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 absence (solid lines) and in the presence (dashed lines) of a height-independent vertical bulk velocity of −10 km s⁻¹. The profiles are obtained both neglecting (black lines) and taking into account (red lines) the impact of a height-independent horizontal magnetic field (θ_B = π/2, χ_B = 0) with strength B = 10 G (left column) and B = 30 G (right column). The calculations are performed in the FAL-C atmospheric model. The reference direction for positive Stokes Q is the parallel to the limb.