Technical note: An assessment of the relative contribution of the Soret effect to open water evaporation

It is standard practice to assume that evaporation depends on the gradient in water vapor concentration as per Fick’s law. However, Fick’s law is only true in an isothermal system. In general, we anticipate an additional mass flux due to the temperature gradient (in a non-isothermal system) and this is known as Soret diffusion or the Soret effect. Here we evaluate the relative magnitude of the Soret effect and find that under typical environmental conditions it is at least two orders of magnitude smaller than classical concentration-dependent mass (‘Fickian’) diffusion. This result justifies the standard practice of ignoring the effect of the temperature gradient by assuming evaporation follows the gradient in water vapor concentration.