If you’re wondering about the best and worst airlines to fly during the holidays, you need to talk to Brent Bowen.

He’s the Embry-Riddle Aeronautical University professor who publishes the Airline Quality Rating (AQR), a respected gauge of airline performance. Bowen, who is based on the university’s Prescott, Arizona campus, uses data collected from the government to generate a report card.

Bowen and his co-researcher, Dean Headley, release the AQR annually in April, but they monitor airline performance year-round. He offered Forbes an exclusive look at his research for the holiday travel season. The surprising results might help you have a better flight during the busy Christmas and New Year’s travel period.

“The airline industry as a whole performs lower during the holiday travel period,” explains Bowen. “There’s a significant downturn. It’s a period of high demand and there are winter storms.”

The U.S. Department of Transportation (DOT) publishes numbers on complaints, lost luggage, on-time departures, and overbooking. To find a list of the best and worst airlines to fly during the holidays, Bowen pulled data from the government’s Air Travel Consumer Reports from November, December and January over the last three years.

“It’s much more difficult to maintain on-time performance during the holidays,” says Bowen. “Consumer complaints go up. That leads to a decline in scores.”

How to fly during the holidays? Here’s advice from an expert

Based on this list of the best and worst airlines, how should you fly during the holidays? While Bowen recommends flying on the better-performing carriers, he says avoiding the worst airlines isn’t always possible. You might need to connect to a regional carrier when you’re flying somewhere or may not have the budget to fly on one of the better airlines.

“If you want to save money, for example, you may still want to fly on Frontier,” he says. “But you need to know what to expect.”

Bowen recommends avoiding “bad weather” hubs like Chicago, Cleveland and San Francisco.

“The airlines intentionally schedule their flights for fair weather conditions,” he says. “If you have to go to instrument-only landing system, because of ground fog or a marine layer, then you have an automatic backlog. That creates an automatic delay.”

Also, don’t check a bag, he says. Winter weather can delay baggage for days or weeks. Instead, carry everything on the plane if possible. That advice is echoed by weather experts who I interviewed for my transportation column in the Washington Post.

Ideally, of course, you would postpone any air travel until after the holidays. That’s the real takeaway from this list of the list of best and worst airlines to fly during the holidays. That’s what Bowen is doing this year.

“I’m not going anywhere during the holidays,” he told me. “And if I do, I’ll drive.”

Christopher Elliott’s latest book is “How To Be The World’s Smartest Traveler” (National Geographic). For help with any consumer problem, please visit https://www.elliott.org/help

This article originally appeared in Forbes. © 2018 Christopher Elliott.