In this paper we demonstrate that a small transverse temperature gradient of a few mK/cm induces the nucleation of a wetting layer at the liquid-vapor surface of the critical binary liquid mixture hexadecane+acetone. The nucleation phenomenon is well described by a classical surface nucleation theory [B. M. Law, Phys. Rev. Lett. 72, 1698 (1994)] where far from the critical temperature the height of the energy barrier is independent of the magnitude of the temperature gradient while the characteristic nucleation time A=1/(attempt frequency) is approximately inversely proportional to the transverse temperature gradient which induces laminar fluid flow.