The two terms on the right-hand side of Eq. (5) as a function of the distance to the surface for the flux-tubes which carry the fastest and the slowest wind flow at three different instants of the cycles (from the top to the bottom panel: t = 0, 3.30, 3.85 yr), as in Fig. 13. The colour scheme for the wind speed is the same as in Figs. 6 and 9. The continuous lines represent the first term (gravitation and inclination), and the dashed lines represent the second term (expansion). The terms switch sign at the sonic point (cf. the denominator in Eq. (5)) at a height varying between 2 and 6 R_⊙.