  Because the opacity of clouds in substellar mass object (SMO) atmospheres
depends on the composition and distribution of particle sizes within the cloud,
a credible cloud model is essential for accurately modeling SMO spectra and
colors. We present a one--dimensional model of cloud particle formation and
subsequent growth based on a consideration of basic cloud microphysics. We
apply this microphysical cloud model to a set of synthetic brown dwarf
atmospheres spanning a broad range of surface gravities and effective
temperatures (g_surf = 1.78 * 10^3 -- 3 * 10^5 cm/s^2 and T_eff = 600 -- 1600
K) to obtain plausible particle sizes for several abundant species (Fe,
Mg2SiO4, and Ca2Al2SiO7). At the base of the clouds, where the particles are
largest, the particle sizes thus computed range from ~5 microns to over 300
microns in radius over the full range of atmospheric conditions considered. We
show that average particle sizes decrease significantly with increasing brown
dwarf surface gravity. We also find that brown dwarfs with higher effective
temperatures have characteristically larger cloud particles than those with
lower effective temperatures. We therefore conclude that it is unrealistic when
modeling SMO spectra to apply a single particle size distribution to the entire
class of objects.
