  The dayside brightness spectrum of a highly irradiated transiting brown dwarf
KELT-1b has been shown to be challenging to explain with the current brown
dwarf atmosphere models. The spectrum has been measured from observations
spanning ten years and covering high-precision secondary eclipses and phase
curves from space in blue-visible (CHaracterising ExOPlanet Satellite, CHEOPS),
red-visible (Transiting Exoplanet Survey Satellite, TESS), and near-infrared
(Spitzer), as well as secondary eclipse observations in near-infrared from the
ground. First, the dayside of KELT-1b was observed to be brighter in the TESS
passband than expected based on earlier near-infrared phase curve observations
with Spitzer, and recently, the dayside was observed to be extremely dark in
the CHEOPS passband. While several theories have been proposed to reconcile the
discrepancy between the TESS and Spitzer bands, explaining the difference
between the largely overlapping CHEOPS and TESS bands has proven to be more
difficult.
  Here, I model the TESS photometry from Sector 17 together with the new TESS
photometry from Sector 57 and show that the discrepancies in KELT-1b's dayside
brightness spectrum are best explained by temporal variability in KELT-1b's
albedo. This variability is most likely due to changing silicate cloud coverage
on the brown dwarf's dayside, that is, weather.
