From extensive radiative transfer calculations we find that clumpy torus models with { \cal N } _ { 0 } \sim 5–15 dusty clouds along radial equatorial rays successfully explain AGN infrared observations .
The dust has standard Galactic composition , with individual cloud optical depth \tau _ { V } \sim 30–100 at visual .
The models naturally explain the observed behavior of the 10 \mu m silicate feature , in particular the lack of deep absorption features in AGN of any type .
The weak 10 \mu m emission feature tentatively detected in type 2 QSO can be reproduced if in these sources { \cal N } _ { 0 } drops to \sim 2 or \tau _ { V } exceeds \sim 100 .
The clouds angular distribution must have a soft-edge , e.g. , Gaussian profile , the radial distribution should decrease as 1 / r or 1 / r ^ { 2 } .
Compact tori can explain all observations , in agreement with the recent interferometric evidence that the ratio of the torus outer to inner radius is perhaps as small as \sim 5–10 .
Clumpy torus models can produce nearly isotropic IR emission together with highly anisotropic obscuration , as required by observations .
In contrast with strict variants of unification schemes where the viewing-angle uniquely determines the classification of an AGN into type 1 or 2 , clumpiness implies that it is only a probabilistic effect ; a source can display type 1 properties even from directions close to the equatorial plane .
The fraction of obscured sources depends not only on the torus angular thickness but also on the cloud number { \cal N } _ { 0 } .
The observed decrease of this fraction at increasing luminosity can be explained with a decrease of either torus angular thickness or cloud number , but only the latter option explains also the possible emergence of a 10 \mu m emission feature in QSO2 .
X-ray obscuration , too , has a probabilistic nature .
Resulting from both dusty and dust-free clouds , X-ray attenuation might be dominated by the dust-free clouds , giving rise to the observed type 1 QSO that are X-ray obscured .
Observations indicate that the obscuring torus and the broad line region form a seamless distribution of clouds , with the transition between the two regimes caused by dust sublimation .
Torus clouds may have been detected in the outflow component of H _ { 2 } O maser emission from two AGN .
Proper motion measurements of the outflow masers , especially in Circinus , are a promising method for probing the morphology and kinematics of torus clouds .