standard atmosphere

(′stan·dərd ′at·mə′sfir)

(meteorology) A hypothetical vertical distribution of atmospheric temperature, pressure, and density which is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, and ballistic tables; the air is assumed to obey the perfect gas law and hydrostatic equation, which, taken together, relate temperature, pressure, and density variations in the vertical; it is further assumed that the air contains no water vapor, and that the acceleration of gravity does not change with height.
(mechanics) atmosphere

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physics SI, Metric-c.g.s. and -m.k.s. A standardized configuration of typical atmospheric conditions, most notably of pressure. More elaborate standard atmospheres, e.g. for artillery or meteorological use, define the lapse rate of temperature with altitude and other particulars, but the main usage of the term relates just to pressure. As such, it is a laboratory standard corresponding with typical ground-level atmospheric pressure, now SI-deprecated as a unit of measure (usually just atmosphere), but not as a reference pressure.

On the traditional barometer of mercury, the head of liquid is about 30 in or 760 mm; the latter figure was adopted as the standard, suitably defined regarding surrounding and intrinsic conditions. Now the derived (non- coherent) unit is defined as 101.325 kPa (14.695 95~ p.s.i.).

The overall picture of the standard atmosphere is shown in Table 52.

	SI	BI
temperature	15°C	59°F
pressure	101.325 kPa, 1.01325 bar	2116.2 lb-f·ft-2
gravitational acceleration	9.806 55 m·s-2	32.173 72 ft·s-2
volumic mass	1.2250 kg·m-3	0.076 474 lb·ft-3
viscosity - dynamic	1.7894 × 10-5 kg·m-1·s-1	1.2024 × 10-5 lb·ft-1·s-1
viscosity - kinematic	1.4607 × 10-5 m2·s-1	15.723 × 10-5 ft2·s-1
mean molecular speed	458.94 m·s-1	1505.7 ft·s-1
speed of sound	340.294 m·s-1	1116.45 ft·s-1

For miscellaneous conversions, see atmosphere.

History

Created by the US National Advisory Committee on Aeronautics in 1922 (the NACA + 1922 standard atmosphere) as ‘The atmospheric pressure registering 760 mm (29.921 3~ in) of head on a mercury barometer at mean sea level with gravitational acceleration at 9.806 6 m·s^-2, assuming the air to be a perfect gas at 15° C (59° F) and the mercury having a volumic mass of 13.595 × 10³ kg m^-3, = 101.325 0~ kPa (14.691~ p.s.i.).’ In 1924 the International Commission for Air Navigation adopted (as the ICAN + 1924 standard atmosphere) this definition with a minor change of gravity to 9.806 2 m·s^-2. In 1941 the International Civil Aviation Organization did likewise (as the ICAO + 1941 standard atmosphere), with gravity at 9.806 55 m·s^-2. The subsequent definition by the 9th CGPM of 1948 as a simple derived unit (1 013 250 dyn·cm^-2) and its re-statement in m.k.s. and the SI were only a re-expression of the implied value of the 1941 definition; so any statement in ICAO+1941 terms is equally valid today.

1954	10th CGPM: ‘The 10th CGPM, having noted the definition of the standard atmosphere given by the 9th CGPM when defining the International Temperature Scale led some physicists to believe that this definition of the standard atmosphere was valid only for accurate work in thermometry, declares that it adopts, for general use, the definition:
	1 standard atmosphere = 1 013 250 dynes per square centimetre, i.e. 101 325 newtons per square metre.’see note below

[Le Système International d'Unités (Sèvres, France: Bureau International de Poids et Mesures, 1985)]

A pressure equivalent to 14.7 lb per sq in. (1.01 × 10⁶ dynes per sq cm).

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