(NewsNation Now) — For air travelers, the new year picked up where the old one left off — with lots of frustration over canceled or delayed flights.

More than 2,500 U.S. flights and more than 4,100 worldwide were grounded Sunday, according to tracking service FlightAware.

Southwest canceled more than 400 flights nationwide or 11% of its schedule Sunday. SkyWest, a regional carrier that operates flights under the names American Eagle, Delta Connection and United Express,  grounded more than 500 flights. American Airlines and Delta Air Lines scrubbed more than 150 flights each, and United Airlines canceled more than 100.

Among international carriers, China Eastern scrubbed more than 468 flights or 23% of its total schedule and Air China canceled more than 150 flights, 14% of its schedule, according to FlightAware.

The Christmas and New Year holidays are typically a peak time for air travel, but the rapid spread of the highly transmissible Omicron variant has led to a sharp increase in COVID-19 infections, forcing airlines to cancel flights as pilots and cabin crew quarantine.

Disruptions weren’t just due to the virus, however. Wintry weather overnight made Chicago the worst place in the country for travelers. More than 800 flights were scrubbed at O’Hare Airport and more than 250 at Midway Airport on Saturday into Sunday. O’Hare received just over 4 inches of snow.

Southwest Airlines suspended operations at both Chicago airports because of the snowstorm, according to an airline spokeswoman. She said Southwest knows from years of operating at Midway that high winds and blowing snow make it hard to get planes back in the air quickly.

Airlines say they are taking steps to reduce cancellations. United is offering to pay pilots triple or more of their usual wages for picking up open flights through most of January. Spirit Airlines reached a deal with the Association of Flight Attendants for double pay for cabin crews through Tuesday, said a union spokeswoman.

When winter weather hit the Pacific Northwest earlier this week, Alaska Airlines urged customers to delay any “non-essential” trips that were planned through this weekend. With full flights over the New Year’s holiday, the airline said it wasn’t sure it could rebook stranded passengers for at least three days.

Airlines hope that extra pay and reduced schedules get them through the holiday crush and into the heart of January, when travel demand usually drops off. The seasonal decline could be sharper than normal this year because most business travelers are still grounded.

Travelers who stuck to the roads instead of the skies faced challenges, too. Transportation officials in the Midwest warned motorists that a mix of rain and snow could make roads slippery and reduce visibility, leading to hazardous driving conditions.