Why do planes fly at high altitudes?

Airplane - An airplane flying at a high altitude
India
August 30, 2007 7:44am CST
Why do planes fly at such high altitudes? Why don't they fly at lower altitudes?
2 responses
uath13
@uath13 (8192)
• United States
30 Aug 07
Thinner air has less resistance. There's also less ecological damage. If they flew lower their sound would be quite disturbing , they'd hit more birds & run into more foul weather which they usually fly above.
Bala
@balasri (26537)
• India
30 Aug 07
In higher altitude the air is thinner. So the flight will be smoother. A lot of times, The low altitude flights are bumpy because of the air.Spying and tracking planes should fly so high to get better pictures.