Atmospheric Composition and Structure
During most of history, the 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 3.6° F per 1000 feet (6.5° C per km), the so called lapse rate. Until de Bort, it was assumed that this lapse rate unchanged continued to the top of the atmosphere. His observations revealed, however, that atmospheric temperatures first level off, then begin to increase, at an altitude of about 8 miles (14 km). de Bort's observations led him in 1908 to divide the atmosphere into two layers: the troposphere, the lower portion where clouds and weather are important, and the stratosphere, the upper layer where temperatures increased with height. The stratosphere begins only a few kilometers beyond the highest mountain peaks and the upper limit of regular cloud formation. The boundary between the troposphere and stratosphere, where the atmospheric temperature is at a minimum, was named the tropopause. Together, the troposphere and stratosphere contain about 99.9% of the atmosphere's mass.
Eventually, scientists discovered that the atmosphere consists of several layers, not just two. The stratosphere, it has been found, continues up to a level of about 28 miles (45 km). The temperature starts at about -76° F (-60° C) at the tropopause and reaches a maximum of over 32° F (0° C) at the top of the stratosphere, called the stratopause. Beyond this, the temperature begins to drop with height again, reaching a low of about-130° F (-90°C) at about 50 miles (80 km). This portion of the atmosphere is called the mesosphere. Most meteors disintegrate in the mesosphere as they approach the Earth, providing the flash we see in the night sky. Above the mesosphere, at about 62 miles (100 km), lies the thermosphere. This outer skin of the atmosphere is most affected by solar radiation and can reach temperatures ranging from 400 to 3100° F (200 to 1700° C), depending on the state of solar activity.
In the thermosphere attention shifts from temperature variations to other phenomena. This layer is characterized by highly energetic particles emitted from the sun which become trapped in the Earth's magnetic field. When these rapidly moving particles collide with the molecules and atoms of the upper atmosphere, visible light is produced. The result is the aurora borealis in the northern hemisphere and the aurora australis in the southern hemisphere. Both occur primarily near the polar regions and at heights of 50 to 190 miles (80 to 300 km).
An area of interest within the thermosphere is the ionosphere, so named because of the presence of ions, or charged particles. This layer of ionized gases both absorbs and reflects radio waves from the ground. When the ionosphere is thin, such as at night, it acts as a good reflector and can bounce radio and other communication signals around the globe. During daylight hours, when solar energy creates more ions and a thicker ionosphere, radio waves are absorbed more; this is the reason that distant radio stations can sometimes be heard at night but not during the day. During periods of high solar activity, when the sun emits large amounts of charged energetic particles, the ionosphere thickens to the point that worldwide radio communications are often disrupted.
The atmosphere has no "top"—it gradually thins out as it approaches the near vacuum of space. The region of this transition is called the exosphere, which begins at about 310 miles (500 km)above the surface of Earth. Here the components of the atmosphere are mostly free atoms, rather than molecular gases.
Since the early pioneering balloon measurements, much information has been gained by high altitude research balloons carrying instruments to remotely study the upper atmosphere. Beyond the limits of balloon flight, knowledge of the upper atmosphere has been made possible by increasingly high airplane flights and orbiting satellites.
Over all, the atmosphere's gaseous composition consists of 78 percent nitrogen, 21 percent oxygen, and about 1 percent mixture of minor gases dominated by argon. Among these minor gases are two that have become the focus of much research. The first, carbon dioxide, is one of the "greenhouse gases", which act to increase the surface temperature of Earth by trapping heat radiation. The levels of carbon dioxide in the atmosphere have been increasing steadily over at least the last fifty years, most likely due to the burning of coal, oil and other fossil fuels. Whether this increased amount of carbon dioxide will cause global temperatures to increase, and by how much, is under intense debate, and is the subject of measurements and modeling studies.
The second trace gas that scientists have focused attention on is ozone. A type of oxygen, ozone is formed in the stratosphere by the absorption of ultraviolet light, the portion of the solar spectrum responsible for sun burns. Concern has been raised over the destruction of the stratospheric ozone layer by the introduction of chlorofluorocarbons (CFCs), human made chemicals that are released at the surface and find their way to the stratosphere. Depletion of the protective ozone layer could increase the level of ultraviolet light at Earth's surface, resulting in increased skin cancer, crop damage, and harm to the tiny plant life of the oceans on which all sea life depends.
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