The isotopic fractionations in the precipitate are equilibrium fractionations and the evolution of a system can be described using Rayleigh destillation! The fractionation type during evaporation is mostly kinetic and the height of the isotope fractionation depends on the vapour pressure and the humidity of the air! But this process can also be also described with the Rayleigh destilation if you assume that the humidity is 0%, right? However, if the humidity is 100%, there is probably an equilibrium between water and water vapour, but wouldn't this then be an equilibrium isotope effect? or isn't it, since the vapour pressure first has to be overcome.