  As an exoplanet orbits its host star it reflects and emits light, forming a
distinctive phase curve. By observing this light, we can study the atmosphere
and surface of distant planets. The planets in our Solar System show a wide
range of atmospheric phenomena, with stable wind patterns, changing storms, and
evolving anomalies. Brown dwarfs also exhibit atmospheric variability. Such
temporal variability in the atmosphere of a giant exoplanet has not to date
been observed. HAT-P-7 b is an exoplanet with a known offset in the peak of its
phase curve. Here we present variations in the peak offset ranging between
-0.086+0.033-0.033 to 0.143+0.040-0.037 in phase, implying that the peak
brightness repeatedly shifts from one side of the planet's substellar point to
the other. The variability occurs on a timescale of tens to hundreds of days.
These shifts in brightness are indicative of variability in the planet's
atmosphere, and result from a changing balance of thermal emission and
reflected flux from the planet's dayside. We suggest that variation in wind
speed in the planetary atmosphere, leading to variable cloud coverage on the
dayside and a changing energy balance, is capable of explaining the observed
variation.
