Physical atmosphere

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Physical unit
Unit name Physical atmosphere
Unit symbol
Physical quantity (s) pressure
Formula symbol
In SI units
See also: Pascal , Technical Atmosphere

The physical atmosphere is a non- SI -conforming unit of pressure . Since January 1, 1978, it is no longer a legal entity in Germany . The unit symbol is atm .

Historically, the unit has been defined so that the pressure of 1 atm is as high as the mean air pressure prevailing at sea ​​level . This is caused by the weight of the earth's atmosphere .

The pressure of the physical atmosphere is one of the standard conditions on which many processes and measured values ​​are based.


The word atmosphere is derived from ancient Greek ἀτμός atmós , German 'steam' , 'haze', 'breath' and σφαῖρα sphaira , German 'ball' ( Latinized sphära ). In the present context it refers to the gaseous envelope above the earth's surface . The pressure unit is based on the amount of normal pressure exerted by this shell.


The standard atmospheric pressure was defined in 1954 as the unit of measurement "physical atmosphere":


The opposite is true

Previously, the standard was an atmospheric pressure of 760  Torr , that is, the pressure that holds a mercury column high in a mercury barometer . Since the pressure also depends on the gravitational acceleration and the density of the mercury , this definition was dependent on the measuring location and the temperature. With the definition of the units of the metric system in 1954 , the physical atmosphere became independent of temperature, location and weather conditions. The value of 101 325 Pa was obtained using:

(Density of mercury at 0 ° C)
( Standard acceleration )

As a result, the unit " Torr " (also no longer legal today) was given a definition via the metric system:


Individual evidence

  1. ^ Resolution 4 of the 10th CGPM (1954). In: Bureau International des Poids et Mesures, accessed June 18, 2020 .
  2. Peter Kurzweil: The Vieweg unit lexicon: formulas and terms from physics, chemistry and technology. Vieweg, 1999, p. 40 f
  3. Hans U. v. Vogel: Chemists Calendar. Springer, 1956, p. 392
  4. Le Système international d'unités . 7e édition, 1998 (the so-called "SI brochure"), chap. 4.2 Table 10, French and English
  5. DIN 1314 printing - basic terms and units. 1977
  6. DIN 1301, Part 3: Units - Conversion of non-SI units , 2018