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A scale height is a term often used in scientific contexts for a distance over which a quantity decreases by a factor of e. It is usually denoted by the capital letter H.

For planetary atmospheres, it is the vertical distance upwards, over which the pressure of the atmosphere decreases by a factor of e. The scale height remains constant for a particular temperature. It can be calculated by

- $H\; =\; frac\{kT\}\{Mg\}$

where:

- k = Boltzmann constant = 1.38 x 10
^{−23}J·K^{−1} - T = mean planetary surface temperature in kelvins
- M = mean molecular mass of dry air (units kg)
- g = acceleration due to gravity on planetary surface (m/s²)

The pressure in the atmosphere is caused by the weight on the atmosphere of the overlying atmosphere [force per unit area]. 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:

- $frac\{dP\}\{dz\}\; =\; -grho$

where g is used to denote 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 a perfect gas of mean molecular mass m at temperature T, the density can be expressed as such:

- $rho\; =\; frac\{mP\}\{kT\}$

Therefore combining the equations gives

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

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

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

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

- $P\; =\; P\_0e^\{(-frac\{z\}\{H\})\}$

This translates as the pressure decreasing exponentially with height.

In the Earth's atmosphere, the pressure at sea level P_{0} roughly equals 1.01×10^{5}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 the Earth's atmosphere is therefore 1.38/(4.808×9.81)×10^{3} = 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 the Earth's atmosphere plotted at NRLMSISE-00, which shows the air density dropping from 1200 g/m^{3} at sea level to 0.5^{3} = .125 g/m^{3} 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 ρ
_{0}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.

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Last updated on Wednesday September 10, 2008 at 05:43:09 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Wednesday September 10, 2008 at 05:43:09 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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