More than 3,000 flights were canceled early Friday as a bitter winter storm engulfed parts of the US.
Disruptions are set to continue, the National Weather Service warned.
The National Weather Service said early Friday that the US would experience a "historic winter storm" with more than 240 million people under winter weather warnings or advisories.
Around a third of the flights to and from Denver International Airport were canceled and around a quarter to and from Chicago O'Hare International Airport were canceled, FlightAware data shows.
Other airports, including Dallas Fort Worth International Airport, Reagan National Airport, and Seattle-Tacoma International Airport, had a large proportion of their flights delayed, although fewer than 10% were canceled.