  NLTE radiative transfer calculations of differentially expanding supernovae
atmospheres are computationally intensive and are almost universally performed
in time-independent snapshot mode. The validity of the steady-state
approximation in the rate equations has recently been questioned. We calculate
the effective recombination time of hydrogen in SN II using our general purpose
model atmosphere code PHOENIX. While we find that the recombination time for
the conditions of SNe II at early times is increased over the classical value
for the case of a simple hydrogen model atom with energy levels corresponding
to just the first 2 principle quantum numbers, the classical value of the
recombination time is recovered in the case of a multi-level hydrogen atom. We
also find that the recombination time at most optical depths is smaller in the
case of a multi-level atom than for a simple two-level hydrogen atom. We find
that time dependence in the rate equations is important in the early epochs of
a supernova's lifetime. The changes due to the time dependent rate equation (at
constant input luminosity) are manifested in physical parameters such as the
level populations which directly affects the spectra. The H-alpha profile is
affected by the time dependent rate equations at early times. At later times
time dependence does not significantly modify the level populations and
therefore the H-alpha profile is roughly independent of whether the
steady-state or time-dependent approach is used.
