The Atmosphere's Thin Shell

Relative to Earth’s radius, the dense lower atmosphere is very thin, often compared with an apple’s skin on a scale model of the planet. NASA uses about 100 km as a convenient atmospheric boundary, but the atmosphere has no sharp edge: air becomes progressively thinner, and traces extend much farther. The analogy conveys thinness, but it is not a precise measurement of the atmosphere’s “thickness.” [1–3]