. I'm going home," says fellow passenger Denny DeJesu, snapping shut his briefcase in disgust. An employee benefits consultant with a company in Itasca, he was supposed to deliver a presentation. He decides he'll just cobble something together by fax and speakerphone.

The Madsens can't leave. They have to get back home to Ft. Wayne. And, unlike the business travelers who start to line the stools at Wolfgang Puck, devouring $8 pizzas, they don't have the money to while away a delay.

They're worried about the extra $8 that parking their car another day at the Ft. Wayne airport will cost them.

"If I did my job this way," Erik mutters, "I wouldn't have one."

On the radar screens, the thunderstorms slide across the country in silence, creeping toward O'Hare.

But, on the ground, they stampede like a posse of drunken cowboys, slamming up to the airport in a swirl of gusts that will top 47 m.p.h. and dump 2½ inches of rain on Chicago in an hour.

The first warning of their arrival is the wind, which begins, just before 1 p.m., to whip around erratically.

In O'Hare's air traffic tower, a 260-foot-tall, glass and steel tube at the airport's center, the volume of the constant chatter suddenly rises. The controllers pace the room, their headsets still attached to cords, blurting out strings of letters and numbers that tell the planes where to go.

They have to "turn the airport," switching which runways will be used for arrivals and departures so planes can take off and land into the wind.

"Let's keep moving, guys," one controller calls out to the pilots, herding planes from one spot to another. "Keep the line tight and keep it moving. ... United 8114, I need you to catch up with the group."

Another controller jumps out of his chair and walks across the room to make sure he can see one of his planes land, striding past the industrial-size bottle of generic aspirin, its lid off, which always sits near the coffee machine.

The day's weather complicates the situation, but the problem with the airport's seven runways is permanent. There aren't enough of them. And they intersect, which means planes must take turns coming in or going out, aggravating delays.

Then, a high-pitched horn starts to blare. It's a wind-shear alarm, warning of a sudden shift in wind direction that can sneak up behind an 80,000-pound plane and, as if it were flicking it with a finger, flip the aircraft into the ground. In the 1970s and '80s, before technology could detect wind shear, it caused several fatal crashes.

"United 687, wind-shear alert," a controller calmly advises one of his planes.

"Roger," the pilot replies. "United 687 is going around."

The plane breaks off its approach and circles back up into the sky, as do at least four others. Soon, other flights, farther from the airfield, realize the storm will arrive before they do and begin asking for help diverting to other airports. Planes can't fly through thunderstorms because lightning can burn holes through the fuselage and turbulence can injure passengers or even cause a pilot to lose control.

On the airfield outside Terminal 3, Kurt Schluter, who runs ground operations at nine of American Airlines' gates, stands near a cluster of empty baggage carts.

"Get ready to batten down the hatches," he says into his hand-held radio.

Workers connect the baggage carts together so they don't roll away in the high winds and hook tow bars to the airplanes so ground crews can move them as soon as possible after the storm lifts.

At United headquarters, weather forecaster Garry Hinds turns away from the computer screens. He walks to the window and watches as rain begins to sprinkle the cars below. "We thought it would take much longer to get here, but we missed it," he says, returning to his desk. "The plan did not work out."

Because everyone misjudged the storm, the airlines missed their chance to plan cancellations early. Now, they will have to do what they hate to do more than almost a a anything else: divert planes to other airports.

It's not just the inconvenience to the passengers they're worried about. Diverting flights puts planes and crews in places they aren't supposed to be, which costs money in wasted fuel and wasted wages, and then the airlines have to figure out how to get them back where they belong for the next flights.

On O'Hare's north side, Bill Lonergan, who manages this airfield for the city's Aviation Department, which runs operations at Chicago's three airports, stands on a grassy berm. Wind whips his airport badge and tie around his neck.

He came out here to meet with wildlife biologists who remove wild birds from the area so they won't fly into planes and cause damage or an accident.

Before he left his office this morning, he pulled up a local weather forecast on his computer and saw storms teetering northwest of Chicago. Thinking, like almost everyone else, they would move farther north, he concluded the weather didn't "look that bad."

Now, he sees clouds the color of charcoal swallow the sky. Gusty cool winds cut swaths through the blanket of heat.

He shuts the door to his car, his tie still flipped over his shoulder, and turns to two co-workers in the back seat.

He says, "Hell is upon us."