Distribution of extra broadening velocities for the B-supergiants in the OB sample. The data derived by assuming different models for extra broadening are highlighted in different colors: red denotes an isotropic Gaussian model (Fraser et al. 2010), whilst green refers to a radial-tangential one (Lefever et al. 2007; Markova & Puls 2008; Simón-Díaz et al. 2010). Note the systematic shift along the vertical axes between the green and the red dots (upper panel) and that this shift disappears after the original Fraser et al. (2010) data for extra broadening have been corrected to make them consistent with the rest (lower panel). For dotted lines, see Sect. 5.1.