CHICAGO — Southwest Airlines was slammed with a record fine by the Transportation Department on Thursday after the airline forced passengers to remain on arriving planes at Chicago Midway International Airport for hours during severe weather last winter.

Delays involving 16 airplanes last January resulted in a fine of $1.6 million.

It’s the largest civil penalty the Department of Transportation has assessed an airline for violating its tarmac delay rules, which require airlines to offer passengers on domestic flights the opportunity to get off the plane within three hours of arrival.

“Airline passengers have rights, and the department’s tarmac delay rules are meant to prevent passengers from being stuck on an aircraft on the ground for hours on end,” Transportation Secretary Anthony Foxx said. “We have aggressively enforced, and will continue to aggressively enforce, our tarmac delay rule to ensure carriers have adequate resources to minimize passengers’ exposure to lengthy tarmac delays.”

The Transportation Department said 16 Southwest flights at Midway experienced tarmac delays of more than three hours overnight Jan. 2-3, 2014.

Southwest’s crew scheduling system malfunctioned, and an unexpected shortage of workers, particularly the carrier’s ramp crew, hurt the carrier’s ability to clear aircraft from gates to accommodate arriving flights.

That day, severe winter weather, including heavy snow and record-setting cold, resulted in thousands of canceled and delayed flights for many airlines at Midway and the city’s busier airport, O’Hare.

Southwest Chief Operating Officer Mike Van de Ven said in a statement he was “disappointed” the government would dole out the fine on top of the penalties imposed by Mother Nature during the winter storms.

“Southwest conducted an extensive internal review of the events,” he said. “Based upon our findings, we made significant investments in our operation to prevent recurrences, including enhancements to our policies and procedures, staffing and airfield monitoring equipment.”

Southwest, which said it “acted reasonably to prevent the delays,” paid affected passengers compensation totaling $269,000 and added staff to Midway at a cost of $2.4 million per year, the company said.

Before the incident, Southwest had no tarmac delay fines from the DOT, the airline noted.