Zenith atmospheric opacity inferred by skydip as a function of the one derived by the ATM model from GNSS PWV data. The opacity values are integrated over the KISS bandwidth. The slope and the intercept of the linear regression are given above the figure. The error bars for the skydip are calculated on the statistics of the pixels, while the ones from the PWV method consider a 10% conservative incertitude on the central value. In the red shaded region, we report the 1-σ bounds of the slope, fixing the intercept at its central value.