  The spectral distribution of field white dwarfs shows a feature called the
"non-DA gap". As defined by Bergeron et al., this is a temperature range
(5100K--6100K) where relatively few non-DA stars are found, even though such
stars are abundant on either side of the gap. It is usually viewed as an
indication that a significant fraction of white dwarfs switch their atmospheric
compositions back and forth between hydrogen-rich and helium-rich as they cool.
In this paper, we present a Monte Carlo model of the Galactic disk white dwarf
population, based on the spectral evolution model of Chen and Hansen. We find
that the non-DA gap emerges naturally, even though our model only allows white
dwarf atmospheres to evolve monotonically from hydrogen-rich to helium-rich
through convective mixing. We conclude by discussing the effects of convective
mixing on the white dwarf luminosity function and the use thereof for
Cosmochronology.
