The Ocean Above Us

We live at the bottom of an ocean of air called the atmosphere and we experience the currents of this ocean as wind. The "waves" above cause our surface pressure to rise and fall. As one ascends from the "ocean floor" to higher levels, the characteristics of the atmosphere change. Pilots must be familiar with the composition and structure of the atmosphere to understand the environment in which they fly. Let's examine this environment.

The atmosphere is about 4/5 nitrogen, with all-important oxygen making up only 1/5 of the atmosphere. Carbon dioxide and other trace chemicals are also present in small amounts. Properties of the atmosphere include the air's temperature, pressure, and density. As one climbs to higher altitudes, the pressure and air density decrease. Air pressure results from the force of gravity pushing the weight of the atmosphere downward. At higher altitudes less pressure exists because there is less atmosphere above pushing down. At 18,000 feet, the pressure is about one-half of that at sea level.

The atmosphere consists of layers, or spheres. The troposphere, the layer closest to the earth, extends from the surface to about 36,000 feet. Within this layer the temperature decreases three degrees Fahrenheit for every 1,000 feet. This happens because the ground is warmed by the sun and the farther one climbs away from the warm earth, the colder the air becomes. The temperature drop levels off at the tropopause, the uppermost level of the troposphere. It is in the tropopause that most commercial jetliners cruise.

Above the tropopause is the stratosphere. Within the lower levels of the stratosphere the temperature remains relatively constant at a frigid -70F. In the upper levels of the stratosphere, the temperature becomes warmer due to the ozone. Ozone absorbs incoming solar ultraviolet radiation, which is good for us here on the ground, and the air becomes warmer. At about 30 miles altitude, far higher than a commercial airliner would fly, the stratosphere meets the mesosphere.

Within the mesosphere, or "middle" sphere, the temperature again drops back down to about -80F. The mesopause, the upper boundary of the mesosphere, is located at an altitude of about 70 miles. Above this is the thermosphere.

Within the thermosphere the temperature increases to as high as 120F, an increase attributed to the extremely thin atmosphere. The atmosphere at this level is so thin that, on average, molecules of air are a half-mile apart. The best way to illustrate the temperature effect caused by the thin atmosphere is to imagine a croissant being warmed in a microwave oven. Since the croissant has such little mass it heats up very quickly compared to something as dense as a cup of coffee. The thin upper atmosphere is like the croissant warming quickly to high temperatures by the solar radiation.

The upper level of the thermosphere is located at an altitude of about 300 miles. This is about as high as the space shuttle orbits. In the extremely high region of the exosphere, above the thermosphere, the high-speed atmospheric molecules can escape the pull of the earth's gravity and escape into space. It is impossible to determine the ultimate end point of the atmosphere as these exiting molecules can be found as one continues upward indefinitely. It is for this reason that there is no absolute limit to the atmosphere, but the molecules are so far apart that the density can be assumed to be zero.

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