Variability in the spectra over the night of 19 July 2007 for some water lines in the band at 5900 Å with the Na i D1 line on the left. The colorbar indicates the airmass. Panel a shows the non-corrected spectra. Green bands indicate the strongest telluric lines. Panel b is the variation of each non-corrected spectrum from the mean spectrum of the night. Panel c shows spectra corrected from the telluric lines using Molecfit. Paneld is the variation of each corrected spectrum from the mean spectrum.