RASCAS simulations of the transfer of Lyman-α photons (x_in = 0) emitted at the centre of a uniform sphere at T = 10² K. Top: variation of the emergent spectrum as a function of τ_H I for a static sphere. The solid coloured lines are the results from RASCAS, while the black dashed curves are the analytic solutions of Dijkstra et al. (2006). Bottom: non-static sphere with a H I column density of 2 × 10²¹ cm⁻² in which the gas velocity increases linearly with radius from 0 at the centre to V_max at the edge of the outflow. The spectra predicted by RASCAS (curves) are compared to Laursen et al. (2009b), who did the same experiments with their code (crosses).