Recently a surface transition driven by the temperature in nematics has been observed and discussed [K. Flatischler et al., Mel. Cryst. Liq. Cryst. 198, 119 (1991)]. Such a transition usually appears some degrees below the clearing point : it consists of a change of alignment in the whole nematic layer, from homeotropic to planar, on heating. The transition could be second or first order, depending on surface treatment and bulk properties and is reversible : in the latter case hysteresis takes place. The importance of such a phenomenon is evident, since the temperature surface transition affects the electro-optical properties of a nematic cell. Here we present a new Landau-de Gennes model describing the surface transition which when the cell walls, covered by a thin layer or silicon monoxide, are dipped or in a very dilute solution of egg-lecithin in chloroform.