Barometric formula

The barometric formula, sometimes called the "exponential atmosphere" or "isothermal atmosphere", is a formula used to model how the pressure (or density) of the air changes with altitude.

Pressure equations

There are two different equations for computing pressure at various height regimes below 86 km (or 278,400 feet). Equation 1 is used when the value of standard temperature lapse rate is not equal to zero and equation 2 is used when standard temperature lapse rate equals zero.

Equation 1::{P}=P_b cdot left [frac{T_b}{T_b + L_bcdot(h-h_b)} ight] ^frac{g_0 cdot M}{R^* cdot L_b}

Equation 2::{P}=P_b cdot exp left [frac{-g_0 cdot M cdot (h-h_b)}{R^* cdot T_b} ight]

where:P = Static pressure (pascals):T = Standard temperature (kelvins):L = Standard temperature lapse rate (kelvins per meter):h = Height above sea level (meters):R^* = Universal gas constant for air: 8.31432 N·m / (mol·K):g_0 = Gravitational acceleration (9.80665 m/s²):M = Molar mass of Earth's air (0.0289644 kg/mol)

Or converted to Imperial units:Mechtly, E. A., 1973: "The International System of Units, Physical Constants and Conversion Factors". NASA SP-7012, Second Revision, National Aeronautics and Space Administration, Washington, D.C.]

where:P = Static pressure (inches of mercury):T = Standard temperature (kelvins):L = Standard temperature lapse rate (kelvins per foot):h = Height above sea level (feet):R^* = Universal gas constant (using feet and kelvins and gram moles: 8.9494596×104 kg·ft2·s-2·K-1·kmol-1):g_0 = Gravitational acceleration (32.17405 ft/s²):M = Molar mass of Earth's air (28.9644 g/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"0, "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," "g"0, 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. [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf U.S. Standard Atmosphere] , 1976, U.S. Government Printing Office, Washington, D.C., 1976. (Linked file is very large.) ] The reference value for "Pb" for "b" = 0 is the defined sea level value, "P0" = 101325 pascals 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 = h_{b+1}.:

Density equations

The expressions for calculating density are nearly identical to calculating pressure. The only difference is the exponent in Equation 1.

There are two different equations for computing density at various height regimes below 86 geometric km (84,852 geopotential meters or 278,385.8 geopotential feet). Equation 1 is used when the value of Standard Temperature Lapse rate is not equal to zero and equation 2 is used when Standard Temperature Lapse rate equals zero.

Equation 1::{ ho}= ho_b cdot left [frac{T_b}{T_b + L_bcdot(h-h_b)} ight] ^{left(frac{g_0 cdot M}{R^* cdot L_b} ight)+1}

Equation 2::{ ho}= ho_b cdot expleft [frac{-g_0 cdot M cdot (h-h_b)}{R^* cdot T_b} ight]

where:{ ho} = Mass density (kg/m³):T = Standard temperature (kelvins):L = Standard temperature lapse rate (kelvins per meter):h = Height above sea level (geopotential meters):R^* = Universal gas constant for air: 8.31432 N·m/(mol·K):g_0 = Gravitational acceleration (9.80665 m/s²):M = Molar mass of Earth's air (0.0289644 kg/mol)

Or converted to English units:Mechtly, E. A., 1973: "The International System of Units, Physical Constants and Conversion Factors". NASA SP-7012, Second Revision, National Aeronautics and Space Administration, Washington, D.C.]

Where:{ ho} = Mass density (slugs/ft³):{T} = Standard temperature (kelvins):{L} = Standard temperature lapse rate (degrees Celsius per foot):{h} = Height above sea level (geopotential feet):{R^*} = Universal gas constant (8.9494596×104 ft²/(s·K):{g_0} = Gravitational acceleration (32.17405 ft/s²):{M} = Molar mass of Earth's air (28.9644 grams per mole) 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. The reference value for ho_b for "b" = 0 is the defined sea level value, ho_o = 1.2250 kg/m³ or 0.0023768908 slugs/ft³. Values of ho_b 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_{b+1} U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976. ] In these equations, "g"0, "M" and "R"* are each single-valued constants, while ho, "L", "T" and "h" are multi-valued constants in accordance with the table below. It should be noted that the values used for "M", "g"0 and "R"* are in accordance with the U.S. Standard Atmosphere, 1976, and that the value for "R"* in particular does not agree with standard values for this constant. [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539_1977009539.pdf U.S. Standard Atmosphere] , 1976, U.S. Government Printing Office, Washington, D.C., 1976. (Linked file is very large.) ] .

Derivation

The barometric formula can be derived fairly easily using the ideal gas law:

: ho = frac{M cdot P}{R^* cdot T}

When density is known:

: P = frac{ ho cdot {R^*} cdot T}{M}

And assuming that all pressure is hydrostatic:

: dP = - ho g,dz,

Substituting the first expression into the second we get:

: frac{dP}{P} = - frac{M g,dz}{RT}

Integrating this expression from the surface to the altitude "z" we get:

: P = P_0 e^{-int_{0}^{z}{M g dz/RT,

Assuming constant temperature, molar mass, and gravitational acceleration, we get the barometric formula:: P = P_0 e^{-M g z/RT},

In this formulation, "R" is the gas constant, and the term RT/M g 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 to further understanding)

References

ee also

*NRLMSISE-00


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