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9.4 Composition of the Atmosphere


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We live at the bottom of an ocean of air that we know as the atmosphere. If you compare the size of the atmosphere to the earth you find it is not very deep. The atmosphere is composed of a variety of gases, mostly nitrogen (78%) oxygen (21%) and carbon dioxide (0.04%). The remainder is made up of minute traces of other gases. We look at the atmosphere in layers. Each layer has a name unique to its molecular characteristic role it plays in the atmosphere. Within aviation we are only concerned with the layers of air that influence our decisions to fly.


The troposphere is the lower portion of the earth's atmosphere where most of the earth’s weather occurs and aircraft fly. Air cools with ascent through the troposphere to the tropopause. Subsequently the troposphere is the focus of this subject.


The tropopause is the level in the atmosphere where the temperature no longer decreases with height. The tropopause varies from approximately 26500 ft (8 km) over the poles to 59500 ft (18 km) over the equator and marks the boundary between the troposphere and the stratosphere.

Deep convection in the Intertropical Convergence Zone, or over mid-latitude continents in summer, continuously push the tropopause upwards and as such deepen the troposphere. This is because thunderstorms mix in the tropospheres air. A deepening reduces the tropopause temperature. Therefore, in areas where (or at times when) the tropopause has an exceptionally high altitude, the tropopause temperature is also very low, sometimes below -80. C. Such low temperatures are not found anywhere else in the Earth's atmosphere, at any level, except in the winter stratosphere over Antarctica.

On the other hand, colder regions have a lower tropopause, obviously because convective overturning is limited there, due to the negative radiation balance at the surface. In fact, convection is very rare in Polar Regions; most of the tropospheres mixing at middle and high latitudes are forced by frontal systems in which uplift is forced rather than spontaneous convection. This explains the paradox that tropopause temperatures are lowest where the surface temperatures are highest. The tropopause height does not gradually drop from low to high latitudes. Rather, it drops rapidly moving closer into the polar regions.


Within the stratosphere the temperature begins to increase with height due to the presence of ozone that absorbs ultra violet solar radiation, which heats the stratosphere in the lower layers. Most weather effects occur below the stratosphere due to the fact that the stratosphere is hotter and less dense than the air below, trapping in moisture and weather conditions.