With the Dalton formula it is possible to calculate the evaporation rate in mm/d over surfaces of water and land areas. The evaporation rate is assumed as a function of the wind speed and the vapour saturation deficit. With increasing distance over the water, the vapour saturation deficit reduces because on the way up, water vapour is absorbed. This is a semi-empirical, aerodynamic procedure.

ET_p = f(v) × (e_s - e_a) in mm/d

f(v) is the wind function in mm/(d×hPa), for which different relations of the common form in dependence of the wind velocity v

f(v) = a + b × v^c

have been developed . For small surfaces of water (evaporation pan) e.g. Penman (1963) gives:

f(v) = 0.13 + 0.14 × v

At least in drier climates and wind velocities v₂ larger 1.2 m/s (Riou 1975) the wind function can be reduced to the form f(v) = b × v₂. [1]