Planes typically fly in the stratosphere, which is the second layer of the Earths atmosphere. The stratosphere begins at an altitude of about 33,000 feet, or 6.25 miles, above the Earth, and extends to about 31 miles above it. The reason planes fly at higher altitudes is that the air gets thinner as planes ascend higher into the sky, which translates to less drag. This means that at higher altitudes, planes can fly faster and use considerably less fuel. The "preferred region" in the airline industry usually begins at 35,000 feet and ends at 42,000 feet. However, it is important to note that particular models of aircraft have various optimal altitudes in which they fly. Flying at high altitudes also means that airplanes can avoid bad weather that is typically found in the lower regions of the atmosphere. It is essential to note that helicopters and light aircraft typically fly in the troposphere, which is the lowest layer of the atmosphere and extends up to about 36,000 feet.