Most airplanes fly in the troposphere, which is the layer closest to the Earth and extends up to approximately 11 km above sea level. The troposphere is the layer where weather occurs and contains about 80% of the total mass of the atmosphere. However, commercial aircraft typically prefer to travel in the stratosphere, which is the layer above the troposphere, starting at an altitude of about 33,000 feet or 10 km. The stratosphere has less turbulence and air traffic, and airplanes can fly faster and use less fuel at higher altitudes because the air is thinner. 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. Traveling at high altitudes also means that airplanes can avoid bad weather that is typically found in the lower regions of the atmosphere. Helicopters and light aircraft typically fly in the troposphere, and light aircraft are unable to operate at higher altitudes because they lack pressurized cabins. Therefore, they stay within a range of about 10,000 feet.