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1.2  COMPOSITION OF THE ATMOSPHERE

As shown in figure 1.1. a given volume of dry air contains approximately 78 percent nitrogen; 21 percent oxygen; and 1 percent argon, carbon dioxide, and minute amounts of other gases. Air contains, in addition to the gases just mentioned a variable amount of water vapor, most of which is concentrated below 30,000 feet. The maximum amount of water vapor the air can hold depends primarily upon the temperature of the air; the higher the temperature, the more water vapor the air can hold. Water vapor in the atmosphere varies from insignificant amounts to 4 percent by volume (100 percent humidity).

Air also contains variable amounts of impurities such as dust, salt particles, and products of combustion. These impurities are important because of their effects on visibility and especially because they are necessary to cause the water vapor to form into clouds and fog and to condense into rain and snow. If the air were absolutely pure there would be little condensation.

Figure 1.1. Composition of the Atmosphere

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