Man lives within the planet Earth, not upon it. For Earth, the planet, embraces not only rock and water but also air and all other substances that through gravity are hled tightly together and compelled to form a globe. And this earth globe is a single heavenly body, moving detached and separate around the sun.

Five billion million tons of air, all but 1 per cent of the total atmosphere, are squeezed into the lowest twenty miles of the air ocean. The particles are packed more and more tightly as the earth is approached. In the densest atmosphere, along the floor of the air ocean, live the air-breathing organisms of the earth. Here the overlying air presses down with a force of fifteen pounds to the square inch. Man, like the other organisms of the ocean floor, is adapted by nature to withstand this mighty force. Nor can he move far from the dense air region without taking with him the pressurized air essential to his existence. He is designed to remain a bottom-crawling creature.

There are four great regions of the ocean of air, each one enveloping the solid-liquid earth, so that among them they comprise a series of concentric, spherical shells. The lowest shell is the familiar troposphere, which reaches from the solid earth to heights of about five miles above the poles and about ten miles above the equator.

Next comes the stratosphere. The top of the stratosphere lies at a height of around fifty miles, after which the ionosphere begins.

The ionosphere, in turn, continues upward to around 400 or 500 miles, where it merges with the exosphere, which extends to the very edges of the atmosphere, some 18,000 miles up.

The exosphere is over one hundred times the size of the solid earth and accounts for more than 99 per cent of the volume of the entire atmosphere, yet it contains less than one-billionth of the total amount of the gases of the atmosphere.

The Troposphere

By volume, 99.99 per cent of dry tropospheric air is composed of just four gases. Nitrogen constitutes about 78 per cent, oxygen about 1 per cent, argon not quite 1 per cent, and carbon dioxide .03 per cent. Dry tropospheric air also contains very small amounts of neon, helium, krypton, hydrogen, xenon, radon, and ozone. Water vapor varies from 5 per cent in the air overlying the tropical oceans to .05 per cent or less at the top of the troposphere. The troposphere also has variable amounts of dust, bacteria, carbon particles, and other solid material, nearly all of which has been blown up from the earth's surface, though an appreciable amount has been derived from meteors and from the dust of outer space.

In middle latitudes, at the top of the troposphere, seven miles up, the air is only three-tenths as dense as at sea level and the air pressure is only one-fifth the sea-level pressure. Temperatures also decrease upward through the troposphere, at an average rate of about 3.3 degrees for every rise of 1,000 feet. At the top of the troposphere the average temperature is 55 degrees below zero in middle latitudes and 100 degrees below at the equator, where the troposphere reaches to a height of about ten miles.

The Stratosphere

In the stratosphere the temperature begins to increase chiefly because of the concentration of ozone, which powerfully absorbs much of the ultraviolet light from the sun and so is warmed itself and helps to warm the other surrounding gases. This ozone is, however, chiefly concentrated in the lower stratosphere, at heights of ten to twenty miles, so that the temperature, after reaching a maximum of around 180 degrees in the middle stratosphere, begins to decrease once again and has fallen to around 20 degrees below zero by the time the top of the stratosphere is reached. At that point the density of the air has become less than one ten-thousandth of that at sea level and the air pressure has become so light as to be almost too small for measurement. Above fifty to sixty miles, no audible sounds can be made -- the waves are damped out immediately because of the extreme scarcity of air molecules.

The Ionosphere

Upon the region of the ionosphere there falls an immense electromagnetic cannonade: light rays, ultraviolet and infrared radiations, radio waves and streams of particles from the sun, similar radiations from other stars, cosmic rays from interstellar space beyond the solar system. The ultraviolet radiations knock electrons from the air particles, split molecules into atoms, and leave ionized molecules and atoms. Cosmic rays, shooting from into the nuclei of air atoms at huge energies, produce a vast, energetic debris which cascades down through the atmosphere, creating powerful secondary showers of radiation.

In the ionosphere nitrogen and oxygen appear as individual atoms, instead of in the pairs that form the usual nitrogen and oxygen molecules of the lower atmosphere. Free electons are produced; these, together with electrically charged atoms, form great layers that are themselves electrically charged. These strata are the scene from time to time of the electromagnetic storms that disrupt radio communication, and they are associated with eruptions of auroral lights. The charged or ionized layers are not constant in thickness, but swell in size beneath the bombardment of the sunlight of daytime and shrink at night. Because they are least developed at night, radio reception is better then than it is by day.

Temperatures soar from a few hundred degrees in the lower regions of the ionosphere to 2,000 degrees or more in the uppermost regions. Yet at these heights an individual would actually freeze to death during the night, since there would be no sunlight to keep him warm and the air particles would be too few to warm him by striking him and so conveying their heat. At the top of the ionosphere, about 50,000 billion air particles would strike each square inch of a person's body each second; but this would convey nothing like the heat conveyed by the atmosphere at sea level, where a square inch is struck by 10,000,000,000,000,000 billion air particles each second.

The Exosphere

In the exosphere the atmosphere has become so fantastically rare that the charged particles formed by action of the sunlight are too widely scattered to produce detectable charged layers of the kind that are found in the ionosphere. Instead, at much higher levels, the exosphere includes two enormously thick charged belts of a different kind. One is centered at a height of about 2,500 miles, the other, at about 10,000. The lower belt is hundreds of miles thick; the upper, thousands. The belts consist of swarms of electrons and protons that have arrived from the sun and have become trapped in the earth's magnetic field. These belts are immersed in an unimaginably rare atmosphere that is almost wholly composed of charged particles of atomic oxygen and nitrogen. Probably throughout the exosphere the particles are at temperatures well in excess of 3,000 degrees. And they extend upward to a height of something like 18,000 miles.

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Our Wonderful World, An Encyclopedic Anthology for the Entire Family. New York:Grolier Incorporated, 1965.

The Composition of Earth's Atmosphere

This page was last updated on 02/10/2010.