十公里高空

长途飞行时望向窗外，你会看到脚下十公里处的世界。飞机飞得这么高，并不是为了看风景，而是因为高空的空气稀薄，稀薄的空气飞行成本更低。

Look out of a plane window during a long flight and the world is ten kilometres below you. Planes do not fly that high to enjoy the view. They do it because the air up there is thin, and thin air is cheap to fly through.

靠近地面的空气比较浓密。飞机穿过浓密的空气，就像游泳的人在水中前进：每前进一米都要消耗能量。到了十公里的高空，空气要稀薄好几倍。飞机穿过稀薄的空气时阻力小得多，发动机要保持同样的速度，消耗的燃料也少得多。稀薄的空气还带来第二个好处：大多数云层、风暴和颠簸的天气都出现在这个高度以下，所以飞行也更平稳。

Near the ground, air is thick. A plane pushing through thick air is like a swimmer pushing through water: every metre costs energy. Ten kilometres up, the air is several times thinner. The plane slips through it with much less resistance, so the engines burn far less fuel to keep the same speed. The thin air has a second gift: most clouds, storms, and bumpy weather live below that height, so the ride is smoother too.

当然，高度也有极限。飞得太高，空气就会稀薄到机翼无法产生足够的升力，发动机也无法正常呼吸。因此，航空公司会瞄准一个最佳区间，通常在十到十二公里之间。既能省油，又能保持飞行。这正是每次安全带指示灯熄灭时，背后进行的那番计算。

There is a limit, of course. Too high, and the air becomes too thin for the wings to hold the plane up, and too thin for the engines to breathe. So airlines aim for the sweet spot, usually between ten and twelve kilometres. High enough to save fuel, low enough to keep flying. That is the quiet calculation happening every time your seatbelt sign turns off.