We present updated atmospheric parameters for all DA, DB, and DO white dwarfs in the complete sample of the Palomar-Green survey. In particular, we perform a spectroscopic analysis of the DO stars using our new set of non-LTE model atmosphere calculations. We then compute an improved luminosity function of the Palomar-Green sample, including the DO stars for the first time, which allows us to study the variations in the numbers of hydrogen- and helium-atmosphere white dwarfs along a large part of the cooling sequence. We demonstrate that the so-called float-up model successfully explains the spectral evolution of the majority of white dwarfs.