Short timescale variations observed on Dec. 11, 2008 at 5000 Å. Panel A) DIMM seeing (the dots indicate the image quality measured on the spectra). Panel B) LOSSAM rms fluctuations. Panel C) airmass. Panel D deviations from the average extinction at 5000 Å. Different symbols refer to different stars: GD108 (circles), GD50 (triangles), and BPM16274 (squares). The horizontal dashed lines trace the average deviation (middle) and  ±1 sigma levels (upper and lower).