I flew back from Spain the other day and the Pilot told us we were flying at an altitude of 37,000 ft.
I can remember on all my other flights the Captains telling me the same thing.
so why do they fly so high ?
couldn’t they just fly at say 4,000 ft ?
and also wouldn’t the lower height make them faster ?
sorry meant less journey time NOT faster.
WAG - the air friction is lower at those altitudes, due to the atmosphere being thinner, meaning they save on fuel costs.
Jet engines get more efficient at higher altitudes. Flying at FL 370 burns MUCH less gas than flying at 4,000 ft. Also, you do fly “faster” - the higher you go, the higher your true airspeed for the same indicated airspeed. High altitude also allows you to avoid most of the weather: clouds=turbulence, so avoiding weather makes for a smoother flight.
Any flight across the ocean uses sophisticated internal navigation systems that don’t need ground-based navaids, but for airplanes that need to tune up these stations, higher altitude means a greater signal range.
thanks 
Plus, if your plane loses engine power, you have a lot more glide distance. There was a thread about this a few weeks ago. Search under ‘jumbo jet gliding’ or something similar and you’ll find it.
And one of my favorites: The Gimli Glider.