Evolution of signals strengths as a function of the number of principal components. Signal strengths are defined as the maximum in the cross-correlation signal-to-noise K_p-V_sys map in a 12 km s⁻¹ by 12 km s⁻¹ region centred on the expected velocity. This is done for an atmospheric model that includes all four species: H₂O, OH, Fe and Mg (black solid line) as well as separately for each species (coloured dashed lines).