ATM model fit to the skydip data obtained with APEX on December 6 2020 using the SEPIA660 receiver. Only three airmasses (1.0, 1.5, and 2.0) are plotted here for clarity (scans at 1.25 and 1.75 air-mass were also taken). Since the observations took over three hours to complete, the PWVC changed significantly and it was necessary to fit an average value plus a slope for it, with a great improvement in the agreement.