# Physical atmosphere

Physical unit
Unit name Physical atmosphere
Unit symbol ${\ displaystyle \ mathrm {atm}}$
Physical quantity (s) pressure
Formula symbol ${\ displaystyle p, p _ {\ mathrm {amb}}}$
dimension ${\ displaystyle {\ mathsf {M \; L ^ {- 1} \; T ^ {- 2}}}}$
In SI units ${\ displaystyle \ mathrm {1 \, atm = 101 \, 325 \; {\ frac {kg} {m \, s ^ ​​{2}}}}}$ ${\ displaystyle \ quad = \ mathrm {101 {,} 325 \; kPa}}$

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.

## etymology

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.

## definition

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

${\ displaystyle 1 \, \ mathrm {atm} = 101 \, 325 \, \ mathrm {Pa} = 1 {,} 013 \, 25 \ \ mathrm {bar} \,}$.

The opposite is true

${\ displaystyle 1 \; \ mathrm {Pa} \ approx 9 {,} 8692 \ cdot 10 ^ {- 6} \; \ mathrm {atm} \ ,.}$

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${\ displaystyle h = 760 \; \ mathrm {mm}}$${\ displaystyle p = \ rho \; g \; h}$${\ displaystyle g}$${\ displaystyle \ rho}$ 325 Pa was obtained using:

${\ displaystyle \ rho = \ mathrm {13 {,} 595 \, 1 \, {\ frac {g} {cm ^ {3}}}} \}$ (Density of mercury at 0 ° C)
${\ displaystyle g = 9 {,} 806 \, 65 \; \ mathrm {\ frac {m} {s ^ {2}}} \}$ ( Standard acceleration )

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

${\ displaystyle \ mathrm {1 \; Torr = {\ frac {1 \; \ mathrm {atm}} {760}} = 133 {,} 322 \ ldots \; Pa}}$.

## Individual evidence

1. ^ Resolution 4 of the 10th CGPM (1954). In: bipm.org. 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