Why do planes fly at different altitudes?

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Tags

1. #flight
2. #requires
3. #moreover
4. #interaction
5. #decreasing
6. #molecules
7. #altitudes
8. #flying
9. #increases
10. #lower
11. #friction
12. #amount
13. #surface
14. #density
15. #aircraft
16. #efficiency
17. #produced
18. #airplane
19. #required