Why do planes fly so high?

+73 votes
asked Jul 15, 2015 in Cars & Transportation by Haeley (970 points)
I just return from a trip overseas. While flying back, I notice that the plane is flying far above the clouds. Why do planes fly so high? Why does it need to go up above the clouds? Shouldn’t it be enough if they just reach a height where it could avoid most obstacles? Does it save fuel because of the thinner air? But if so, wouldn’t they waste more fuel flying up that high in the first place?

2 Answers

+19 votes
answered Jul 24, 2015 by Heath (930 points)
Planes fly high in order to save fuels. When we travel by air we are at the altitude of 37,000 feet high on the average commercial jet. To fly at that high altitude, the planes would need to climb all the way up there first. This requires a lift and forward velocity, which burns a lot of fuels. So why do we try to fly up that high rather than just maintain a height high enough clear most obstacles? It’s all about cost-effectiveness really. Airplanes burn less fuel at a much higher altitude. High altitudes have thinner air, which is much kinder to the jet engines. Jet engines require a constant supply air, working better based on the stoichiometric ratio, in layman’s term, air to fuel ratio. In higher altitudes, there is less air and so the engine needs less fuel, so effectively less fuel is used. In addition, the lower air density makes the plane fly smoother and as such faster, as there is less drag force against the aircraft. So by burning more fuel to get to the higher altitudes, the jet plane could use up much fewer fuels for the rest of the journey.
+7 votes
answered Jul 24, 2015 by Couture (1,140 points)
So why do planes fly so high? Because planes fly higher than the clouds to avoid harsh weather. The usual weather we know of, rain, snow and thunderstorms, all occurs in the troposphere, which is at the average of 36,000 feet, the closest atmosphere layer to the Earth surface. This is where all the clouds are, essentially the thickest layer of the air concentrated with water vapor. Planes fly above the layer at 37,000 feet, so we avoid facing the danger of weather. When your planes are having turbulences, it is often caused by the airplane slicing through this troposphere layer, which rich is water vapor and ice. In addition, the clouds and water droplets might reduce the visibility of the pilots. Hence, the flight at the high altitude is some much safer.
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