Atmosphere
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atmosphere
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The atmosphere is the nearly transparent envelope of gases and suspended particles that surrounds the Earth, profoundly influencing environmental conditions on the planet's surface. Without chemical processes involving several of the atmospheric gases, life could not exist. The physical processes that operate in the atmosphere are also of vital importance because they are responsible for the Earth's varied climates.History of StudiesWind vanes were known to the ancient Chinese and Egyptians, and an atmospheric observatory, the Tower of the Winds, was built by the Greeks in the 2d century BC. Aristotle's interpretations of atmospheric phenomena in his Meteorologica, written about 340 BC, dominated Western thinking for almost 2,000 years (until the end of the 16th century).Modern study of the atmosphere began with the invention of the thermometer (c.1600) by GALILEO and of the barometer (1643) by TORRICELLI. Early scientists quickly discovered that weather conditions are related to pressure changes, and by the end of the 17th century, records of atmospheric conditions were being kept at many locations. Using such records, Benjamin FRANKLIN first deduced (1743) that storms are traveling systems, but this discovery could not result in useful weather forecasts until the telegraph was invented in the 1850s.During the latter half of the 19th century, balloonS were used to probe the upper levels of the atmosphere. These efforts culminated in the discovery (1899) of the STRATOSPHERE by a French meteorologist, Teisserenc de Bort. With the development in 1927 of the radiosonde (an inexpensive balloon-borne instrument package radioing back to Earth information on upper-level temperature, pressure, and humidity), systematic mapping of the atmosphere's structure and circulation began.At about the same time the theory that cyclonic storms arise from disturbances in polar fronts was developed by the Bergen school of meteorologists in Norway. Further key theoretical advances were made during the 1930s and '40s by Carl-Gustaf Rossby, a Swedish-American meteorologist who clarified the role of traveling weather disturbances in the circulation of the atmosphere as a whole. During world War II, bomber pilots discovered JET STREAMS, narrow "rivers" of high-speed winds encircling the globe at altitudes of approximately 10 km (6 mi). With the launching (1960) of the first weather satellite, Tiros I, scientists were able for the first time to obtain a truly global view of the atmosphere. The concurrent development of high-speed computers has made it possible to construct mathematical models of complex atmospheric processes and to forecast the weather with increasing accuracy by numerical methods.COMPOSITION AND STRUCTUREMany of the physical and chemical processes that occur in the atmosphere are directly related to its composition. The atmosphere is now composed almost entirely of oxygen and nitrogen in their diatomic forms (two atoms bound together by chemical forces). Diatomic nitrogen accounts for approximately 78% of the total molecules in the atmosphere, and diatomic oxygen represents nearly 21%. The inert noble gas, argon, accounts for about 0.9%, and the remaining 0.1% is composed of many trace gases, the most significant of which are carbon dioxide and water vapor.Although carbon dioxide makes up only 325 parts per million of the atmosphere by volume, it is vital in maintaining the Earth-atmosphere system's heat balance because it strongly absorbs infrared (thermal) radiation. Water vapor, which is present in highly variable quantities ranging from 0 to 4% by volume, also absorbs considerable infrared radiation and, additionally, is an essential link in the HYDROLOGIC CYCLE. Another important trace gas is the triatomic form of oxygen, ozone, which is concentrated in a layer centered at about 25 km (16 mi) above the surface. Although present in maximum concentrations of only about 12 parts per million, ozone absorbs radiation in the ultraviolet region of the spectrum so effectively that the ozone layer can almost completely shield life on Earth from harmful ultraviolet rays.Meteorologists usually divide the atmosphere into four layers. In order of increasing elevation these are the TROPOSPHERE, the stratosphere, the MESOSPHERE, and the THERMOSPHERE. Each has a different temperature range. Temperatures decrease with altitude in the troposphere and mesosphere and increase with altitude in the stratosphere and thermosphere. The troposphere and stratosphere are separated by the TROPOPAUSE, a level of minimum temperature that varies in altitude from about 16 km (10 mi) near the equator to 9 km (5 mi) near the poles. The stratosphere and mesosphere are separated by the stratopause, a level of temperature maximum at an altitude near 50 km (30 mi). The mesosphere and thermosphere are in turn separated by a temperature minimum, the mesopause, which oc?urs near 80 km (50 mi). These temperature layers are created primarily by the selective absorption of SOLAR radiation at various levels in the atmosphere. radiation in the extreme ultraviolet (wavelength less than 100 nanometers) is absorbed by atoms of oxygen above 100 km (60 mi). This process not only maintains the high temperatures of the thermosphere but also produces electrically charged particles, called ions. For this reason the region of the atmosphere above 80 km (50 mi) is also referred to as the ionosphere. Ultraviolet radiation of somewhat longer wavelengths (200-300 nanometers) penetrates into the stratosphere, where it is absorbed by ozone to produce the temperature maximum near 50 km (30 mi). Visible radiation, on the other hand, penetrates to the surface of the Earth and produces the temperature maximum at the ground level.Stratification and Static StabilityTemperature distribution is not alone in determining the state of the atmosphere. pressure and density also are important. Atmospheric pressure, usually expressed in units called millibars, is the force that the total Mass of air in an imaginary vertical column exerts on a given horizontal area of the Earth's surface. Standard sea-level pressure, 1,013.25 millibars, is equivalent to the pressure exerted by a column of mercury 760 mm (30 in) high. If, like water, the atmosphere were incompressible, pressure would decrease uniformly with height, and the atmosphere, like the ocean, would have a definite upper limit. In reality the atmosphere is compressible; that is, density (Mass per unit volume) is proportional to pressure. This relationship, called BOYLE'S LAW, implies that density decreases with height in the atmosphere: as height increases, less Mass remains above a given point; therefore less pressure is exerted. At sea level the density of air is about 1 kg per cu m (8 oz per cu ft). Both pressure and density decrease by about a factor of 10 for every 16 km (10 mi) increase in altitude.density does not depend only on pressure; for a given pressure, it is inversely proportional to temperature. This relationship, known as Charles's law, implies that the depth of an air column bounded by two constant-pressure surfaces will increase as the temperature in the column increases. Thus the vertical distance over which pressure decreases to half of its surface value ranges from about 5,800 m (19,850 ft) in the tropics to 5,100 m (16,575 ft) near the poles.When an AIR Mass rises, it expands (because of the reduction in pressure). In expanding, it must work against the pressure force exerted by the surrounding air. According to the principle of conservation of energy (the first law of thermodynamics), the work done in expansion must be balanced by an equal reduction in the internal energy of the air Mass. Since the internal energy is proportional to temperature, an expanding air Mass must cool. Conversely, an air Mass that is compressed must warm. Temperature changes caused by compression or expansion of a gas in the absence of heat exchange with the surroundings are called adiabatic changes.The process of adiabatic cooling or heating is essential to an understanding of vertical convection in the atmosphere. A Mass of dry air rising adiabatically in the atmosphere cools at a rate of about 10 deg C per km (17 deg F per mi). Since, on the average, the decrease of temperature with height (the LAPSE RATE) in the troposphere is only 6.5 deg C per km (11 deg F per mi), an adiabatically ascending air Mass becomes cooler and denser than its surroundings and tends to sink back toward its original level. Thus the mean vertical temperature structure is said to be statically stable with respect to a dry adiabatic displacement. In regions such as desertS, where the air near the ground is strongly heated, the lapse rate of temperature in the lower troposphere may exceed 10 deg C per km (17 deg F per mi). In such situations an air Mass displaced vertically upward will become warmer than its surroundings and will be accelerated farther upward. S?ch statically unstable conditions result in vigorous mixing and upward heat transport, which tends to reduce the lapse rate toward the average value observed elsewhere.Moist ProcessesWater can exist in all three chemical phases--solid, liquid, and gas--at atmospheric temperatures and pressures. During phase changes, water exchanges heat with its surroundings, a process called latent heating. Condensation and freezing release heat and thus warm the surroundings, whereas evaporation and melting absorb heat and cool the surroundings. CLOUDS and PRECIPITATION are thus essential to the heat balance of the atmosphere.