Preprint typeset using LATEX style emulateapj v. 08/22/09 STRONG BRIGHTNESS VARIATIONS SIGNAL CLOUDY-TO-CLEAR TRANSITION OF BROWN DWARFS

We report the results of a J band search for cloud-related variability in the atmospheres of 62 L4-T9 dwarfs using the Du Pont 2.5-m telescope at Las Campanas Observatory and the Canada France Hawaii Telescope on Mauna Kea. We find 9 of 57 objects included in our final analysis to be significantly variable with>99 % confidence, 5 of which are new discoveries. In our study, strong sinusoidal signals (peak-to-peak amplitudes>2%) are confined to the L/T transition (4/16 objects with L9-T3.5 spectral types and 0/41 objects for all other spectral types). The probability that the observed occurrence rates for strong variability inside and outside the L/T transition originate from the same underlying true occurrence rate is excluded at>99.7 % confidence. Based on a careful assessment of our sensitivity to astrophysical signals, we infer that 39+16−14 % of L9-T3.5 dwarfs are strong variables on rotational timescales. If we consider only L9-T3.5 dwarfs with 0.8< J −Ks <1.5, and assume an isotropic distribution of spin axes for our targets, we find that 80+18−19 % would be strong variables if viewed edge-on; azimuthal symmetry and/or binarity may account for non-variable objects in this group. These observations suggest that the settling of condensate clouds below the photosphere in brown dwarf atmospheres does not occur in a spatially uniform manner. Rather, the formatio