The earth’s atmosphere is the envelope of gases that surrounds our planet and revolves with it. Most of the gases are within about 300 miles of the earth’s surface, but a thin atmosphere extends some 22,000 miles!
Up to a height of 60 miles above the earth, the atmosphere is mostly nitrogen and oxygen. About 78 percent of the gas is nitrogen, and 20 percent is oxygen. But above 60 miles, it’s made up of layers of oxygen, hydrogen, and helium gas.
So, the atmosphere close to the earth is much different from the upper atmosphere. When people say that the atmosphere is mostly nitrogen, they’re really talking about the lower atmosphere.