It’s heat wave season. That’s bad news for your flights.

How extreme heat affects planes

Planes absorb energy from the sun and ground, and passengers add to the heat and humidity once they’re onboard, too.

In a post on the school’s website, experts at Embry-Riddle Aeronautical University explain that hot air is thinner and less dense than cooler air. As a result, planes produce less thrust as they’re taking off, need more speed to get in the air and require more room on the runway to reach that speed.

“The only way to make a takeoff possible on shorter runways would be to lower the weight of the aircraft,” Bob Thomas, an assistant professor of aeronautical science at Embry-Riddle, said in the post.

On days when weight restrictions are in effect, airlines generally shed fuel, bags or people. The number of those days is expected to multiply, according to research from Columbia University that examined four high-risk airports: Denver, New York’s LaGuardia, Phoenix and Reagan National near D.C.

Sometimes flights are delayed until the temperature is cooler or are canceled outright. When planes do fly on hot days, the experience can be miserable if passengers are stuck on the tarmac. During a stretch of intense heat last summer, travelers on multiple airlines complained that they were stuck on overheated planes. In at least one case, passengers needed medical attention.

How planes can (and can’t) cool down

While federal regulations say travelers have the right to “comfortable cabin temperatures” during a tarmac delay, the regulations don’t define “comfortable.” The Federal Aviation Administration reauthorization bill, signed into law last month, includes a mandate to study and evaluate standards for temperatures on planes.

Plane cooling systems work best when aircraft are flying, not when they’re on the ground. If a plane is stuck on the tarmac, the captain might shut down a main engine, which is the best source to power the air conditioning. In those cases, an auxiliary power unit might not be capable of cooling the plane as much as needed.

Airlines often will pump cooler air into a plane before boarding, but that might not be enough to make the cabin comfortable on extremely hot days.

“At some point on the temperature scale, the very best unit can only do so much,” Billy Nolen, a former FAA acting administrator and the chief safety officer at the aircraft company Archer, told The Washington Post last year.

Airlines have said they take steps to keep planes cool enough, while acknowledging that those measures are required more often.

“We’re really taking this seriously, and we’re going to have to as we go forward,” American Airlines CEO Robert Isom said during a call with investors last summer.

Experts recommend that air travelers avoid booking flights during the hottest part of a day. Embry-Riddle’s post says flights in the early morning and late evening are the least likely to experience heat-related issues.

Most air-travel insiders already suggest booking the first flight of the day to avoid weather delays, since those tend to build up throughout the day.

“No one likes to fly at 5 a.m., but it’s cooler then, and the plane should leave on time,” Brian Dilse, an academic instructor in the University of North Dakota’s Department of Aviation, told The Washington Post last year.

Andrea Sachs contributed to this report.