In April 2020, at the height of the coronavirus pandemic, the number of daily flights overseen by the Federal Aviation Administration dipped below 11,000. Now, with coronavirus vaccination rates trending upward and case numbers relatively low, Americans are ready to travel again. The FAA is managing 40,000 flights a day at this point and expects to handle as many as 48,000 flights daily by the end of the summer.

“We’re already seeing traffic is back,” said Daniel Murphy, director of National Airspace System Operations at the FAA. “If you have money, you want to go on vacation. You're going to places in the United States like Florida.”

For the past year, meager air traffic has meant minimal delays, even in the face of inclement weather. With so few flights, planes waylaid by storms weren’t likely to end up in a holding pattern at their destination waiting for an open runway. The recent surge in air traffic, however, has led to more delays, particularly in Florida.

Murphy said the number of flights to Florida is actually 10 percent higher than it was at this time in 2019, before the pandemic, posing an unusual challenge for air traffic controllers.

Typically, air travel to Florida peaks in the winter, when the weather is mild, and falls off in the summer, when high temperatures give rise to thunderstorms, he said. The usual drop in summer air traffic helps minimize thunderstorm-related delays, but this year is different.

“We can't necessarily rely on our historical experience to manage the traffic,” Murphy said. “There are more delays in Florida than we've seen historically for the summer.”

Thunderstorms, also called “convective storms,” produce severe turbulence and wind shear (changing wind speed and direction with altitude) that are hazardous to aircraft, Murphy said. As such, if a thunderstorm forms near an airport, air traffic controllers will pause takeoffs, causing delays. If a thunderstorm forms along a flight path, they will route planes around the storm, which can extend flights, also contributing to delays.

The large, well-organized thunderstorms that form over the Great Plains can sometimes extend for hundreds of miles, slowing air traffic across the country, he said. Florida tends to see thunderstorms that are smaller and shorter-lived, but also less predictable, creating challenges for air travel.

“In Florida, they get a lot of thunderstorms that are more associated with interactions with the ocean,” said Jennifer Stroozas, warning coordination meteorologist at the National Weather Service’s Aviation Weather Center in Kansas City, Mo. “They’re what we call ‘pulse thunderstorms.’ They kind of pulse up and bubble up and then die back down.”

The Aviation Weather Center supplies forecasts for air travel, focusing on weather phenomena that are relevant to aircraft, such as turbulence, icing and thunderstorms. From March to October, it works with the Center Weather Service Units, which provide forecasts to the 21 Air Route Traffic Control Centers, industry meteorologists and others to produce four-, six- and eight-hour forecasts for convective storms. Murphy said he begins every day by looking over the convective weather forecast for the United States.

“Especially this time of year, it’s all about thunderstorms,” Stroozas said. “Whether it's on the smaller scale, or whether it's a more organized storm, it's still a hazard and a threat for anybody flying an airplane.”

The return of air travel this summer means airports will be more crowded, which will exacerbate weather delays in vacation hot spots, like Florida, that are prone to thunderstorms.

“We’re already seeing delays because of weather,” Murphy said. “If we add a little bit more traffic into that, that’s going to make things interesting.”