Earth scientists assume that evaporation depends on the gradient in water vapour concentration (Fick’s law), but this is only true in an isothermal system. Temperature gradients can impact evaporation via the Soret effect. Here we evaluate the relative magnitude of the Soret effect and find that it is at least 2 orders of magnitude smaller than classical concentration-dependent mass (“Fickian”) diffusion. This result justifies the standard practice of assuming evaporation follows Fick’s law.