Barometric formula: Difference between revisions
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The '''barometric formula''' is a [[formula]] used to model how the [[air pressure]] (or [[air density]]) changes with [[altitude]]. | The '''barometric formula''' is a [[formula]] used to model how the [[air pressure]] (or [[air density]]) changes with [[altitude]]. | ||
== | == Model equations == | ||
{{see also|Atmospheric pressure}} | {{see also|Atmospheric pressure}} | ||
[[File:Pressure air.svg|thumb|300px|Pressure as a function of the height above the sea level]] | [[File:Pressure air.svg|thumb|300px|Pressure as a function of the height above the sea level]] | ||
The [[U.S. Standard Atmosphere]] gives two equations for computing pressure as a function of height, valid from sea level to 86 km altitude. The first equation is applicable to the atmospheric layers in which the temperature is assumed to vary with altitude at a non null temperature gradient of <math>L_{M,b}</math>: | |||
{{anchor|Non-zero lapse rate}} | {{anchor|Non-zero lapse rate}} | ||
<math display="block">P = P_{b} \left[ | <math display="block">P = P_{b} \cdot \left[ \frac{ T_{M,b} }{ T_{M,b} + L_{M,b} \cdot \left(H - H_{b}\right) }\right]^{\frac{g_{0}' \cdot M_{0}}{R^{*} \cdot L_{M,b}}}</math> | ||
The second equation is applicable to the atmospheric layers in which the temperature is assumed not to vary | .<ref name="USSA1976">{{cite report |author=<!-- not stated --> |date=October 1976 |title=U.S. Standard Atmosphere, 1976 |url=https://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/us-standard-atmosphere-1976/us-standard-atmosphere_st76-1562_noaa.pdf |location=Washington, D.C. |publisher=U.S. Government Printing Office |id=NTRS 19770009539 |access-date=2025-06-29}}</ref>{{rp|p=12}} | ||
The second equation is applicable to the atmospheric layers in which the temperature is assumed not to vary with altitude (zero temperature gradient): | |||
{{anchor|Zero lapse rate}} | {{anchor|Zero lapse rate}} | ||
<math display="block">P = P_b \exp \left[\frac{- | <math display="block">P = P_b \cdot \exp \left[\frac{-g_{0}' \cdot M_{0} \left(H-H_b\right)}{R^* \cdot T_{M,b}}\right]</math> | ||
where: | ,<ref name="USSA1976"/>{{rp|p=12}} where: | ||
*<math>P_b</math> = reference pressure | *<math>P_b</math> = reference pressure | ||
*<math>T_{M,b}</math> = reference temperature ([[kelvin|K]]) | *<math>T_{M,b}</math> = reference temperature ([[kelvin|K]]) | ||
*<math>L_{M,b}</math> = temperature | *<math>L_{M,b}</math> = temperature gradient (K/m), e.g. -6.5 K/km at sea level. This is the [[lapse rate]] with the opposite sign convention. | ||
*<math> | *<math>H</math> = [[geopotential height]] at which pressure is calculated (m) | ||
*<math> | *<math>H_b</math> = geopotential height of reference level ''b'' (meters; e.g., ''H<sub>b</sub>'' = 11 000 m) | ||
*<math>R^*</math> = [[universal gas constant]]: 8. | *<math>R^*</math> = [[universal gas constant]]: 8.31432 N·m/(kmol·K)<ref name="USSA1976"/>{{rp|p=3}} | ||
*<math> | *<math>g_{0}'</math> = The [[standard gravity|gravitational acceleration]] in units of geopotential height, 9.80665 m/s<sup>2</sup><ref name="USSA1976"/>{{rp|p=3}} | ||
*<math> | *<math>M_{0}</math> = mean molecular weight of air at sea level: 28.9644 kg/kmol<ref name="USSA1976"/>{{rp|p=9}} | ||
Or converted to [[imperial units]]:<ref name="conversion">Mechtly, E. A., 1973: ''[https://ntrs.nasa.gov/api/citations/19730018242/downloads/19730018242.pdf The International System of Units, Physical Constants and Conversion Factors]''. NASA SP-7012, Second Revision, National Aeronautics and Space Administration, Washington, D.C.</ref> | Or converted to [[imperial units]]:<ref name="conversion">Mechtly, E. A., 1973: ''[https://ntrs.nasa.gov/api/citations/19730018242/downloads/19730018242.pdf The International System of Units, Physical Constants and Conversion Factors]''. NASA SP-7012, Second Revision, National Aeronautics and Space Administration, Washington, D.C.</ref> | ||
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*<math>P_b</math> = reference pressure | *<math>P_b</math> = reference pressure | ||
*<math>T_{M,b}</math> = reference temperature ([[kelvin|K]]) | *<math>T_{M,b}</math> = reference temperature ([[kelvin|K]]) | ||
*<math>L_{M,b}</math> = temperature | *<math>L_{M,b}</math> = temperature gradient (K/ft) | ||
*<math> | *<math>H</math> = height at which pressure is calculated (ft) | ||
*<math> | *<math>H_b</math> = height of reference level ''b'' (feet; e.g., ''H<sub>b</sub>'' = 36,089 ft) | ||
*<math>R^*</math> = [[universal gas constant]]; using feet, kelvins, and (SI) [[mole (unit)|moles]]: {{val|8.9494596e4|u=lb·ft<sup>2</sup>/(lb-mol·K·s<sup>2</sup>)}} | *<math>R^*</math> = [[universal gas constant]]; using feet, kelvins, and (SI) [[mole (unit)|moles]]: {{val|8.9494596e4|u=lb·ft<sup>2</sup>/(lb-mol·K·s<sup>2</sup>)}} | ||
*<math>g_0</math> = [[standard gravity|gravitational acceleration]]: 32.17405 ft/s<sup>2</sup> | *<math>g_0</math> = [[standard gravity|gravitational acceleration]]: 32.17405 ft/s<sup>2</sup> | ||
*<math>M</math> = molar mass of Earth's air: 28.9644 lb/lb-mol | *<math>M</math> = molar mass of Earth's air: 28.9644 lb/lb-mol | ||
The value of subscript ''b'' ranges from 0 to 6 in accordance with each of seven successive layers of the atmosphere shown in the table below. In these equations, ''g''<sub>0</sub>, ''M'' and ''R''<sup>*</sup> are each single-valued constants, while ''P'', ''L,'' ''T,'' and '' | The value of subscript ''b'' ranges from 0 to 6 in accordance with each of seven successive layers of the atmosphere shown in the table below. In these equations, ''g''<sub>0</sub>, ''M'' and ''R''<sup>*</sup> are each single-valued constants, while ''P'', ''L,'' ''T,'' and ''H'' are multivalued constants in accordance with the table below. The values used for ''M'', ''g''<sub>0</sub>, and ''R''<sup>*</sup> are in accordance with the [[U.S. Standard Atmosphere]], 1976, and the value for ''R''<sup>*</sup> in particular does not agree with standard values for this constant.<ref name="USSA1976"/>{{rp|p=3}} The reference value for ''P<sub>b</sub>'' for ''b'' = 0 is the defined sea level value, ''P''<sub>0</sub> = 101 325 [[pascal (unit)|Pa]] or 29.92126 inHg. Values of ''P<sub>b</sub>'' of ''b'' = 1 through ''b'' = 6 are obtained from the application of the appropriate member of the pair equations 1 and 2 for the case when ''H'' = ''H''<sub>''b''+1</sub>.<ref name=USSA1976/>{{rp|p=12}} | ||
{| class="wikitable" style="text-align: center" | {| class="wikitable" style="text-align: center" | ||
| Line 41: | Line 43: | ||
! colspan="2"|Geopotential | ! colspan="2"|Geopotential | ||
height above MSL | height above MSL | ||
( | (H)<ref name="USSA1976" />{{rp|p=3}} | ||
! colspan="2"|Static pressure | ! colspan="2"|Static pressure | ||
! rowspan="2"|Standard temperature<br> (K) | ! rowspan="2"|Standard temperature<br> (K) | ||
! colspan="2"|Temperature | ! colspan="2"|Temperature gradient<ref name="USSA1976" />{{rp|p=3}} | ||
! rowspan="2"|Exponent <br> g0 M / R L | ! rowspan="2"|Exponent <br> g0 M / R L | ||
|- | |- | ||
! ( | ! (km) !! (ft)!! (Pa) !! (inHg) !! (K/km) !! (K/ft) | ||
|- | |- | ||
| 0 || 0 || 0 || 101 325 | | 0 || 0 || 0 || 101 325 || 29.9213 || 288.15 || -6.5 || -0.0019812 || -5.25588 | ||
|- | |- | ||
| 1 || | | 1 || 11 || 36 089 || 22 632.1 || 6.68324 || 216.65 || 0.0 || 0.0 || — | ||
|- | |- | ||
| 2 || | | 2 || 20 || 65 617 || 5 474.89 || 1.616734 || 216.65 || 1.0 || 0.0003048 || 34.1626 | ||
|- | |- | ||
| 3 || | | 3 || 32 || 104 987 || 868.019 || 0.256326 || 228.65 || 2.8 || 0.00085344 || 12.2009 | ||
|- | |- | ||
| 4 || | | 4 || 47 || 154 199 || 110.9063 || 0.0327506 || 270.65 || 0.0 || 0.0 || — | ||
|- | |- | ||
| 5 || | | 5 || 51 || 167 323 || 66.9389 || 0.0197670 || 270.65 || -2.8 || -0.00085344 || -12.2009 | ||
|- | |- | ||
| 6 || | | 6 || 71 || 232 940 || 3.95642 || 0.00116833 || 214.65 || -2 || -0.0006096 || -17.0813 | ||
|} | |} | ||
Density can be calculated from pressure and temperature using | |||
<math>\rho = \frac{ P \cdot M_{0} }{ R^* \cdot T_{M} } = \frac{ P \cdot M }{ R^* \cdot T }</math> | |||
<math | ,<ref name="USSA1976" />{{rp|p=15}} where | ||
* <math>M_{0}</math> is the molecular weight at sea level | |||
*<math>{ | * <math>M</math> is the mean molecular weight at the altitude of interest | ||
*<math> | * <math>T</math> is the temperature at the altitude of interest | ||
*<math> | * <math>T_{M} = T \cdot \frac {M_0}{M}</math> is the molecular-scale temperature.<ref name="USSA1976" />{{rp|p=9}} | ||
*<math> | |||
The atmosphere is assumed to be fully mixed up to about 80 km, so <math>M = M_{0}</math> within the region of validity of the equations presented here.<ref name="USSA1976" />{{rp|p=9}} | |||
Alternatively, density equations can be derived in the same form as those for pressure, using reference densities instead of reference pressures.{{citation needed|date=June 2025}} | |||
This model, with its simple linearly segmented temperature profile, does not closely agree with the physically observed atmosphere at altitudes below 20 km. From 51 km to 81 km it is closer to observed conditions.<ref name="USSA1976" />{{rp|p=1}} | |||
{ | |||
==Derivation== | ==Derivation== | ||
Latest revision as of 05:55, 29 June 2025
Template:Short description Template:Broader
The barometric formula is a formula used to model how the air pressure (or air density) changes with altitude.
Model equations
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The U.S. Standard Atmosphere gives two equations for computing pressure as a function of height, valid from sea level to 86 km altitude. The first equation is applicable to the atmospheric layers in which the temperature is assumed to vary with altitude at a non null temperature gradient of : Script error: No such module "anchor". .[1]Template:Rp
The second equation is applicable to the atmospheric layers in which the temperature is assumed not to vary with altitude (zero temperature gradient): Script error: No such module "anchor". ,[1]Template:Rp where:
- = reference pressure
- = reference temperature (K)
- = temperature gradient (K/m), e.g. -6.5 K/km at sea level. This is the lapse rate with the opposite sign convention.
- = geopotential height at which pressure is calculated (m)
- = geopotential height of reference level b (meters; e.g., Hb = 11 000 m)
- = universal gas constant: 8.31432 N·m/(kmol·K)[1]Template:Rp
- = The gravitational acceleration in units of geopotential height, 9.80665 m/s2[1]Template:Rp
- = mean molecular weight of air at sea level: 28.9644 kg/kmol[1]Template:Rp
Or converted to imperial units:[2]
- = reference pressure
- = reference temperature (K)
- = temperature gradient (K/ft)
- = height at which pressure is calculated (ft)
- = height of reference level b (feet; e.g., Hb = 36,089 ft)
- = universal gas constant; using feet, kelvins, and (SI) moles: Template:Val
- = gravitational acceleration: 32.17405 ft/s2
- = molar mass of Earth's air: 28.9644 lb/lb-mol
The value of subscript b ranges from 0 to 6 in accordance with each of seven successive layers of the atmosphere shown in the table below. In these equations, g0, M and R* are each single-valued constants, while P, L, T, and H are multivalued constants in accordance with the table below. The values used for M, g0, and R* are in accordance with the U.S. Standard Atmosphere, 1976, and the value for R* in particular does not agree with standard values for this constant.[1]Template:Rp The reference value for Pb for b = 0 is the defined sea level value, P0 = 101 325 Pa or 29.92126 inHg. Values of Pb of b = 1 through b = 6 are obtained from the application of the appropriate member of the pair equations 1 and 2 for the case when H = Hb+1.[1]Template:Rp
| Subscript b | Geopotential
height above MSL (H)[1]Template:Rp |
Static pressure | Standard temperature (K) |
Temperature gradient[1]Template:Rp | Exponent g0 M / R L | |||
|---|---|---|---|---|---|---|---|---|
| (km) | (ft) | (Pa) | (inHg) | (K/km) | (K/ft) | |||
| 0 | 0 | 0 | 101 325 | 29.9213 | 288.15 | -6.5 | -0.0019812 | -5.25588 |
| 1 | 11 | 36 089 | 22 632.1 | 6.68324 | 216.65 | 0.0 | 0.0 | — |
| 2 | 20 | 65 617 | 5 474.89 | 1.616734 | 216.65 | 1.0 | 0.0003048 | 34.1626 |
| 3 | 32 | 104 987 | 868.019 | 0.256326 | 228.65 | 2.8 | 0.00085344 | 12.2009 |
| 4 | 47 | 154 199 | 110.9063 | 0.0327506 | 270.65 | 0.0 | 0.0 | — |
| 5 | 51 | 167 323 | 66.9389 | 0.0197670 | 270.65 | -2.8 | -0.00085344 | -12.2009 |
| 6 | 71 | 232 940 | 3.95642 | 0.00116833 | 214.65 | -2 | -0.0006096 | -17.0813 |
Density can be calculated from pressure and temperature using
,[1]Template:Rp where
- is the molecular weight at sea level
- is the mean molecular weight at the altitude of interest
- is the temperature at the altitude of interest
- is the molecular-scale temperature.[1]Template:Rp
The atmosphere is assumed to be fully mixed up to about 80 km, so within the region of validity of the equations presented here.[1]Template:Rp
Alternatively, density equations can be derived in the same form as those for pressure, using reference densities instead of reference pressures.Script error: No such module "Unsubst".
This model, with its simple linearly segmented temperature profile, does not closely agree with the physically observed atmosphere at altitudes below 20 km. From 51 km to 81 km it is closer to observed conditions.[1]Template:Rp
Derivation
The barometric formula can be derived using the ideal gas law:
Assuming that all pressure is hydrostatic: and dividing this equation by we get:
Integrating this expression from the surface to the altitude z we get:
Assuming linear temperature change and constant molar mass and gravitational acceleration, we get the first barometric formula:
Instead, assuming constant temperature, integrating gives the second barometric formula:
In this formulation, R* is the gas constant, and the term R*T/Mg gives the scale height (approximately equal to 8.4 km for the troposphere).
(For exact results, it should be remembered that atmospheres containing water do not behave as an ideal gas. See real gas or perfect gas or gas for further understanding.)