PENETRATION OF RADIATION THROUGH THE ATMOSPHERE
The earth's atmosphere is an envelope of gas consisting mainly of nitrogen and oxygen. In addition, there are smaller amounts of other gases, the chief of which are carbon dioxide, argon, neon, and other rare gases. There are also other substances, such as radioactive emanations, oxides of nitrogen, ozone, and particularly water vapor, present in varying amounts. Of these water vapor alone forms an appreciable percentage of the atmosphere, and the quantity present is dependent on the temperature. The higher the temperature, the larger is the amount of water vapor which the air can hold. The composition of the atmosphere near the surface of the earth, apart from its content of water vapor, appears to be essentially the same at all parts of the earth, although there is some change with elevation above the surface.
An atmosphere consisting only of gases and water vapor would be known as pure or clean air. The normal air, however, is never pure, for it contains innumerable particles of solid matter suspended in it. These particles come from the surface of the earth itself and from outer space, and are very varied in character. Dust caught up by the wind from the earth's surface or emitted from volcanoes is there in abundance; vegetable fibers, pollen, spores, bacteria, and other microscopic organisms permeate the atmosphere; fires emit considerable quantities of ash, and smoke in the form of particles of carbon, tarry matter, and other materials; the evaporation of the spray of the ocean leaves tiny particles of salt behind; dust of cosmic origin inhabits the upper layers known as the stratosphere; minute droplets are present owing to the union of water with gases produced by electric discharges, ultraviolet radiation, and the burning of coal. At times,