  We present a comprehensive analysis of the Hubble Space Telescope
observations of the atmosphere of WASP-121 b, a ultra-hot Jupiter. After
reducing the transit, eclipse, and phase-curve observations with a uniform
methodology and addressing the biases from instrument systematics,
sophisticated atmospheric retrievals are used to extract robust constraints on
the thermal structure, chemistry, and cloud properties of the atmosphere. Our
analysis shows that the observations are consistent with a strong thermal
inversion beginning at ~0.1 bar on the dayside, solar to subsolar metallicity Z
(i.e., -0.77 < log(Z) < 0.05), and super-solar C/O ratio (i.e., 0.59 < C/O <
0.87). More importantly, utilizing the high signal-to-noise ratio and repeated
observations of the planet, we identify the following unambiguous time-varying
signals in the data: i) a shift of the putative hotspot offset between the two
phase-curves and ii) varying spectral signatures in the transits and eclipses.
By simulating the global dynamics of WASP-121 b atmosphere at high-resolution,
we show that the identified signals are consistent with quasi-periodic weather
patterns, hence atmospheric variability, with signatures at the level probed by
the observations (~5% to ~10%) that change on a timescale of ~5 planet days; in
the simulations, the weather patterns arise from the formation and movement of
storms and fronts, causing hot (as well as cold) patches of atmosphere to
deform, separate, and mix in time.
