*w*= (0.007 + 0.005

*v*)(

*x*

_{s}−

*x*),

where

*w*is the evaporation rate in grams per second for each square meter of water surface (g/m

^{2}s),

*v*is the velocity of the air in meter per second (m/s),

*x*

_{s}is the saturation concentration of water vapor in grams of water per kilogram of dry air (g/kg) for air at the same temperature as the water, and

*x*is the actual concentration of water vapor in grams per kilogram of dry air (g/kg) in the air above the water.

In our simulation, we know the temperature of the surface water, and when we simulate a planet with a water surface using CC8, we find that the surface gas cells are within a few degrees of the temperature of the surface water. Our previous work on impetus for circulation suggests that the velocity of our gas cells is of order 4 m/s. When we implement evaporation, we will keep track of the water vapor concentration in each cell, so we will know

*x*. What remains for us to determine is

*x*

_{s}, the saturation concentration of water vapor in air at the surface temperature.

The following graph shows measured values of saturation concentration in g/kg for a range of temperatures in Kelvin, based upon data we found here. To see the same plot in Centigrade, see here.

Also plotted on the graph is a parabolic approximation to the measured data, which is based upon two reference points: 0 g/kg at 250 K and 45 g/kg at 310 K. This approximation is good enough for our purposes, and will simplify our program. Thus our evaporation equation becomes:

*w*= [(

*T*−250)

^{2}/80 −

*x*] / 40

For example, if we have dry air over a lake at 290 K (14°C), water will evaporate at 0.5 g/m

^{2}s. In one hour, 1.8 kg of water will evaporate from each square meter. Our gas cells have mass 330 kg/m

^{2}, so after an hour over the lake, the gas will acquire water vapor concentration 5 g/kg, which is well below the saturation concentration of 20 g/kg given by our approximation. Its relative humidity will be 25%.

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