We propose to use time-resolved grism observations of three spectroscopically-variable T dwarfs to test theoretical models of cloud evolution in cool brown dwarfs. The overall spectral evolution of brown dwarfs is driven by two main factors: changes in temperatures and changes in the cloud structure. Models predict that the drastic change in spectral characteristics between L and T dwarfs stems primarily from cloud dispersal, either through hole growth or thinning due to rainout. We have used the WFC3 IR grism to carry out the first successful spectroscopic variability study of brown dwarfs with intriguing results: two T2 dwarfs show evidence for a mixed thick and thin cloud structure, but no cloud holes; a T6 dwarf exhibits multiple out-of-phase variable components, with the phase lag correlating with the pressure probed at that wavelength. Do these results show a systematic pattern of cloud evolution? To further probe this complex behavior, we propose to expand coverage to a broader range of spectral types. We target 3 T dwarfs (T3.5, T4.5, T6) that are identified as spectroscopic variables from our SNAP survey. We will combine time-resolved WFC3/G141 grism data with simultaneous Spitzer/IRAC photometry to follow their variability over a substantial fraction of a rotation period. We will get unique horizontal and vertical structure information by probing several pressure levels simultaneously. We can then compare the heterogeneous atmospheres of T dwarfs at three distinct cloud dissipation stages for two objects each: the still cloudy T2s, the T4s near the end of the L/T transition, and the T6s, where silicate clouds should have disappeared but new clouds may appear. We are observing 3 Brown Dwarfs with WFC3 IR slitless spectroscopy with grism G141 to search for time variability. Each object is observed for 5 consecutive orbits. At the beginning of each orbit, a direct image is taken for wavelength calibration. 2 of the 3 objects will be observed simulateously with Spitzer and scheduling must be coordinated. 2 target changes were approved by STScI. COOL STARS Default false false 18 false false false SUPPORTED STAR BROWN DWARF T This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 J=13.8 STAR BROWN DWARF T This object was generated by the targetselector and retrieved from the SIMBAD database. SDSSP-J162414.37+002915.6 J2000 J=15.5 STAR BROWN DWARF L T Coordinates from Luhman et al. 2013, last WISE measurement, Jan 6 2011 WISE-J104915.57-531906.1 J2000 J=11.2, H=10.4 WFC3/IR Spectroscopic No No G141 WFC3/IR Spectroscopic No No G141 WFC3/IR Spectroscopic No No G141 The scheduling data was generated by CASM 21.2. CASM was run in Phase II mode. 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