One of the mechanisms suggested for the L to T dwarf spectral type transition
is the appearance of relatively cloud-free regions across the disk of brown
dwarfs as they cool. The existence of partly cloudy regions has been supported
by evidence for variability in dwarfs in the late L to early T spectral range,
but no self-consistent atmosphere models of such partly cloudy objects have yet
been constructed. Here we present a new approach for consistently modeling
partly cloudy brown dwarfs and giant planets. We find that even a small
fraction of cloud holes dramatically alter the atmospheric thermal profile,
spectra, and photometric colors of a given object. With decreasing cloudiness
objects briskly become bluer in J - K and brighten in J band, as is observed at
the L/T transition. Model spectra of partly cloudy objects are similar to our
models with globally homogenous, but thinner, clouds. Hence spectra alone may
not be sufficient to distinguish partial cloudiness although variability and
polarization measurements are potential observational signatures. Finally we
note that partial cloud cover may be an alternative explanation for the blue L
dwarfs.Comment: 14 pages, 3 figures, Ap. J. Let. in pres
