Nearly 7 million travelers flooded U.S. airports on Friday, Saturday and Sunday, with a total of 13 million air travelers to, from or within the United States expected to have passed through by the end of this Fourth of July weekend.

For many of those passengers, however, travel plans were upended because of flight delays and cancellations caused by a boom in travel demand coupled with widespread staffing shortages. From Friday through Sunday, airlines that fly within, into, or out of the United States canceled more than 1,400 flights, according to FlightAware, a flight-tracking website, stranding and angering some passengers headed for long-awaited summer vacations. In addition, more than 14,000 U.S. flights were delayed this holiday weekend, according to the site’s data.

The experience has been frustrating for some passengers on U.S. carriers. On Saturday, 1,048 — or 29% — of Southwest Airlines flights were delayed, as were 28% of American Airlines flights, according to FlightAware. United Airlines and Delta Air Lines had similar problems, with 21% and 19% of their flights delayed. On Sunday, the middle of the holiday weekend, travelers seemed to be getting a respite from the worst of the troubles, with roughly three-quarters of the delays and half as many cancellations as the day before.

As of noon Eastern time on Monday, there were more than 1,000 delays and about 180 cancellations at U.S. airports.

In a typical month, about 20% of flights are delayed or canceled, according to Robert W. Mann Jr., a former airline executive who now runs the airline consultancy R.W. Mann & Co. But this holiday weekend, he said, it was about 30%. “It’s a little bit worse than usual,” he said.

As airlines face a pilot shortage, bad weather conditions and air traffic control delays, some seemed to struggle to handle passenger volume that approached or in some cases exceeded prepandemic levels. On Friday, the Transportation Safety Administration screened more passengers — 2.49 million people — than on any other day this year. That surpassed the 2.18 million travelers screened on July 1, 2019, before the pandemic.

Still, travel to and from airports in the United States seemed to be going better than in many other parts of the world. On Sunday, airlines had delayed about half of all flights departing from Toronto Pearson International Airport, Charles de Gaulle Airport in Paris and Frankfurt Airport, while about 40% of flights from London Heathrow were delayed.

On Monday, Australian airports were hit hard, with nearly 60% of departing flights from Sydney Airport delayed, while airports in Brisbane and Melbourne did not fare much better. SAS, the Scandinavian airline, said on Monday that its pilots’ union had called a strike over pay, which would lead to the cancellation of 50% of its flights, affecting about 30,000 passengers daily. The money-losing carrier, which serves as the national airline of Denmark, Norway and Sweden, called the move “devastating.”