The free surface of the liquid sodium is a narrow rectangle surrounded by a large isothermal plate on the same level, temperature of which is fairly lower than the sodium. Numerical calculations of the convection are performed for the range of Rayleigh number up to 1.6×10⁵ The temperature fields are observed by an interferometer. The evaporation rates are measured. The analogy between Nusselt and Sherwood number is valid for a small difference of the temperature between the free surface and the surroundings. Otherwise, the sodium fog is formed in the cover gas and Sherwood number becomes larger.