A wave of flight cancellations and delays is already rippling across U.S. airports after the Federal Aviation Administration ordered airlines to reduce 4% of flights at 40 of the nation's busiest airports to ease pressure on unpaid air traffic controllers amid the record-long government shutdown. The airlines most at risk today include Spirit, Frontier, and United.

The FAA's unprecedented directive will cut flight operations by 4% today, rising to 6% by next Tuesday and reaching 10% by mid-month, that's if the federal government remains shut down.

For some context, a 10% reduction will result in about 4,400 flights canceled per day, causing widespread travel chaos nationwide ahead of the holiday period later this month.