Variability in the Si iv λ1394–λ1403 doublet of λ Cep in October 1994 showing the cyclic appearance of DACs. Vertical dotted lines denote the rest wavelengths. Lower panel: Grayscale representation of quotient spectra in the panel above it, obtained with a template spectrum constructed from the spectra in the top panel (flux units in 10^-10 erg cm^-2 s^-1 Å^-1) using the method by Kaper et al. (1999). The template represents the undisturbed, underlying wind profile, which makes the DACs appear as absorption features. The second panel from the top gives the temporal variance spectrum (TVS^1/2) expressed as the ratio of the observed to the expected standard deviation (see Sect. 4).