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An atmosphere (〔(ἀτμός ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus Digital Library〕〔(σφαῖρα ), Henry George Liddell, Robert Scott, ''A Greek-English Lexicon'', on Perseus Digital Library〕) is a layer of gases surrounding a planet or other material body of sufficient mass〔(【引用サイトリンク】url=http://www.ontariosciencecentre.ca/School/Challenger-Learning-Centre/Visits/Glossary/ )〕 that is held in place by the gravity of the body. An atmosphere is more likely to be retained if the gravity is high and the atmosphere's temperature is low. The atmosphere of Earth is mostly composed of nitrogen. It also contains oxygen used by most organisms for respiration and carbon dioxide used by plants, algae and cyanobacteria for photosynthesis. It protects living organisms from genetic damage by solar ultraviolet radiation, solar wind and cosmic rays. Its current composition is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms. The term stellar atmosphere describes the outer region of a star, and typically includes the portion starting from the opaque photosphere outwards. Stars with sufficiently low temperatures may form compound molecules in their outer atmosphere. ==Pressure== (詳細はforce per unit area that is always applied perpendicularly to a surface by the surrounding gas. It is determined by a planet's gravitational force in combination with the total mass of a column of gas above a location. On Earth, units of air pressure are based on the internationally recognized standard atmosphere (atm), which is defined as 101,325 Pa (760 Torr or 14.696 psi). The pressure of an atmospheric gas decreases with altitude due to the diminishing mass of gas above each location. The height at which the pressure from an atmosphere declines by a factor of ''e'' (an irrational number with a value of 2.71828..) is called the scale height and is denoted by ''H''. For an atmosphere with a uniform temperature, the scale height is proportional to the temperature and inversely proportional to the mean molecular mass of dry air times the planet's gravitational force per unit area at the surface of Earth. For such a model atmosphere, the pressure declines exponentially with increasing altitude. However, atmospheres are not uniform in temperature, so the exact determination of the atmospheric pressure at any particular altitude is more complex. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atmosphere」の詳細全文を読む スポンサード リンク
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