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Earth's atmosphere is a layer of gases surrounding the planet Earth and retained by the Earth's Gravity . It contains roughly 78% Nitrogen and 21% Oxygen , with trace amounts of Other Gases . This mixture of gases is commonly known as '''air'''. The atmosphere protects Life on Earth by absorbing Ultraviolet Solar Radiation and reducing Temperature extremes between Day and Night . The atmosphere has no abrupt cut-off. It slowly becomes thinner and fades away into Space . There is no definite boundary between the atmosphere and Outer Space . Three-quarters of the atmosphere's mass is within 11 km of the Planetary surface. In the United States , persons who travel above an Altitude of 50.0 miles (80.5 km) are designated as Astronauts . An altitude of 120 km (75 mi or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The Karman Line , at 100 km (62 mi), is also frequently used as the boundary between atmosphere and Space . TEMPERATURE AND THE ATMOSPHERIC LAYERS The Temperature of the Earth's atmosphere varies with Altitude ; the Mathematical Relationship between temperature and altitude varies between the different atmospheric layers:
The boundaries between these regions are named the Tropopause , Stratopause , and Mesopause . The average temperature of the atmosphere at the surface of earth is 14 °C. Various atmospheric regions Atmospheric regions are also named in other ways:
PRESSURE Barometric Formula: (used for airplane flight) Barometric Formula Main article: Atmospheric Pressure Nasa mathematical model: NRLMSISE-00 Atmospheric pressure is a direct result of the weight of the air. This means that air pressure varies with location and time, because the amount (and weight) of air above the earth varies with location and time. Atmospheric pressure drops by ~50% at an altitude of about 5 km (equivalently, about 50% of the total atmospheric mass is within the lowest 5 km). The average atmospheric pressure, at Sea Level , is about 101.3 Kilopascal s (about 14.7 pounds per square inch). THICKNESS OF THE ATMOSPHERE Although the atmosphere exists at heights of 1000 km and more, it is so thin as to be considered nonexistant.
Therefore, most of the atmosphere (99.9999%) is below 100 km, although in the rarified region above this there are Aurora s and other atmospheric effects. COMPOSITION ''Source for figures above: NASA . Carbon dioxide and methane updated (to 1998) by IPCC TAR table 6.1 [http://www.grida.no/climate/ipcc_tar/wg1/221.htm]. The NASA total was 17 ppmv over 100%, and CO2 was increased here by 15 ppmv. To normalize, N2 should be reduced by about 25 ppmv and O2 by about 7 ppmv.'' Minor components of air not listed above include:
Heterosphere Below the . Thus higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as Helium , molecular Hydrogen , and atomic hydrogen. Thus there is a layer, called the heterosphere, in which the earth's atmosphere has varying composition. As the altitude increases, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature. {Link without Title} DENSITY AND MASS Main article: Density Of Air The density of air at sea level is about 1.2 kg/m3. Natural variations of the Barometric Pressure occur at any one altitude as a consequence of Weather . This variation is relatively small for inhabited altitudes but much more pronounced in the outer atmosphere and space due to variable solar radiation. Standard Atmosphere model]] The atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the Barometric Formula . More sophisticated models are used by meteorologists and space agencies to predict weather and orbital decay of satellites. The average mass of the atmosphere is about 5,000 trillion metric tons. According to the National Center for Atmospheric Research, "The total mean mass of the atmosphere is 5.1480 x 1018 kg with an annual range due to water vapor of 1.2 or 1.5 x 1015 kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27 x 1016 kg and the dry air mass as 5.1352 ±0.0003 x 1018 kg." The above composition percentages are done by volume. Assuming that the gases act like ideal gases, we can add the percentages p multiplied by their molar masses m, to get a total t = sum (p·m). Any element's percent by mass is then p·m/t. When we do this to the above percentages, we get that, by mass, the composition of the atmosphere is 75.523% nitrogen, 23.133% oxygen, 1.288% argon, 0.053% carbon dioxide, 0.001267% neon, 0.00029% methane, 0.00033% krypton, 0.000724% helium, and 0.0000038 % hydrogen. THE EVOLUTION OF THE EARTH'S ATMOSPHERE See Also: History of Earth The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research. The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current Chemical composition from two notably different previous compositions. The original atmosphere was primarily Helium and Hydrogen . Heat (from the still-molten crust, and the sun) dissipated this atmosphere. About 3.5 billion years ago, the surface had cooled enough to form a as much Gas as the current atmosphere. It is generally believed that the Greenhouse Effect , caused by high levels of carbon dioxide, kept the Earth from Freezing . During the next few million years, water vapor Condensed to form Rain and Ocean s, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria were the Cyanobacteria . Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the earth's atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen). Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere. Photosynthesizing plants would later Evolve and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in Fossil Fuels , Sedimentary Rock s (notably Limestone ), and Animal Shell s. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, Bacteria would also convert ammonia into nitrogen. As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various Element s (such as Iron ), but eventually oxygen accumulated in the atmosphere, resulting in Mass Extinction s and further evolution. With the appearance of an Ozone Layer (ozone is an Allotrope of oxygen) Life forms were better protected from Ultraviolet Radiation . This oxygen-nitrogen atmosphere is the "third atmosphere". REFERENCES
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