Hundreds of flights were delayed and dozens canceled at San Diego International Airport on Sunday, but most airline passengers appeared to take the interruptions in stride.

As of 5 p.m., San Diego had seen 100 cancellations for flights within, into or out of the United States, and nearly 300 flights delayed, far more than the last couple of days, according to data provided by the website FlightAware.com.

Many of the delays and canceled flights were the result of a federal mandate to cut air travel due to a shortage of air traffic controllers across the nation because of the federal government shutdown. But severe weather, including a snowstorm in Chicago, was also a factor.

In addition, the San Diego airport was under a daylong Federal Aviation Administration ground delay due in part to morning fog that had a ripple effect on flights throughout the day, said airport spokesperson Jonathan Heller. Southwest Airlines also confirmed that the ground delay was responsible for many delays and canceled flights.

“It’s not that bad,” said Ethan Perdue, of Oceanside, who drove his girlfriend to the airport for a flight on Southwest Airlines to Buffalo, N.Y. Her flight was delayed more than an hour, but she received an email in the morning with the updated departure time.

“We had time to prepare,” Perdue said.

Sunday was the third day since the FAA ordered flights cut at the nation’s 40 busiest airports because of the shortage of air traffic controllers, who are going unpaid during the ongoing government shutdown.

The disruptions could get worse. The FAA reductions started Friday at 4% and will increase to 10% by Nov. 14. They are in effect from 6 a.m. to 10 p.m. local time and impact all commercial airlines. International flights are unaffected.

And U.S. Transportation Secretary Sean Duffy warned Sunday in an appearance on “Fox News Sunday” that air traffic across the nation could “slow to a trickle” if the federal government shutdown lingers into the busy Thanksgiving travel holiday season.

A constantly updating information screen at the Southwest ticket terminal in San Diego’s Terminal 1 at 12:30 p.m. showed only three Southwest departures canceled, but about three-quarters of the nearly 100 flights listed were delayed. Some delays were as brief as 15 minutes, most were less than an hour, and some were as long as two hours or more.

The delays included a 9:40 a.m. flight to Austin, Texas, delayed to 12:28 p.m., an 8:05 a.m. departure to Portland, Ore., held until 11:40 a.m., and a 7:35 p.m. bound for Chicago delayed to 9:06 p.m.

Southwest is the largest carrier at the San Diego airport, with more than one-third of the total traffic. Early Sunday afternoon the lines to check in were long, but people moved steadily and orderly through the gates.

Over at Terminal 2, which has most of the other airlines serving San Diego, there were fewer people and only short lines for service. Still, an LED information screen there about 1 p.m. showed eight cancellations and more than two-thirds of the other flights were delayed, though most delays were brief.

“A lot of these people are understanding, but you can tell they are irritated,” said an Alaskan Airlines employee who was helping people check in. “Everybody’s been pretty good about it though.”

AnneMarie Ausland, a shuttle employee directing arrivals to their ground transportation, said she had greeted dozens of people coming in from across the country for a golf tournament in Carlsbad.

Many of her passengers were on delayed flights, she said. If they seem grumpy, she told them to look at the sunshine all around and tried to cheer them up.

“You can take your attitude to the negative side or the positive side,” Ausland said. “Keep a positive attitude. The air traffic controllers will get paid.”

More than 2,100 flights were canceled and 7,000 were delayed Sunday across the United States, according to the Associated Press.

Hartsfield-Jackson International Airport in Atlanta had the most cancellations Sunday, with more than 570, the news service said. Chicago O’Hare International was second with about 400 cancellations, and Newark Liberty International was third with about 350 cancellations.

The San Diego airport and others are advising passengers to arrive at least two hours before their departure time.

The Associated Press contributed to this report.