# Homosphere and heterosphere

Average temperature and molar mass of the air as a function of altitude.
The characteristics of the two curves change at an altitude of approx. 100 km. Here lies the boundary between the homosphere (below) and the heterosphere (above).

In addition to the usual classification of an atmosphere and especially the earth's atmosphere according to its vertical temperature gradient , meteorology also distinguishes between homosphere (from Greek ὁμός homós “equal”) and heterosphere (from Greek ἕτερος héteros “the other”, “unequal”) common. In the homosphere, the atmospheric gases are well mixed; in the heterosphere above, the atmospheric constituents increasingly begin to separate with increasing altitude.

The transition area between the two is referred to as the homopause or turbo pause , which in the case of Earth is at an altitude of around 80 to 120 kilometers.

## Scale height and segregation

According to the Boltzmann statistics assumes the density of a gas which in a uniform gravitational field in the thermal equilibrium is, exponentially with height. The height difference  H , over which the density  falls by a factor of e = 2.718 ..., is the scale height of the atmosphere concerned, and the following applies:

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

With

 R. : universal gas constant T : absolute temperature M. : molar mass G : Acceleration due to gravity

Since the gases that make up the earth's atmosphere have different molar masses, the scale heights of these air components are sometimes very different. For example, for a temperature of 0 ° C, the scale heights are from

 argon : H = 5,980 m molecular oxygen : H = 7,480 m molecular hydrogen : H = 119,500 m.

Even if the idealized prerequisites for these exemplary calculations are not strictly met in the real earth atmosphere (in particular the air temperature is very variable at different altitudes ), it would still be expected that the atmosphere would be partially separated, i.e. that the heavy components with less Scale height concentrate close to the ground and at higher heights the lighter components with greater scale height predominate.

## Homosphere

The observation shows, however, that in the troposphere and essentially also in the stratosphere and mesosphere the composition of the atmosphere is practically independent of the altitude. Turbulence and large-scale vertical movements mix the atmosphere in this area convectively so effectively that no segregation can prevail. This well-mixed layer, which extends up to a height of about 100 km, is known as the homosphere.

Due to the constant composition, the “air” mixture always has the same average molar mass of about 29 g / mol up to an altitude of about 80 km . Barometric height formulas can therefore treat the air up to these heights as a uniform gas with this molar mass.

However, the water vapor content in the troposphere decreases sharply with altitude, because the temperature, which drops with altitude, allows ever lower maximum vapor pressures . For example, the water vapor pressure at a height of 10 km has typically fallen to around 1  ‰ of the soil value (the air pressure, on the other hand, has only fallen to 25% of the soil pressure).

The altitude-dependent formation of ozone and other trace gases also leads to an altitude-dependent concentration of these air components.

With over 99.9% of the particles, the earth's homosphere contains most of the atmosphere. The Fédération Aéronautique Internationale considers the height of 100 km ( Kármán line ) to be the boundary to space , because here (at a temperature of −80 ° C and an air pressure of 0.03  Pascal , i.e. 0.00003% of the air pressure elevation) is to obtain the buoyancy required airspeed the value of the web velocity reaches that for obtaining an orbit is required. This definition of the boundary between air and space is widely recognized internationally, although NASA , for example , already uses this boundary at the mesopause .

## Heterosphere

Molecular diffusion processes that are dependent on the molar mass occur at higher altitudes due to the increasing free path lengths (near the ground: about 0.06 μm, at 100 km height: about 15 cm, at 200 km height: about 200 m) . Thus, in this area known as the heterosphere, the gas kinetic separation required by the Boltzmann statistics actually takes place. For example, the mean molar mass of the air at an altitude of 700 km has decreased to around 16 g / mol (corresponds to atomic oxygen ) and at even higher altitudes it sinks to 4 g / mol ( helium ) and finally 1 g / mol ( hydrogen ).

The large free paths mean that the gas particles can hardly come together to form collective movement processes, so that there are practically no more winds in the heterosphere. Exhaust plumes from ascending rockets are therefore usually dragged away in the homosphere by winds at high altitude in alternating directions without their diameter changing significantly, while they quickly diffuse apart in the heterosphere.

In addition to the gas kinetic segregation, the dissociation processes triggered by the sun's ultraviolet radiation also influence the composition of the atmosphere. Above about 80 km altitude, carbon dioxide and the remaining residues of water vapor dissociate . The dissociation of O 2 molecules into O atoms, which begins at an altitude of about 30 km and leads to the formation of ozone there , only generates free O atoms from an altitude of about 100 km. Only traces of O 2 remain above about 150 km .

## Individual evidence

1. ^ A b c W. Roedel: Physics of our environment: The atmosphere . 3. Edition. Springer, Berlin 2000, ISBN 3-540-67180-3 . P. 63
2. GH Liljequist, K. Cehak: General Meteorology. 3. Edition. Vieweg, Braunschweig / Wiesbaden 1994, ISBN 3-528-23555-1 , p. 380
3. ^ A b F. Möller: Introduction to Meteorology. Volume 1, BI-Wissenschaftsverlag, Mannheim / Vienna / Zurich 1991, ISBN 3-411-00276-X , p. 52
4. ^ F. Möller: Introduction to Meteorology. Volume 1, BI-Wissenschaftsverlag, Mannheim / Vienna / Zurich 1991, ISBN 3-411-00276-X , p. 140
5. NOAA, NASA, USAF: US Standard Atmosphere, 1976 , Washington, DC 1976, p. 68: for Z = 100000 m: t = -78.07 ° C, P = 3,2011e-4 mb, P / P0 = 3, 1593e-7
6. ^ A b F. Möller: Introduction to Meteorology. Volume 1, BI-Wissenschaftsverlag, Mannheim / Vienna / Zurich 1991, ISBN 3-411-00276-X , p. 55
7. ^ F. Möller: Introduction to Meteorology. Volume 1, BI-Wissenschaftsverlag, Mannheim / Vienna / Zurich 1991, ISBN 3-411-00276-X , p. 54