Why extreme heat might cancel your flight
Credit: Jaromir Chalabala/ Shutterstock. Jennifer Griswold, associate professor in Atmospheric Sciences, was interviewed by Discover Magazine recently due to her research on the impacts weather and climate can have aviation. Below is an excerpt of the resulting article.
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Every spring and summer, the headlines appear more familiar: Airlines are grounding flights because it’s simply too hot to fly. These declarations often come out of the toastiest U.S. cities, like Phoenix, which last year set a personal record of “most number of days over 110 degrees Fahrenheit.” Like humans on particularly scorching days, planes often can’t operate properly in extreme heat. In these cases, the aircraft is physically unable to take off because of the temperature.
It’s an awkward situation for airlines and their customers, who might not get why the perfect-seeming weather is stopping their trip. “You can see snow, you can see ice on the wings, you can see heavy rain or lightning, and you don’t want to get in a plane when that’s happening,” says Jennifer Griswold, an atmospheric scientist at the University of Hawai’i at Manoa. “But if it’s just sunny and really bright out and there’s not a cloud in the sky, it’s like, what’s wrong?” Meanwhile, flight operators are keenly aware of how quickly extreme heat can derail flight plans — and know that it’s going to become an even larger stumbling block in the future.
When Physics Keeps You Grounded
To understand why air temperature can make or break a functional flight, you have to know that pilots and flight engineers think of the gas-filled atmosphere instead as a fluid: Planes interact with the air similar to how we interact with water when we float. As a plane cruises down a runway, it pushes against the air. Due to one of the main rules in physics — every action comes with an equal and opposite reaction — the air pushes back. Some of that response translates into lift, the official name for the force that pushes planes into the sky.
But higher-than-expected temperatures interfere with how the air pushes back. Like with water, adding heat to air separates the molecules and forces more space between them. A hot summer day therefore means that the density of the air hovering just above the tarmac drops. More broadly, as high temperatures thin the atmosphere, oxygen molecules spread farther apart from one another. With fewer air molecules pushing back beneath the wings of the plane, the air fails to generate enough force for takeoff.
If the heat is too intense and the air too thin, the flight will stay grounded and might get a chance to take off once the sun sets and the temperatures drop. In some milder cases, there are a few options for pilots to compensate for the lower-density air and still take off on-schedule, some more theoretical than others.
When examining how El Niño and La Niña shifted conditions at 11 U.S. airports from 1979 to 2015, Griswold and her co-author found that the air density, or how tightly packed the air molecules were over the ground, was all over the place depending on location. Contingent on the geography, fluctuations in temperature and moisture can go in either direction during El Niño and La Niña. Some airports saw hotter, more humid conditions — a bad combo for flights, since higher levels of water in the air make the atmosphere less dense, too. Others got lucky with cooler, dryer weather. “The take-home from that particular work was that it’s very regional,” Griswold says. “Each airport needs to know what they would expect.”
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Read the full article at Discover Magazine.