Summer travel is back. So back, in fact, that the federal Transportation Security Administration has projected that more folks will board flights this summer than in 2019. Just how many of them will make it to their destinations without hitting delays, canceled flights, and other snags is far less certain.

The busy travel season so far has been continuously disturbed by unpredictable weather and staffing issues. On Sunday alone, as severe thunderstorms rolled through the East, more than 33,000 flights were delayed and nearly 2,700 were canceled, according to Flight Aware, which tracks flight traffic. Delays over the past few days have been particularly bad across the Northeast, but impacted the Southeast, too, as severe thunderstorms struck: This past weekend, that and a shortage of air traffic controllers contributed to significant delays at Orlando International Airport.

That’s a lot of flight disruptions — the culmination of a perfect storm of bad weather, a barrage of travelers unlike any we’ve seen in the past few years, and lingering staffing and scheduling issues in the airline industry. These delays have left thousands of those travelers stranded at airports in recent weeks awaiting flights, or without clarity on whether they’ll get to their destinations at all.

Just before the July 4th holiday weekend, more than 9,000 flights were also delayed or canceled, mostly due to severe storms that threatened the Northeast, followed by another 7,800 flights delayed by storms and tornadoes that ripped through the Southeast and Ohio Valley.

The Federal Aviation Administration, which enforces airline industry regulations and also manages air traffic faces a shortage of critical workers. This summer’s snarled and canceled flights, however, have mostly been the result of the weather. Weather is the cause of about three-quarters of air-traffic delays, and there’s not much airlines can do when a series of thunderstorms gets in the way. Severe weather in one part of the country doesn’t just stay localized, either. Interruptions in major airports have ripple effects everywhere else.

Staffing problems, along with equipment issues, have only made bad weather harder to accommodate. Slightly more than 19 percent of flights were delayed last year, in contrast to 2018, when about 17 percent of flights were delayed, according to the Bureau of Transportation Statistics. Almost 4 percent of flights were canceled in 2022. In 2023 so far, delays have affected about 22 percent of flights.

Widespread flight delays that have impeded airports and left vacationers stranded have been making news since late last year, and combined with busier airports and the potential for more severe weather to come, this summer could continue to be particularly bumpy for air travel.

“Anything well into the thousands is definitely newsworthy,” says Katy Nastro, a travel expert at the flight deals site Going.com (formerly Scott’s Cheap Flights).

“Everybody just assumes that the winter is the time period where you’re going to encounter tons of delays and cancellations,” Nastro adds. Weather-related delays are actually more common in the summer. Not only do rain and thunderstorms impact flights, but heat waves can also render airport tarmacs unsafe for workers, while high temperatures change the density of the air and affect whether a plane can take off. Hot weather can even lead to passengers getting booted off flights to adjust for changing weight limits.

Smoke and ash from wildfires will throw a wrench in flying, too. Smoke is more disruptive than fog or rain, the FAA’s Kevin Morris explained in a recent tweet, because flight navigation systems are designed to work well even against water droplets, but fare less well with solid particles such as those in smoke. In early June, the FAA briefly grounded flights at New York’s LaGuardia airport when smoke from a Canadian wildfire turned the city’s air quality into the worst in the world. Later in the month, smoke from the ongoing wildfires heavily impacted Chicago, whose O’Hare airport faced higher disruptions.

Planes can operate during extreme weather, and aircraft can safely fly above thunderstorms. Delays happen because airports and airlines have to be more careful during takeoff and landing, which means they move more slowly. That might mean they increase the space between planes or increase the time between one plane landing and another one taking off. Wind can postpone a plane flying or landing, too. With everything operating at a slower pace, the delays start to pile up and can even lead to cancellations.

There’s one other factor at play as we head into summer travel season: Climate scientists have long been telling us that extreme weather is more likely to occur, and likely to be more intense, due to the climate crisis. While the West Coast is no stranger to life disruptions due to wildfire smoke, there are signs that smoky skies could soon become a common feature of life in the Northeast, too. Experts predict, too, that summer 2023 will be particularly hot, and that’s all but guaranteed to affect travel.

Beyond weather and our increasingly dire climate, persistent issues with old technology and labor shortages have made air travel less reliable over the past year. Frustrations came to a head last December when Southwest Airlines canceled almost 3,000 flights during the peak holiday travel week. Inefficient scheduling, miscommunication, and staffing shortages all contributed to the mass cancellations. “During the pandemic, a lot of airlines gave buyouts to some of these older pilots,” explains Nastro, reducing their labor costs, but creating a dearth of experienced staffers just as travel ramped back up.

Travel disruptions aren’t solely the fault of foul weather or the airlines — the FAA shortage is also very real — but the federal government is trying to hold the industry more accountable. The Department of Transportation plans on proposing a new rule requiring airlines to compensate passengers for excessive delays and cancellations. In response to the mishaps and mayhem of 2022, airlines and the government have also been trying to gird themselves for this summer and the unprecedented demand for air travel. That has meant hiring more pilots and air traffic controllers, and reducing the total number of flights while increasing the capacity of aircraft.

Still, these problems don’t go away overnight. “Pilots take time to train,” Nastro says.

Early this month, United Airlines’ flights had the most disruptions. In an internal memo, United CEO Scott Kirby blamed the FAA, writing that it had “failed us this weekend” and pointing to its lack of experienced staff at the administration. The FAA told Vox in an emailed statement that it would “always collaborate with anyone seriously willing to join us to solve a problem.” It also said that it is taking steps to keep air travel “safe and smooth” this summer, including by encouraging airlines to fly fewer but bigger planes, which helps reduce traffic while still being able to deliver the same number of passengers to their destinations.

For passengers, there’s no hard and fast rule to avoid interruptions to planned air travel. Weather patterns can shift quickly and unexpectedly. “Try to take a morning flight, or try to take a nonstop flight,” Nastro advises. Early morning flights rank 25 percentage points higher on their on-time arrival rate than afternoon or evening flights. Delays and cancellations often occur due to snowballing effects of slowdowns, getting worse as the day drags on.

Nastro argues that it’s not all that helpful to look at which airports or airlines have the worst track records of delays and cancellations in a given period. That’s not an indication that bad weather won’t hit a usually on-time airport next. “It changes year to year. It can change month to month,” she says.

She also advises fliers to avoid assuming the worst will happen. Strict regulations around commercial air travel are the reason why it’s one of the safest modes of travel — far more than private flight or driving, and definitely more than private submersible excursions.

Update, July 10, 3:10 pm ET: This story was originally published on June 29 and has been updated to reflect additional flight disruptions in July.