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Recent photometry of L and T dwarfs revealed that the infrared colors show alarge variation at a given Teff and, within the framework of our Unified CloudyModel (UCM), this result can be interpreted as due to a variation of thecritical temperature (Tcr) which is essentially a measure of the thickness ofthe 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 isassociated with a drastic change of the thickness of the dust cloud at aroundTeff = 1400K, but the reason for this change is unknown. Once we allow Tcr tovary at given Teff and log g, the two-color and color-magnitude diagrams can bewell explained as the effect of Teff, log g, and Tcr, but not by that of Teffand log g alone. In general, the effects of Teff and Tcr are difficult todiscriminate on individual spectra, but this degeneracy of Teff and Tcr can beremoved to some extent by the analysis of the SED on an absolute scale. Thereanalysis of a selected sample of spectra revealed that the L-T spectralsequence may not necessarily be a sequence of Teff, but may reflect a change ofthe 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 spectraltypes may also be a manifestation that the L-T spectral sequence is not atemperature sequence, since Mbol also shows a similar 'brightening'. Then, the'J-brightening' may not be due to any atmospheric effect and hence should notbe a problem to be solved by model atmospheres including the UCMs. Thus, almostall the available observed data are reasonably well interpreted with the UCMsin which the cloud thickness varies, and the problem now is how to understandwhy the cloud thickness (or Tcr) changes independently of Teff and log g.Comment: 40 pages, 12 Postscript figures, to be published in The Astrophysical  Journal. A short note discussing the effect of methane opacities based on the  line list by R. Freedman is added in the revised versio

Dust in the Photospheric Environment. III. A Fundamental Element in the Characterization of Ultracool Dwarfs