We report experimental observations of a horizontally vibrated granular medium made out of sand grains. For large enough acceleration, two counter-rotating convection rolls appear in the upper part of the granular packing, whereas the bed remains unperturbed in the lower part. For increasing acceleration, the free surface exhibits different shapes: slight dome, two ridges and single roof. A quantitative characterization is performed by mean of PIV and image analysis. It shows that both relaxed and dynamic densities present a maximum at Γ = 3⋅8 and that the measured thickness of the fluidized region scales with a square root law (as a function of the dimensionless acceleration). In the discussion, a convection mechanism is proposed, as well as an analogy with classical Newtonian fluids.