How thick is Earth’s atmosphere? It’s difficult to know because the planet’s air simply fades away to nothingness at the highest altitude, above which satellites orbit. It is known, however, that 99 percent of Earth’s air is below an altitude of 18 miles. It is sobering to consider that life is sustained within this relatively thin envelope of air. Thanks to gravity, the atmosphere largely remains in place, even though a few molecules, constantly in motion, attain enough escape velocity to slip away into the void. Weather and cloud formations are created within this envelope, and much water is stored in these clouds. For example, a cube of a few hundred yards of clouds contains about a bathtub full of water. Commercial and military aircraft can cruise at altitudes of five to seven miles.
Source: Dewdney, C., 18 Miles, 2018, review of Dewdney’s new book in Science News, Oct. 28, 2018