This year's weather has been among the worst in a decade for air travel in the United States, aviation authorities say, with the most volatile period possibly still ahead as the summer thunderstorm season is underway.
In June, 15 percent of flights by all airlines at O'Hare International Airport were delayed because of bad weather, up from 3 to 6 percent over the previous two Junes, according to calculations that United Airlines provided to the Tribune, based on U.S. Department of Transportation statistics that will be publicly released in August.
Total delays at the airport haven't been this bad since before a new runway opened in late 2008, airline officials said.
"It has just been thunderstorm after thunderstorm rolling through," said Glen Martin, who oversees air traffic control in the central U.S. for the Federal Aviation Administration. "We are hoping it gets better over the next three or four months."
They're hoping for — but not counting on — improvements.
In interviews with the Tribune, officials from the FAA, United and American Airlines laid out their strategies to deal with mounting flight delays and keep the nation's busiest airports like O'Hare operating when storms move in.
Computer software upgrades will help controllers more equitably spread delays throughout the nation's airspace, rather than overpenalizing O'Hare by too frequently halting traffic to and from the airport — with the dreaded "ground stop" — until the weather clears, officials said.
"In the past, the thinking was it was easier to shut off O'Hare and take pressure off the whole system because O'Hare handles so much traffic," Martin said.
The airlines also have pledged to do a better job communicating with passengers about flight delays so that even if trips take much longer to complete, the information is put out there as early as possible.
New initiatives should also help reduce the frequency of flights being diverted to other airports during severe weather, airline officials said.
In Chicago, the airlines have been adding flights at O'Hare and Midway airports. Both can handle more flights on blue-sky days when the air is relatively calm and visibility is good, according to the FAA. The problem is that the weather hasn't cooperated.
From January through spring, O'Hare has been at the bottom of government rankings for on-time departures and arrivals.
Departures at Midway also have been prone to frequent delays, while arrivals rank in the middle of the pack among the nation's busiest airports.
The share of delayed flights because of bad weather has increased. In April and May at O'Hare, about 65 percent of delay minutes were attributed to weather, up from 29 percent in April 2012 and 58 percent in May 2012, according to the U.S. Bureau of Transportation Statistics.
United in June canceled twice as many flights as it did a year ago systemwide because of severe weather, the airline said. Meanwhile, on-time arrivals of the flights that did operate improved by almost 4 percent over 2012, the carrier reported.
At Midway, the number of flight delays caused by weather increased to a total of 1,295 flights in April and May this year, from 374 weather-delayed flights for the same two months last year, the transportation statistics bureau reported.
In an average year, bad weather accounts for about 90 percent of air traffic delays and canceled flights, according to the airlines and FAA.
"This is one of the worst years I have seen in the last decade. The storms, they come wave after wave and they won't go away," said Bob Flynn, FAA air traffic manager at O'Hare tower.
"Instead of raining themselves out and dying like they usually do, the thunderstorms sit out there for an hour or two and grow," Flynn said. "Nobody wants to fly anywhere near it obviously with the updrafts inside the convective weather."
Unlike during winter, when snowstorms can be reliably forecast a day or more in advance and airlines proactively cancel flights, summer thunderstorms tend to be much more unpredictable and the airlines wait it out.