  Secondary eclipse observations of several of the hottest hot Jupiters show
featureless, blackbody-like spectra or molecular emission features, which are
consistent with thermal inversions being present in those atmospheres. Theory
predicts a transition between warmer atmospheres with thermal inversions and
cooler atmospheres without inversions, but the exact transition point is
unknown. In order to further investigate this issue, we observed two secondary
eclipses of the hot Jupiter HAT-P-7b with the Hubble Space Telescope (HST) WFC3
instrument and combined these data with previous Spitzer and Kepler secondary
eclipse observations. The HST and Spitzer data can be well fit by a blackbody
with $T=2692 \pm 14$ K, and the Kepler data point constrains the geometric
albedo to $A_{g}=0.077 \pm 0.006$. We modeled these data with a 3D GCM and 1D
self-consistent forward models. The 1D models indicate that the atmosphere has
a thermal inversion, weak heat redistribution, and water dissociation that
limits the range of pressures probed. This result suggests that WFC3
observations of HAT-P-7b and possibly some other ultra-hot Jupiters appear
blackbody-like because they probe a region near the tropopause where the
atmospheric temperature changes slowly with pressure. Additionally, the 1D
models constrain the atmospheric metallicity
($[\text{M/H}]=-0.87^{+0.38}_{-0.34}$) and the carbon-to-oxygen ratio (C/O $<1$
at 99 % confidence). The solar composition 3D GCM matches the Spitzer data but
generally underpredicts the flux in the WFC3 bandpass and cannot reproduce its
featureless shape. This discrepancy could be explained by high atmospheric drag
or nightside clouds, and may be better understood through further observation
with the James Webb Space Telescope (JWST).
