Fractional polarization Q/I (upper panels) and U/I (lower panels) profiles for the Ca I 4227 Å line calculated at μ = 0.38 in the static case (solid lines, label v₀) and in the presence of vertical bulk velocities with constant gradients (dashed lines). The velocity profiles of Fig. 2, with gradients α (label v_α) and 2α (label v_2α) are considered. The calculations are performed both for the unmagnetized case (black lines, label B₀) and including a height-independent horizontal (θ_B = π/2, χ_B = 0) magnetic field (red lines) with strength B = 10 G (label B₁) and B = 30 G (label B₂). The calculations are performed in the FAL-C atmospheric model. The reference direction for positive Stokes Q is the parallel to the limb.