Most airplanes fly in the troposphere, which is the layer closest to the Earths surface and extends up to approximately 11 km above sea level. The troposphere is where most of the Earths weather occurs, and it contains the air we breathe. However, some commercial aircraft prefer to travel in the stratosphere, which has less turbulence and air traffic. The stratosphere begins at an altitude of about 33,000 feet, or 6.25 miles, and extends up to about 160,000 feet. Operating at very high altitudes means that the amount of oxygen becomes very low for proper combustion in the engines, so it is essential to note that particular models of aircraft have various optimal altitudes in which they fly. For example, the Concorde was known to fly at heights of between 50,000 and 60,000 feet, which is a region where commercial planes do not regularly fly. Helicopters and light aircraft typically fly in the troposphere, and light aircraft are unable to operate at higher altitudes because they lack pressurized cabins.