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A warmer world could make current airport runways too short

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Next time you're in an airplane racing down the runway for takeoff, make a mental note of that moment when the plane leaves the ground. In a warmer world, that moment of liftoff will come later.

Here's why:

Airplanes take off when the air passing over their wings creates enough lift — lift greater than the aircraft's weight. That moment is determined by, among other things, the air's density. Air is denser at sea level where there's greater atmospheric pressure. It's also denser in colder conditions; a cold molecule takes up less space than a warmer one.

Somewhat counter-intuitively, humidity decreases air density as well. A water molecule occupies as much space as other molecules in the atmosphere — chiefly oxygen and nitrogen — but its molecular weight, one oxygen atom and two hydrogen, is much lower. (Nitrogen and oxygen molecules contain two identical atoms; O2 is nearly double the mass of H2O, while N2 is almost half again as heavy.) You may be sweating and feeling suffocated, but all that humidity actually makes the air thinner.

By now, you've probably put two and two together: Temperatures are rising, and they're predicted to rise more this century. Warmer air holds more humidity. Perhaps more importantly, days of extreme highs have — and will likely continue to — become more frequent. All of which changes that moment when a plane speeding down the runway generates enough lift to take off.

Everything else being equal, in warmer conditions, planes take off later. Few, and perhaps none, of the nation's airports were built with a warmer future in mind. The question: In a warmer world, will planes have enough runway for liftoff?

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