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- 天同Lv 71 十年前最愛解答
Modern aeroplanes are propelled by jet engines. The air inside a jet engine , after heated to high temperature and high pressure, is forced to exhaust from the engine as a stream of fast moving hot air, thus exerting a forward propelling force on the aeroplane.
When the plane is flying at high altitude, the air in the surrounding atmosphere is of low temperature and low pressure. As the high presseure hot air gets out of the engine, it expands rapidly. The air thus undergo adiabatic expansion and cools down quickly. The water vapour originally present in the hot exhaust air therefore condenses out and forms tiny water droplets, which leaves a trail in the air. This trail formed by tiny water droplets is seen on the ground as a white line following the plane.