>My understanding is that passenger planes fly at high altitudes because at lower altitudes (higher pressures) they would expend most of their energy pushing ai

>My understanding is that passenger planes fly at high altitudes because at lower altitudes (higher pressures) they would expend most of their energy pushing air out of the way. The way I think about it is that commercial flights spend the vast majority of the time at "cruising speed", where the engines are most efficient on a power/fuel basis. The flight already dissipates 100% of engine power output into pushing air out of the way, so what altitude does is determine what airspeed that happens at. At lower altitudes, there's more air molecules displaced per foot of travel, so the airspeed is lower. So airplanes fly up at 30k feet because it's faster at the same power output, which also happens to be more fuel efficient, but it's not like the plane runs the engines any harder if they cruise at a lower altitude.