Scale height

For planetary atmospheres, scale height is the increase in altitude for which the atmospheric pressure decreases by a factor of e. The scale height remains constant for a particular temperature. It can be calculated by[1][2]

${\displaystyle H={\frac {kT}{mg}}}$

or equivalently

${\displaystyle H={\frac {RT}{Mg}}}$

where:

• k = Boltzmann constant = 1.38 x 10⁻²³ J·K⁻¹
• R = gas constant
• T = mean atmospheric temperature in kelvins = 250 K[3] for Earth
• m = mean mass of a molecule (units kg)
• M = mean mass of one mol of atmospheric particles = 0.029 kg/mol for Earth
• g = acceleration due to gravity at the current location (m/s²)

The pressure (force per unit area) at a given altitude is a result of the weight of the overlying atmosphere. If at a height of z the atmosphere has density ρ and pressure P, then moving upwards an infinitesimally small height dz will decrease the pressure by amount dP, equal to the weight of a layer of atmosphere of thickness dz.

Thus:

${\displaystyle {\frac {dP}{dz}}=-g\rho }$

where g is the acceleration due to gravity. For small dz it is possible to assume g to be constant; the minus sign indicates that as the height increases the pressure decreases. Therefore, using the equation of state for an ideal gas of mean molecular mass M at temperature T, the density can be expressed as

${\displaystyle \rho ={\frac {MP}{RT}}}$

Combining these equations gives

${\displaystyle {\frac {dP}{P}}={\frac {-dz}{\frac {kT}{mg}}}}$

which can then be incorporated with the equation for H given above to give:

${\displaystyle {\frac {dP}{P}}=-{\frac {dz}{H}}}$

which will not change unless the temperature does. Integrating the above and assuming P₀ is the pressure at height z = 0 (pressure at sea level) the pressure at height z can be written as:

${\displaystyle P=P_{0}\exp \left(-{\frac {z}{H}}\right)}$

This translates as the pressure decreasing exponentially with height.[4]

In Earth's atmosphere, the pressure at sea level P₀ averages about 1.01×10⁵ Pa, the mean molecular mass of dry air is 28.964 u and hence 28.964 × 1.660×10⁻²⁷ = 4.808×10⁻²⁶ kg, and g = 9.81 m/s². As a function of temperature the scale height of Earth's atmosphere is therefore 1.38/(4.808×9.81)×10³ = 29.26 m/deg. This yields the following scale heights for representative air temperatures.

T = 290 K, H = 8500 m

T = 273 K, H = 8000 m

T = 260 K, H = 7610 m

T = 210 K, H = 6000 m

These figures should be compared with the temperature and density of Earth's atmosphere plotted at NRLMSISE-00, which shows the air density dropping from 1200 g/m³ at sea level to 0.5³ = .125 g/m³ at 70 km, a factor of 9600, indicating an average scale height of 70/ln(9600) = 7.64 km, consistent with the indicated average air temperature over that range of close to 260 K.

Note:

• Density is related to pressure by the ideal gas laws. Therefore density will also decrease exponentially with height from a sea level value of ρ₀ roughly equal to 1.2 kg m⁻³
• At heights over 100 km, an atmosphere may no longer be well mixed. Then each chemical species has its own scale height.
• Here temperature and gravitational acceleration were assumed to be constant but both may vary over large distances.

Approximate atmospheric scale heights for selected Solar System bodies follow.

• Venus: 15.9 km[5]
• Earth: 8.5 km[6]
• Mars: 11.1 km[7]
• Jupiter: 27 km[8]
• Saturn: 59.5 km[9]

• Titan: 21 km[10]

• Uranus: 27.7 km[11]
• Neptune: 19.1–20.3 km[12]
• Pluto: ~50 km[13]

• Time constant