Feel the Pump Physics

Biotic pump of atmospheric moisture functions like this. Water vapor from the forest canopy undergoes condensation in the atmosphere and disappears from the gas phase. For this reason, the air rarifies and its pressure drops. In the result, air is sucked from below to compensate for this pressure drop. This, in its turn, leads to the drop of pressure at the surface, so that surface air is drawn from the neighboring areas to the area of the upwelling. If there is ocean in the neighborhood, then the air which is drawn to the continent will be enriched by evaporated moisture.

Pump physics is simple. One needs to have an idea of what ideal gas is, what diffusion is and how it differs from the dynamic flow of gases and, finally, how water vapor pressure depends on temperature (Clausius-Clapeyron law). All these phenomena can be vividly illustrated with use of interactive Flash models. You can change and monitor gas pressure, make water evaporate, make gas mixtures diffuse or flow dynamically. You can also get an idea of the major components of atmospheric circulation over forests, clear-cuts or deserts, by switching the circulation on and off yourself. All models were created for our website by S.K. Buruchenko.

Why do condensation and precipitation occur in the atmosphere?

Read the explanations to the right of the model and move the slide up and down to study the conditions when the condensation and precipitation are possible.

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If you have problems with viewing the Flash object,
click here for free installation of the latest version of Flash Player.

1. Water vapor — the gaseous state of water — in the atmosphere finds itself under the action of two independent physical factors, gravity and temperature. In the motionless, equilibrium atmosphere partial pressure of water vapor at each height is compensated by its weight in the atmospheric column above that height. Drop of gas pressure with height leads to the rarification of the gas, which further enhances the pressure drop. In the result, the equilibrium (aerostatic) pressure of water vapor drops with height in the geometric progression — by twofold per each nine kilometers of temperature rise (shown with the blue line). On the other hand, water vapor pressure cannot exceed the saturated one. Saturated pressure arises in the contact with liquid water — at the oceanic surface, moist soil surface, surface of green leaves. Saturated (i.e. maximum) pressure drops with decreasing temperature in the geometric progression as well — by approximately twofold per each 10 oC (shown with the red line).

2. Therefore, water vapor could be in the static gravitational equilibrium above the wet surface if only air temperature decreased with height not faster than by 10 oC per each 9 km of ascent — in this case the blue line finds itself below the red line, for example, when air temperature drops by 0.5 degrees per km (temperature gradient = 0.5 K/km). With increasing temperature drop (move the model slide down!) the red line approaches the blue line from above and finally coincides with the latter at the critical value of temperature gradient equal to 1.2 degrees Ņelsius per km.

3. In the real atmosphere air temperature drops by nearly 60 oC per 9 km of ascent (temperature gradient = 6.5 K/km, move the model slide down to the utmost)! This means that the upper atmospheric layers are too cold for the weight of there residing water vapor to compensate partial pressure of water vapor in the lower, warmer atmospheric layers. The excessive water vapor immediately undergoes condensation and disappears from the gas phase. In the absence of a sufficiently large vertical gradient of air temperature, there would be neither evaporation, nor condensation, nor precipitation on Earth! The uncompensated partial pressure of water vapor at the surface (equal to the difference between the blue and red lines) exceeds the weight of water vapor in the atmospheric column by many times.

4. This results in the apperance of an upward directed force acting on moist air. This force creates fluxes of air and water vapor from the Earth's surface to the upper atmosphere, where water vapor condenses and precipitates in the form of rain or snow. The on-going disapperance of water vapor from the gas phase is compensated by evaporation from the moist surface. In those regions where evaporation is higher, the upwelling fluxes of moist air are more intense. This leads to the following most important regularity of atmospheric circulation: moist air at the surface is sucked from areas with lower evaporation to areas where evaporation is more intense. This is the essence of how the biotic pump works. Evaporation above natural, undisturbed forest canopy is always higher than evaporation from the open oceanic surface, so forest can easily draw moist air from the ocean in the amounts necessary for its normal functioning.

If you have any questions, you are welcome to ask them here.

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