Why do commercial aircraft fly at 35000 feet?

Commercial aircraft fly at 25,000 to 35,000 feet, where the air is thin enough to reduce air friction and where atmospheric weather turbulence is less. The higher altitudes do not provide sufficient lift and there is less oxygen for the jet engines. Supersonic aircraft such as the Concorde and Tu-144 operated at even higher levels (up to 65,000 feet) because of the heating caused by very high flight speeds.