# Scale height

The earth atmosphere's scale height is about 8.5km, as can be confirmed from this diagram of air pressure p by altitude h: At an altitude of 0, 8.5, and 17 km, the pressure is about 1000, 370, and 140 hPa, respectively.

In various scientific contexts, a scale height is a distance over which a quantity decreases by a factor of e (approximately 2.72, the base of natural logarithms). It is usually denoted by the capital letter H.

## Scale height used in a simple atmospheric pressure model

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:

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 at 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—with some departures caused by varying temperature—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, molecular diffusion means that each molecular atomic species has its own scale height.

## Planetary examples

Approximate atmospheric scale heights for selected Solar System bodies follow.

## References

1. ^ "Glossary of Meteorology - scale height". American Meteorological Society (AMS).
2. ^
3. ^
4. ^ "Example: The scale height of the Earth's atmosphere" (PDF). Archived from the original (PDF) on 2011-07-16.
5. ^ "Venus Fact Sheet". NASA. Retrieved 28 September 2013.
6. ^ "Earth Fact Sheet". NASA. Retrieved 28 September 2013.
7. ^ "Mars Fact Sheet". NASA. Retrieved 28 September 2013.
8. ^ "Jupiter Fact Sheet". NASA. Archived from the original on 5 October 2011. Retrieved 28 September 2013.
9. ^ "Saturn Fact Sheet". NASA. Archived from the original on 21 August 2011. Retrieved 28 September 2013.
10. ^ Justus, C. G.; Aleta Duvall; Vernon W. Keller (1 August 2003). "Engineering-Level Model Atmospheres For Titan and Mars". International Workshop on Planetary Probe Atmospheric Entry and Descent Trajectory Analysis and Science, Lisbon, Portugal, October 6–9, 2003, Proceedings: ESA SP-544. ESA. Retrieved 28 September 2013.
11. ^ "Uranus Fact Sheet". NASA. Retrieved 28 September 2013.
12. ^ "Neptune Fact Sheet". NASA. Retrieved 28 September 2013.
13. ^ "Pluto Fact Sheet". NASA. Retrieved 2016-10-20.