ZZ Ceti stars are variable DA white dwarfs (WDs) with effective temperatures . Their amplitudes range from to . The pulsations are non-radial g modes with periods of the order of . A well-known property of g modes is that radius variations are negligible because of the high surface gravity ( ) of WDs (Robinson et al. 1982; hereafter RKN). Therefore, one can think of these oscillations as temperature waves.
Recently, Brassard et al. (1995; hereafter BFW) calculated spectra semi-analytically for temperature distributions across the surface expressed as where is a sum of spherical harmonics. The aim of this analysis was to investigate non-linear effects that occur since the specific intensity I is not a linear function of . We have developed a program that calculates the time-variable spectrum by integrating I over the visible disk of the WD for a distribution of across the WD surface. For now, we use for an expression similar to that of BFW. However, we can adapt our program to non-linear variations of , which were suggested by Brickhill (1992) and could be caused by the response of the convective flux to linear pressure perturbations in deeper layers of these stars. We interpolate I from a grid containing I as a function of , , , and different parametrizations of the mixing length theory (MLT). The grid was calculated with a stellar atmospheres code.