Cloud structure of brown dwarfs from spectroscopic variability observations

Recent discoveries of variable brown dwarfs have provided us with a new window into their three dimensional cloud structure. The highest variables are found at the L/T transition where the cloud cover is thought to break up but variability has been found to occur also for both cloudy L dwarfs and (mostly) cloud free mid T dwarfs. We summarize results from recent HST programs measuring the spectral variability of brown dwarfs in the near infrared and compare to results from ground based programs. We discuss the patchy cloud structure of L/T transition objects for which it is becoming increasingly certain that the variability does not arise from cloud holes into the deep hot regions but from varying cloud thickness. We present a new patchy cloud model to explain the spectral variability of 2MASSJ21392676+0220226. We also discuss the curious multi wavelength variability behavior of the recently discovered very nearby early T dwarf WISE J104915.57 531906.1B (Luhman 16B) and the mid T dwarf 2MASS J22282889 431026.