Background Information: Composition of the Atmosphere
The atmosphere is 1000 kilometers above mean sea level. In fact, only about 1 percent of the total mass of the atmosphere is above an altitude of approximately 30 kilometers above mean sea level. However, most scientists regard the upper limit of the atmosphere to merge gradually with the extremely rarefied gases and dust that occupy the space between the planets. In this case, there is no sharp boundary between the atmosphere and outer space.
Structure of the Atmosphere
Meteorologists realize that it is important to consider the atmosphere as a whole if they are to predict its future behavior. So satellites and rockets bearing electronic equipment are used to study the upper atmosphere. At the same time, surface meteorological networks are being extended throughout the world. While it is often convenient to study a particular region of the atmosphere, it is necessary to keep in mind that events in one region affect the environment of other parts of the atmosphere.
Composition of the Atmosphere
The mixture of gases that make up the atmosphere is given in Table 1.1 as the composition of dry air by volume.
| Table 1.1 Composition of Dry Air | |
| Gas | Percentage by Volume |
| Nitrogen | 78.084 |
| Oxygen | 20.946 |
| Argon | 0.934 |
| Carbon Dioxide | 0.033 |
| Neon | 0.00182 |
| Helium | 0.00052 |
| Krypton, Hydrogen, Xenon, Ozone, Radon, Etc. | 0.00066 |
| Note: The gases above are the components in pollution-free dry air at ground level expressed as percent by volume. |
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In general, the atmospheric relative composition is virtually constant up to an altitude of approximately 80 kilometers. There are some exceptions: ozone and water vapor. Some variability of carbon dioxide also occurs at lower levels.
Water vapor is a minor constituent in the atmosphere, but has a major role in atmospheric circulation. In fact, water vapor is found to be lighter than nitrogen the major constituent of air. Thus, adding water vapor to air makes it lighter, and it tends to rise. However, removing water vapor makes air heavier (more dense) causing it to sink. Also, the release of heat caused by condensing water vapor warms the surrounding air and is an important energy source for atmospheric processes. Thus, warming the atmosphere causes air to become less dense, while cooling it causes air to become denser.
