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METEOROLOGICAL CLASSIFICATION.— In the meteorological classification (com-mencing with Earth’s surface and proceeding upward) we have the troposphere, tropopause, stratosphere, stratopause, mesosphere, meso-pause, thermosphere, and the exosphere. These classifications are based on temperature charac-teristics. (See fig. 1-4-1 for some examples.)

Troposphere.— The troposphere is the layer of air enveloping Earth immediately above Earth’s surface. It is approximately 5 1/2 miles (29,000 ft or 9 km) thick over the poles, about 7 1/2 miles (40,000 ft or 12.5 km) thick in the mid-latitudes, and about 11 1/2 miles (61,000 ft or 19 km) thick over the Equator. The figures for thickness are average figures; they change somewhat from day to day and from season to season. The troposphere is thicker in summer than in winter and is thicker during the day than during the night. Almost all weather occurs in the troposphere. However, some phenomena such as turbulence, cloudiness (caused by ice crystals), and the occasional severe thunderstorm top occur within the tropopause or stratosphere.

The troposphere is composed of a mixture of several different gases. By volume, the com-position of dry air in the troposphere is as follows:

78 percent nitrogen, 21 percent oxygen, nearly 1 percent argon, and about 0.03 percent carbon dioxide. In addition, it contains minute traces of other gases, such as helium, hydrogen, neon, krypton, and others.

The air in the troposphere also contains a variable amount of water vapor. The maximum amount of water vapor that the air can hold depends on the temperature of the air and the pressure. The higher the temperature, the more water vapor it can hold at a given pressure.

The air also contains variable amounts of impurities, such as dust, salt particles, soot, and chemicals. These impurities in the air are important because of their effect on visibility and the part they play in the condensation of water vapor. If the air were absolutely pure, there would belittle condensation. These minute particles act as nuclei for the condensation of water vapor. Nuclei which have an affinity for water vapor are called HYDROSCOPIC NUCLEI.

The temperature in the troposphere usually decreases with height, but there may be inversions for relatively thin layers at any level.

Tropopause.— The tropopause is a transi-tion layer between the troposphere and the stratosphere. It is not uniformly thick, and it is not continuous from the equator to the poles. In each hemisphere the existence of three distinct tropopauses is generally agreed upon—one in the subtropical latitudes, one in middle latitudes, and one in subpolar latitudes. They overlap each other where they meet.

The tropopause is characterized by little or no increase or decrease in temperature with increasing altitude. The composition of gases is about the same as that for the troposphere. However, water vapor is found only in very minute quantities at the tropopause and above it. 

Stratosphere.— The stratosphere directly overlies the tropopause and extends to about 30 miles (160,000 ft or 48 kilometers). Temperature varies little with height in the stratosphere through the first 30,000 feet (9,000 meters); however, in the upper portion the temperature increases approximately linearly to values nearly equal to surface temperatures. This increase in temperature through this zone is attributed to the presence of ozone that absorbs incoming ultraviolet radiation.

Stratopause.— The stratopause is the top of the stratosphere. It is the zone marking another reversal with increasing altitude (temperature begins to decrease with height).

Mesosphere.— The mesosphere is a layer approximately 20 miles (100,000 ft or 32 kilo-meters) thick directly overlaying the stratopause. The temperature decreases with height.

Mesopause.— The mesopause is the thin boundary zone between the mesosphere and the thermosphere. It is marked by a reversal of temperatures; i.e., temperature again increases with altitude.

Thermosphere.— The thermosphere, a second region in which the temperature increases with height, extends from the mesospause to the exosphere.

Exosphere.— The very outer limit of Earth’s atmosphere is regarded as the exosphere. It is the zone in which gas atoms are so widely spaced they rarely collide with one another and have individual orbits around Earth.

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