  We present a method to calculate vertical profiles of particle size
distributions in condensation clouds of giant planets and brown dwarfs. The
method assumes a balance between turbulent diffusion and sedimentation in
horizontally uniform cloud decks. Calculations for the Jovian ammonia cloud are
compared with results from previous methods. An adjustable parameter describing
the efficiency of sedimentation allows the new model to span the range of
predictions made by previous models. Calculations for the Jovian ammonia cloud
are consistent with observations. Example calculations are provided for water,
silicate, and iron clouds on brown dwarfs and on a cool extrasolar giant
planet. We find that precipitating cloud decks naturally account for the
characteristic trends seen in the spectra and colors of L- and T-type ultracool
dwarfs.
