Airline passengers know to expect possible flight delays when weather forecasts call for thunderstorms, heavy snow or strong wind. The omens are less obvious on a hot summer day with clear skies.

Yet extreme heat also disrupts flights and travel plans. On the hottest days, thinner air can make it harder for aircraft to get off the ground safely. Airlines may have to lighten the load by removing passengers or cargo, or carrying less fuel.

The snag is well known at airports in desert climates around the world, from the U.S. Southwest to the Middle East. But even less extreme heat can create similar challenges at airports with higher elevations or shorter runways. Researchers say human-caused climate change could make such disruptions more common.

With heat waves searing parts of the United States and Europe this summer, Las Vegas illustrated how extreme heat can interfere with air travel. American Airlines offered compensation to customers who voluntarily gave up their seats on a July 24 flight from Harry Reid International Airport after temperatures reached 114 degrees Fahrenheit (47 degrees Celsius).

The airline described reducing a plane’s weight as a standard industry practice in hot-weather destinations when conditions require it to ensure a safe takeoff. The explanation comes down to physics.

Hot air is thinner than cool air, making it harder for a plane’s wings to generate lift and for jet engines to generate the thrust needed for takeoff, explained Ross Aimer, a retired United Airlines captain and CEO of Aero Consulting Experts.

“There is a science behind all this,” Aimer said. “You can’t deny the science of flight and what it takes for an airplane to operate.”

Airplanes struggle in hot, thin air

Aimer likens the effect of high temperatures on airplane performance to athletes competing at high altitude. During this year’s World Cup, foreign teams competing in Mexico City — located more than 7,300 feet (2,225 meters) above sea level — had to contend with thinner air that can leave players fatigued more quickly.

“Just like human beings, jet engines require lots of cool, dense air to perform well,” Aimer said.

Before every departure, pilots and airlines determine whether an aircraft can safely take off by calculating several key factors, including the airplane’s weight, the airport’s elevation and the outside temperature, said Alan Price, a retired Delta Air Lines captain.

Aircraft at higher elevations are already operating in thinner air, so temperatures don’t have to climb as high to affect takeoff performance. Denver, for example, sits more than 5,000 feet (1,524 meters) above sea level.

Runway length also matters. Airliners need room to build enough speed for takeoff when temperatures soar, and shorter runways leave less distance to do that, Price said. That’s one reason airports such as New York’s LaGuardia and Washington’s Reagan National can be more likely to face takeoff restrictions or weight limits during very hot weather.

If the pilot’s preflight math shows an aircraft cannot safely depart as planned, airlines may choose to reduce cargo, carry less fuel and make a refueling stop later, wait for cooler temperatures or ask passengers to volunteer for a later flight.

But there comes a point when removing weight isn’t enough to make a safe takeoff possible.

“At some point, the numbers just don’t make sense,” Aimer said. “You could roll down a runway as long as you want, and you still can’t take off.”

Delays due to hot weather could become more common

The Federal Aviation Administration does not set a single maximum temperature for takeoff. Instead, the FAA says, operating limits vary based on an aircraft’s make and model, its weight and the airport’s altitude.

A 2017 study by researchers from Columbia University, NASA’s Goddard Institute for Space Studies and the Logistics Management Institute projected that some airports could see weight restrictions on 10% to 30% of flights departing during the hottest part of the day by midcentury if greenhouse gas emissions continued to rise. The study examined 19 airports around the world and found that airports with short runways, high elevations and hotter climates would be affected most.

A more recent study concluded that current climate prediction models indicated planes taking off from four European airports would have to shed the weight equivalent of 10 passengers per flight by 2065.

While triple-digit temperatures come with desert living, the kind of hot days that might affect flights are becoming more frequent in both Phoenix and Las Vegas.

Phoenix Sky Harbor International Airport has averaged 43 days each year since 2020 when the temperature reached at least 110 F (43 C), according to an analysis of weather station data by The Associated Press. The number of such days increased gradually over several decades before surging in the 2020s to more than triple the average during the middle of the last century.

At Harry Reid International Airport in Las Vegas, temperatures reached that threshold an average of about 18 days a year between 2020 and 2025 — more than double the annual average during the 1940s through 1960s, the AP analysis found.

The 2020s have also been exceedingly hot at Palm Springs International Airport. The airport in Southern California has recorded an annual average this decade of 55 days with temperatures reaching 110 F — an 83% increase compared to the 2000s, when the weather station there began recording data.

Safety determines when airlines reduce weight

Airlines have dealt with these issues for years. During a stretch of scorching heat in June 2016, American canceled several round-trip regional flights between Palm Springs, California, and Phoenix. The following year, the airline canceled dozens of regional flights in Arizona as temperatures in Phoenix climbed toward 120 F (49 C).

In July 2018, some airlines bumped passengers at Denver International Airport to later flights as temperatures reached 98 F (37 C), KDVR-TV in Denver reported.

Delta Air Lines also asked for volunteers to step off a plane in Las Vegas during an excessive heat warning in July 2023 because of weight restrictions tied to the extreme temperatures. That flight was ultimately canceled after passengers spent hours aboard the aircraft in the heat.

Airports can be especially vulnerable to the urban heat island effect, which is the phenomenon of built-up areas becoming hotter than their surroundings, said Jennifer Brady, a data scientist with Climate Central.

With little shade or green space, vast expanses of heat-absorbing pavement and constant activity from airplanes, ground vehicles and air conditioning systems that generate waste heat, “all of the elements are there for it being one of the hottest spaces in a city,” Brady said.

Wildeman reported from Hartford, Connecticut.