Some days ago I flew from Frankfurt to San Francisco. On the return flight, which was to Zurich, I noticed that the route was way lower than the outward one: from Europe I flew over Greenland, but when coming back it was much lower over the Atlantic ocean, passing over Spain at the end. I know that Zurich is lower than Frankfurt, but it doesn't feel like it's enough to justify such a difference . There may be other reasons why the route is different? I was thinking about some possible explanations involving Earth rotation or something like that, but I can't really figure it out.