Correlations between Doppler shift of spectral features of Mg II k and the vertical velocity in the atmosphere. Panel (a): Correlation between Doppler shift of k_2v and the vertical component of the velocity at the formation height. Panel (b): Similar relation, but for k₃. Panel (c): Same as panel (a), but for k_2r. The Pearson correlation coefficient R_P is given in each panel. The blue solid lines indicate x = 0 and y = 0. The black dashed line indicates x = y. The red, green, and blue contours enclose 25%,50%, and 90% of the data.