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A warmer world could make current airport runways too short

Global warming

By Moises Velasquez-Manoff / September 10, 2009



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.

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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?

The US Department of Transportation noted [PDF] global warming's impact on aviation as far back as 2002 :

Higher or more frequent extreme temperatures associated with climate change may, in conjunction with aircraft type (rated cargo and passenger capacity, engine size and efficiency), runway length, destination elevation and location (requirements for additional fuel storage) and other factors, reduce aircraft cargo carrying capacities.

A follow-up report last year elaborates [PDF]:

[H]igh temperature, combined with moisture and field elevation is used to calculate “density altitude”, used to quantify engine combustion efficiency and the needed runway length for take-off and landing at specified aircraft loads. On hot summer days at high altitude airports, such as at Denver International, aircraft may have to burn fuel, unloading weight, in order to have a safe take-off roll. Airport runways are, of course, designed to the climatological conditions of temperature, moisture, wind velocity, and visibility; therefore, for example, higher altitude airports have longer runways. Still, with more days of higher temperatures, the number of days of limited operations at high altitude airports will increase, essentially at airports in the intermountain west (Arizona, Colorado, Idaho, Montana, New Mexico, Nevada, Utah and Wyoming). Moist air, being less dense than dry air, also contributes to higher density altitude, but temperature is a more important factor in the calculation.

Then comes the kicker: "For aircraft that use up most of the pavement on even the longest runways, even a 1 or 2 percent increase in density altitude from increased moisture may put those aircraft out of commission for daytime operations on certain days. With more days of higher temperatures, the number of days of limited operations at high altitude airports would be expected to increase."

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