Twenty years of PWV study in Chajnantor Plateau. Monthly quartiles (25%, 50%, and 75%) of PWV measurements over the full period (upper panel). On the plateau, the best observing conditions are shown over June, July, and August. Cumulative distribution fraction of PWV for Chajnantor Plateau is shown in the lower panel. Total fraction, along with the extreme cases of January and August, are included to limit the typical range of PWV for the remaining months.