  Transmission spectroscopy provides a window to study exoplanetary
atmospheres, but that window is fogged by clouds and hazes. Clouds and haze
introduce a degeneracy between the strength of gaseous absorption features and
planetary physical parameters such as abundances. One way to break that
degeneracy is via statistical studies. We collect all published HST/WFC3
transit spectra for 1.1-1.65 $\mu$m water vapor absorption, and perform a
statistical study on potential correlations between the water absorption
feature and planetary parameters. We fit the observed spectra with a template
calculated for each planet using the Exo-Transmit code. We express the
magnitude of the water absorption in scale heights, thereby removing the known
dependence on temperature, surface gravity, and mean molecular weight. We find
that the absorption in scale heights has a positive baseline correlation with
planetary equilibrium temperature; our hypothesis is that decreasing cloud
condensation with increasing temperature is responsible for this baseline
slope. However, the observed sample is also intrinsically degenerate in the
sense that equilibrium temperature correlates with planetary mass. We compile
the distribution of absorption in scale heights, and we find that this
distribution is closer to log-normal than Gaussian. However, we also find that
the distribution of equilibrium temperatures for the observed planets is
similarly log-normal. This indicates that the absorption values are affected by
observational bias, whereby observers have not yet targeted a sufficient sample
of the hottest planets.
