View Full Version : Leavin' on a jet plane...
OK, a question posed by my daughter. When you see a big commercial jetliner high up inna sky, it leaves a vapor trail. Howcome it don't leave a vapor trail when it's not up so high, like when you're watching it take off or land?
I'm not sure of the physics, but part of the reason is that it's very, very cold up there.
My company has a jet, with a display inside the cabin showing altitude and outside temperature. The time I got to ride on it, the outside temperature was -75 degrees when we hit cruising altitude. Brrrr.
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zyada is right. It's the cold. There's a certain amount of water vapor in jet exhaust, and when it gets up high enough it freezes. Atmospheric pressure might have something to do with it, too.
Additional helpful info:
When it is cold outside, open your mouth wide and exhale. See how much condensation there is?
When it is warm outside, open your mouth wide and exhale. Hey, how come I don't see anything!
If you look closely, you'll also see the condensation trails do not begin immediately behind the jet exhaust - it takes some time for the heated, compressed air to cool and contract, shedding its water vapor. The vapor trail consists of water already in the air the engine ingests (same holds true for piston engines) and is forced out of solution, so to speak, by the heating (compression) and cooling (contraction) processes. An similar phenomenon can be seen from the ground (or from your window over the wing) at the wingtips of fast airplanes landing or taking off in cool, humid conditions. As the air is accelerated over the wing, it is rapidly cooled, (venturi effect) and sheds its water vapor. (The wingtip vortices you're seeing can persist for hundreds of yards behind a departing or arriving aircraft, and can flip an unwary small plane pilot on his back in a big hurry). Sometimes you'll see the entire wing forming a condensate trail intermittently as the plane passes through varying layers of air at the correct temperature and relative humidity, but the effect is most pronounced at the wingtips.
The "vapor trails" that you see behind <<a big commercial jetliner high up inna sky...>> are properly called contrails.
In the book 'From Takeoff to Landing' (Sternstein & Gold) it says:
They are formed when heated hydrogen, a by-product of burned jet fuel, mixes with the atmospheric oxygen to make water. If the temperature is cold enough, the water will immediately freeze into ice crystals. These tiny ice crystals are what you actually see.
The trails behind the wingtips (vortices) mentioned by Nickrz, although similar, are created by an entirely different effect (Nickrz explained this one correctly). These vortices are the result of "induced drag" which is produced by the different pressures on the lower vs upper wing cambers. This effect increases proportionately to the weight, slowliness, and flap/slat configuration of the aircraft. The reason for "wingtips" on modern aircraft (Learjet "Longhorn", Airbus A320/321/319, etc.) is to reduce this induced drag and increase the aircraft's performance.
And let me tell you something, the "turbulence" behind a landing B757, can scare the sh.. out of most pilots flying behind one of those monsters, notwithstanding the size of your plane!
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