Relative importance of the variables’ air pressure, air temperature, seeing, and zenith angle. In the left column, seeing is independent of wavelength, whilst the right column shows the results for wavelength-dependent seeing. The varying parameter is indicated on the right hand side. The other three parameters in each panel are set at mean values using non-repeated MJD settings in Table 2. The arrows in each panel indicate the direction in which the correction function moves as the parameter value increases. Line thickness also increases with the increase in the parameter value. See Sect. 2.5 for details.