Atmospheric Water and Cloud Microphysics

As discussed in Chapter 6, water forms strong hydrogen bonds and these lead to a number of important features of its atmospheric behavior. All three phases of water exist in the atmosphere, and the condensed phases can exist in equilibrium with the gas phase. The equilibria between these phases is summarized by the phase diagram for water. Fig. 7-9. 

We see from this diagram that partial pressures of H2O at ordinary conditions range from very small values to perhaps 30 or 40 mbar. This corresponds to a mass concentration range up to about 25 g H20/m. In typical clouds, relatively little of this is in the condensed phase. Liquid water contents in the wettest of cumulus clouds are around a few grams per cubic meter ordinary mid-latitude stratus clouds have 0.3-1 g/m.  

Water clouds play two key roles in biogeo-chemical cycles on the Earth  

They deliver water from the atmosphere to the Earth s surface as rain or snow, and are thus a key step in the hydrologic cycle. 

They scavenge a variety of materials (e.g. nutrients and trace elements) from the air 

They scavenge a variety of materials from the air and make them available for delivery in precipitation. 

Thus we proceed to examine the physicochemical nature of the cloud nucleation process. 

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