The Atmosphere of the Earth

Abstract

The atmosphere of the earth is a thin spherical shroud composed of a mixture of gases and retained by gravitational attraction. It extends to a great height, but conventional flight is possible only in its denser layers. In fact, about 99% of the total mass of the air is found below about 40 km (25 miles). Commercial airliners fly considerably below this height at roughly 30,000 to 50,000 ft (9 to 15 km), with the supersonic Concorde going to about 60,000 ft. General aviation* is concentrated at much lower altitudes. If you compare these altitudes to horizontal distances on the earth’s surface, the shallowness of tangible air becomes apparent; it is no more than the distance between a couple of rural towns. If the earth were a ball with a radius of one meter (roughly three feet), breathable air would extend only one millimeter (one twenty-fifth inch) outward. This thin layer of air makes life on earth possible. Planets such as Mars with a smaller mass and consequently a weaker gravitational attraction have a more tenuous atmosphere than ours, while yet smaller heavenly bodies such as our moon cannot retain a gaseous shell at all. This is in contrast to Jupiter, the largest planet in our solar system. With its mass 315 times that of the earth, its acceleration of gravity is about 3 times greater, and consequently heavy gases can be retained at high surface pressures. On earth, gases with the lightest molecules, like hydrogen, have flown off into space. By attaining escape speeds of over 11 km/s (7 mi/s), they can escape from the gravitational grip of the earth to fly into space, just like a spaceship on its way to the moon.

We live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight, and so much, indeed, that near the surface of the earth where it is most dense, it weighs (volume for volume) about the four-hundredth part of the weight of water… whereas…on the tops of high mountains it begins to be distinctly rare and of much less weight.

Evangelista Torricelli (1608-47)