Dispersions σ_U8 (red lines), σ_V8 (blue lines) and σ_W8 (black lines) in U, V and W velocity components, respectively, calculated for stars around R = 8 kpc in a BGS simulation (i.e. with bar, GMCs and spiral arms included), full lines, and when crossing the Galactic plane (for σ_W). Also plotted are the stellar observations of Holmberg et al. (2009) as dots (U: full, V: open) and asterisks (W), with a colour coding corresponding to that used for the models.