Cumulative column density as a function of the disk height integrating from the surface to the midplane in a disk radial cut (r = 300 AU). Only T Tauri models with f_large = 0.99 are shown. Different line colors represent models with various disk masses. Dotted lines show the scaled total gas cumulative column density (see text), solid lines that of C¹⁸O. The black lines show where the dust temperature is lower than 20 K for the different disk models, i.e., where CO freeze-out becomes important.