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The International Standard Atmosphere (ISA) is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a common reference for temperature and pressure and consists of tables of values at various altitudes, plus some formulas by which those values were derived. The International Organization for Standardization (ISO) publishes the ISA as an international standard, ISO 2533:1975.〔International Organization for Standardization, ''(Standard Atmosphere )'', ISO 2533:1975, 1975.〕 Other standards organizations, such as the International Civil Aviation Organization (ICAO) and the United States Government, publish extensions or subsets of the same atmospheric model under their own standards-making authority. ==Description== The ISA mathematical model divides the atmosphere into layers with linear temperature distributions against geopotential altitude.〔Gyatt, Graham (2006-01-14): ("The Standard Atmosphere" ). A mathematical model of the 1976 U.S. Standard Atmosphere.〕 The other two values (pressure and density) are computed by simultaneously solving the equations resulting from: * the vertical pressure variation (which relates pressure, density and geopotential altitude), using a standard pressure of at mean sea level as a boundary condition, and * the ideal gas law (which relates pressure, density, and temperature), at each geopotential altitude. Air density must be calculated in order to solve for the pressure, and is used in calculating dynamic pressure for moving vehicles. Dynamic viscosity is an empirical function of temperature, and kinematic viscosity is calculated by dividing dynamic viscosity by the density. Thus the standard consists of a tabulation of values at various altitudes, plus some formulas by which those values were derived. For example, at mean sea level the standard gives a pressure of (1 atm), a temperature of , a temperature lapse rate of per km (roughly −2°C (-3.6°F) per 1,000 ft), and a density of . The tropospheric tabulation continues to , where the pressure has fallen to , the temperature to , and the density to . Between 11 km and 20 km, the temperature remains constant.〔Batchelor, G. K., ''An Introduction to Fluid Dynamics'', Cambridge Univ. Press, 1967.〕 To allow modeling conditions below mean sea level, the troposphere actually extends to about , where the temperature is , pressure is , and density is . :lapse rate given per kilometer of ''geopotential altitude'' In the above table, ''geopotential altitude'' is calculated from a mathematical model that adjusts the altitude to include the variation of gravity with height, while ''geometric altitude'' is the standard direct vertical distance above mean sea level.〔Gyatt, Graham (2006-01-14): ("The Standard Atmosphere" ). A mathematical model of the 1976 U.S. Standard Atmosphere.〕 Note that the Lapse Rates cited in the table are given as °C per kilometer of geopotential altitude, not geometric altitude. The ISA model is based on average conditions at mid latitudes, as determined by the ISO's TC 20/SC 6 technical committee. It has been revised from time to time since the middle of the 20th century. 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「International Standard Atmosphere」の詳細全文を読む スポンサード リンク
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