Gorshkov V.G., Makarieva A.M. (PNPI)

 

Thermocondensational osmosis of water vapor in the gravitational field of Earth

 

The phenomenon of osmosis in the mixture of condensable and non-condensable gases in the presence of a spatial temperature gradient is studied. Condensation of one gas in the mixture leads to local drop of partial pressure, the appearance of a pressure gradient force and dynamic fluxes of gas mixture to the considered area. In the gravitational field of the planet each atmospheric gas tends to be distributed along height over a scale determined by its molecular mass. For water vapor, the condensable gas in the atmosphere of Earth, static equilibrium is possible when its partial pressure drops e-fold over 13.5 km. At the same time, maximum (saturated) concentration of water vapor depends on temperature and decreases approximately twofold per each ten degrees of temperature drop. Therefore, it appears that there exists a critical vertical gradient of air temperature, beyond which atmospheric water vapor cannot be in static equilibrium. (As we showed, this critical gradient is 1.2 K/km for terrestrial atmosphere, while the global mean decrease of air temperature with height corresponds to a gradient of 6.5 K/km, i.e. it significantly exceeds the critical value). When the critical gradient is exceeded, water vapor starts to undergo condensation. Its local partial pressure drops, as well as the total pressure of gas mixture. In the result, there appears an upward-directed force of osmotic nature, as far as the partial pressure of water vapor ever tends to return to the equilibrium. This force acts on moist air making it rise. The role of semi-permeable membrane inherent to the phenomenon of osmosis is, in this case, played by the vertical temperature gradient, which selectively removes water vapor from the gas mixture. Condensation of water vapor is compensated by continuous evaporation from the surface of the hydrosphere. The magnitude of the resulting force is enough to explain the power of the observed atmospheric dynamic fluxes. The horizontal flow of moist air is directed from the area with lower, to the area with higher, evaporation. Thus, in order to ensure that moist air flows from ocean to land (and, thus, compensates for the gravitational river runoff), it is necessary to ensure that evaporation on land be higher than in the ocean. This situation is only possible if there is a contiguous forest cover, which features a very large evaporative surface per unit projection area.