  We show that the two groups of ultracool dwarfs classified as L and T types
can be understood as a temperature effect in a unique sequence of the model
photospheres in which dust always exists but only in the restricted region deep
in the photosphere. Under the high density of the photospheres of cool dwarfs,
dust forms easily at the condensation temperature T_cond (about 2000 K), but
the dust will soon grow larger than the critical radius r_cr at the critical
temperature T_cr (about 1800 K). The large dust grains with r > r_cr will soon
segregate from the gas and precipitate below the photosphere. For this reason,
dust exists only in the limited region of T_cr < T < T_cond in the photo-
sphere which we referred to as an active dust zone, and this means that a cloud
layer is formed deep in the photosphere rather than in the cooler surface
region. With the simple assumption that only small dust grains (r < r_cr) can
be sustained in the photosphere, a self- consistent non-grey model photosphere
in radiative-convective equilibrium can be developed without any other ad-hoc
assumption. We show that the spectra and colours of L (the cloud layer appears
in the optically thin region) and T (the dust cloud is situated too deep to be
observable) dwarfs can consistently be reproduced by a single grid of our
unified model photospheres.
