As of 9 p.m. ET Monday, more than 6,000 flights in, out of and across the U.S. have been delayed, according to the flight tracking website FlightAware. Another 640 flights were canceled.

Cancellations, in particular, are still down compared with the first three months of the year when airlines battled staffing issues related to the omicron variant of COVID-19 as well as winter storms, FlightAware spokesperson Kathleen Bangs told FOX Business.

Meanwhile, delays have increased throughout the second quarter and even into July and August, she continued.

Although airlines are still batting staffing issues, delays and cancellations were particularly heavy all weekend due to thunderstorm activity and weather ground stops, according to Bangs.

Travelers queue up at the south security checkpoint in the main terminal of Denver International Airport, May 26, 2022.

Passengers faced a spike in cancellations, particularly on Thursday and Friday.

On Thursday, 7,612 flights were delayed followed by another 8,865 on Friday, which was the highest number of delays all weekend, according to the data. Meanwhile, 1,239 flights were canceled on Thursday followed by another 1,614 flights on Friday.

According to the data, delays on Saturday and Sunday surpassed 14,200 while cancellations notched about 1,585 for both days.

A Delta Air Lines spokesperson told FOX Business that any disruptions the airline faced over the weekend resulted from weather and air traffic control challenges.

Representatives for American, Southwest and JetBlue did not immediately respond to FOX Business' request for comment.

Despite these issues, Bangs said we could have seen even more cancellations if airlines hadn't pared back their summer schedules.

Earlier this summer, Nicholas Calio, president of the trade group Airlines for America, said its member carriers cut 15% of the flights they originally planned for through August while also ramping up hiring and training to combat issues and become more reliable for passengers.

A pilot walks through Hartsfield-Jackson Atlanta International Airport in Georgia, Dec. 21, 2021.

Problems have still persisted as demand ramps up to pre-pandemic levels, forcing some carriers to reduce their schedule for the fall.

Last week, American announced that it has already taken steps to reduce its overall September system capacity.