  Recent photometry of L and T dwarfs revealed that the infrared colors show a
large variation at a given Teff and, within the framework of our Unified Cloudy
Model (UCM), this result can be interpreted as due to a variation of the
critical temperature (Tcr) which is essentially a measure of the thickness of
the dust cloud. It appears that the L/T transition takes place at around Teff =
1400K, where Tcr shows a particularly large change. Thus the L/T transition is
associated with a drastic change of the thickness of the dust cloud at around
Teff = 1400K, but the reason for this change is unknown. Once we allow Tcr to
vary at given Teff and log g, the two-color and color-magnitude diagrams can be
well explained as the effect of Teff, log g, and Tcr, but not by that of Teff
and log g alone. In general, the effects of Teff and Tcr are difficult to
discriminate on individual spectra, but this degeneracy of Teff and Tcr can be
removed to some extent by the analysis of the SED on an absolute scale. The
reanalysis of a selected sample of spectra revealed that the L-T spectral
sequence may not necessarily be a sequence of Teff, but may reflect a change of
the thickness of the dust cloud, represented by Tcr in our UCM. Also, an odd
'brightening' of the absolute J magnitudes plotted against the L-T spectral
types may also be a manifestation that the L-T spectral sequence is not a
temperature sequence, since Mbol also shows a similar 'brightening'. Then, the
'J-brightening' may not be due to any atmospheric effect and hence should not
be a problem to be solved by model atmospheres including the UCMs. Thus, almost
all the available observed data are reasonably well interpreted with the UCMs
in which the cloud thickness varies, and the problem now is how to understand
why the cloud thickness (or Tcr) changes independently of Teff and log g.
