Evaporation Processes in the Atmosphere

2°C (90°FJ the saturation concentration for the free stream is obtained from the steam tables  

We have already described some evaporation processes and indicated relations between heat and mass transfer. In the atmosphere, the continuous evaporation and condensation of water from the soil, oceans, and lakes influences every form of life and provides many of the day-to-day varieties of climate that govern the environment on earth. These processes are very complicated because in practice they are governed by substantial atmospheric convection currents that are difficult to describe analytically. 

Let us first consider the diffusion of water vapor from a horizontal surface into quiescent air as indicated in Fig. 11-5. At the surface, the partial pressure of the vapor is ps. The vapor pressure steadily drops with a rise in elevation z to the free atmosphere value of px. The molecular diffusion of the water vapor may be written in the form of Eq. (11-13) as 

Evaporation processes in the atmosphere are much more complicated than indicated by the simple form of Eq. (11-35) for two reasons  

The diffusion process involves substantial turbulent eddy motion so that the diffusion coefficient Dw may vary significantly with the height z. 

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