Atmosphere

The atmosphere is the layer of air that surrounds the Earth.

Deciding exactly how far "up" the atmosphere extends above the Earth's surface is somewhat arbitrary.   Better than 99.9% of the air that makes up the Earth's atmosphere is below an altitude of 50 kilometers.   Nonetheless, traces of our atmosphere can be found thousands of kilometers above the surface.

If we consider that the atmosphere at even a mere 20 kilometers above the Earth's surface is too thin to sustain breathing, and then we compare this to the size of the Earth, which has a diameter of slightly less than 12,800 kilometers, it quickly becomes apparent how relatively thin the "sea of air" is within which we live.