  We present $Spitzer$/IRAC Ch1 and Ch2 monitoring of six brown dwarfs during 8
different epochs over the course of 20 months. For four brown dwarfs, we also
obtained simulataneous $HST$/WFC3 G141 Grism spectra during two epochs and
derived light curves in five narrow-band filters. Probing different pressure
levels in the atmospheres, the multi-wavelength light curves of our six targets
all exhibit variations, and the shape of the light curves evolves over the
timescale of a rotation period, ranging from 1.4 h to 13 h. We compare the
shapes of the light curves and estimate the phase shifts between the light
curves observed at different wavelengths by comparing the phase of the primary
Fourier components. We use state-of-the-art atmosphere models to determine the
flux contribution of different pressure layers to the observed flux in each
filter. We find that the light curves that probe higher pressures are similar
and in phase, but are offset and often different from the light curves that
probe lower pressures. The phase differences between the two groups of light
curves suggest that the modulations seen at lower and higher pressures may be
introduced by different cloud layers.
