Top: relative temperature fluctuation Θ_rms (in %) as a function of the Mach number ℳ_a. The filled circles correspond to the values obtained for each 3D model (see Table 1). Theses values were computed at the photosphere (i.e. at the optical depth τ = 2/3). The red curve corresponds to the polynomial function given by Eq. (12). The two upper points that deviate most from the polynomial function correspond to the two 3D models with log   g ≤ 2, i.e. the most evolved RG 3D models of our grid. Bottom: ℳ_a as a function of the quantity z₁ given by Eq. (17). The red line corresponds to a linear scaling in z₁.