Atmospheric composition and structure

During most of history, Earth's atmosphere was regarded as little more than a mass of air and clouds . Simple observations from the ground yielded little more than a basic understanding of the atmosphere's characteristics.

Manned balloon ascents in the late 1700s and early 1800s were restricted to about 5 miles (8 km), the limit of life-supporting oxygen . There were also risks to human life. From a practical standpoint, it was difficult to make widespread observations over space and time.

Breakthroughs in atmospheric research came as new inventions made it possible to obtain information from unmanned balloon flights. The first of these was the theodolite, a viewing instrument used to survey distances and angles, invented by Gustave Hermite in 1896. This device increased the range to which a ground observer could follow a balloon's ascent pattern. The information-gathering packages delivered to the atmosphere by balloons became known as radiosondes, with each balloon journey referred to as a sounding. The maximum height at which the atmosphere will sustain a balloon is about 28 miles (35 km).

In 1893, George Besancon developed a recording thermometer and barometer capable of making in-flight observations during unmanned ascents. Beginning in 1897, Léon Phillipe Teisserenc de Bort made improvements to these instruments at an observatory he established near Paris. Of equal importance, this was the first organized effort to obtain repeated readings of high-altitude phenomena.

It was generally known that temperatures decreased with elevation at a rate of about 33.8°F (18.6°C) per 590 feet (180 m). Until de Bort, it was assumed that this rate continued out into space. His observations revealed, however, that temperatures first level off, then increase, beginning at about 8 miles (14 km). This warm region is located only a few kilometers beyond the highest mountain peaks and the upper limit of regular cloud formation.

In 1908, de Bort's observations led him to divide the atmosphere into two layers, the lower being the troposphere and the upper being the stratosphere . The area where the two meet, where the temperature begins to modify, he named the tropopause.

Eventually, scientists discovered that the atmosphere consists of several layers, not just two. The warm region, it has been found, continues to a level for about 28 miles (45 km). The temperature starts at about −76°F (−60°C) at the tropopause and exceeds 32°F (0°C) at about 28 miles (45 km). Beyond this, the temperature drops again to about −130°F (−90°C) at about 50 miles (80 km). This area has been called the stratopause, but it is also known as the mesosphere . This area is where most meteors disintegrate as they approach the earth.

Above the stratopause, at about 62 miles (100 km), the temperature rises sharply again to 212°F (100°C) and continues to rise to levels that can only be theoretically estimated, perhaps 1200° K. This area is called the thermosphere .

In the thermosphere, attention shifts from temperature variations to other phenomena. This layer is characterized by highly energized particles (cosmic rays), which enter from outer space and became electronically charged. The result is the aurora borealis in the Northern Hemisphere and the aurora australis in the Southern Hemisphere. Both occur at about 20°–25°latitude and at heights of 50–190 miles (80–300 km).

The ionosphere coincides with, but is not confined to, the thermosphere. Its only distinguishing property is its layers of ionized gases that reflect radio waves, as opposed to other atmospheric regions, which do not. It exists from 50 miles (80 km) upward to 620 miles (1,000 km) and beyond. The ionosphere is influenced greatly by solar activity, which can rearrange or eliminate the reflective layers.

The exosphere begins at about 310 miles (500 km). Here the atmospheric components lose their molecular structure and become atomic in nature. These components cannot be considered gaseous beyond this point.

Beyond 620 miles (1,000 km), particle structures further degrade into electrons and protons. The earth's gravity gives way to magnetic fields as the dominant distributor of particles. The Van Allen belts at 2,500 miles (4,000 km) and at 12,400 miles (20,000 km) mark the outer limit of the magnetosphere, the most remote known sphere of Earth's influence.

Far beyond the limits of balloon flight, knowledge of the upper atmosphere has been made possible by increasingly high airplane flights and orbiting satellites.

Overall, the atmosphere's gaseous composition consists of 78% nitrogen, 21% oxygen, and a 1% mixture of minor gases dominated by argon. This composition not only sustains life, but is also determined by it. Also, there is a general distribution of dust particles carried from Earth's surface or entering from space.

Recently, scientists have been giving attention to two areas of atmospheric research. Concern has been raised over the destruction of the ozone layer (near the stratospheric warm region), which absorbs ultraviolet radiation, by the introduction of chlorofluorocarbons (CFCs) by man.

A new area of research involves exploration of atmospheres of neighboring planets. With an expanding solar system , the dense atmosphere of Venus may hold clues to Earth's atmospheric past, while the thin atmosphere of Mars may be a clue to its future.

See also Atmospheric circulation; Atmospheric inversion layers; Atmospheric lapse rate; Atmospheric pollution; Atmospheric pressure; Ozone layer and hole dynamics