Most airplanes fly in the troposphere, which is the lowest layer of Earths atmosphere and extends up to approximately 11 km above sea level. Within the troposphere, each type of aircraft uses a different altitude, but the "preferred region" in the airline industry usually begins at 35,000 feet and ends at 42,000 feet. Operating at very high altitudes means that airplanes can fly faster and use considerably less fuel. Traveling at high altitudes also means that airplanes can avoid bad weather that is typically found in the lower regions of the atmosphere. However, particular models of aircraft have various optimal altitudes in which they fly. It is essential to note that helicopters and light aircraft typically fly in the troposphere as well, but they are unable to operate at higher altitudes because they lack pressurized cabins.