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More than a third of flights leaving Midway International Airport were more than 15 minutes late in January, the 21st consecutive month the airport posted departures well below the national average.

The flights for the nation’s busiest airports are detailed in a report released Monday by the U.S. Department of Transportation.

In January, just 60 percent of flights out of Midway left within 15 minutes of their scheduled departure times — the standard the report uses to distinguish an on-time flight from a delayed flight — putting the airport at the bottom of the list for on-time departures.

And while this number was an improvement from December, when just 49.9 percent of flights left on time, it fell well below the 76.6 percent average. By comparison, 73.9 percent of flights left O’Hare International Airport on time in January, according to the report.

These numbers come after Southwest Airlines, which accounts for 85 percent of flight operations at Midway, acknowledged its on-time performance, particularly in the Chicago market, has slipped in recent months. In January, the airline ranked 13th out of 16 airlines for on-time performance, with 74.4 percent of its operations on time, according to the report.

“We’re not satisfied with our current on-time performance,” Southwest spokesman Chris Mainz said. “We see room for improvement.”

Mainz pointed to three causes of the delays: the weather, record numbers of overall passengers and record numbers of connecting passengers, due in part to the airline’s lower prices and “Bags Fly Free” campaign.

Indeed, in January, the airline served more than 6.8 million passengers, up from 6.3 million the previous year. Similarly, last month, the airline saw a 7.1 increase in passengers, according to a press release.

Because of Chicago’s centralized location, Mainz said Midway has become a major connection hub for the airline over the years and, with the recent surge in business, has experienced more delays than other airports in the Southwest system. This is because more passengers — and their luggage — are having to transfer from one plane to another at the airport.

“As you carry more connecting customers than you have in the past, it’s harder to do everything in the same amount of time,” Mainz said.

In an effort to combat the delays, Southwest spokeswoman Ashley Dillon said the airline is adding three departure gates at Midway in the coming months.

“We are still finalizing the details of which gates and where, but we think that will help with customer connectivity,” she said.

Southwest is also adjusting its schedule, allowing for more time between connecting flights, Dillon said. She added that the airline’s current schedule extends to October, so any changes made in the coming months would apply primarily to post-October operations.

“We don’t want to have average on-time performance. We want to have better than average on-time performance,” Dillon said. “We have to make adjustments, and I think toward the end of the year, we’ll begin to see changes.”

Chicago Department of Aviation offices were closed Monday and city officials could not be reached for comment.