  Over the last decade, precise exoplanet transmission spectroscopy has
revealed the atmospheres of dozens of exoplanets, driven largely by
observatories like the Hubble Space Telescope. One major discovery has been the
ubiquity of atmospheric aerosols, often blocking access to exoplanet chemical
inventories. Tentative trends have been identified, showing that the clarity of
planetary atmospheres may depend on equilibrium temperature. Previous work has
often grouped dissimilar planets together in order to increase the statistical
power of any trends, but it remains unclear from observed transmission spectra
whether these planets exhibit the same atmospheric physics and chemistry. We
present a re-analysis of a smaller, more physically similar sample of 15
exo-Neptune transmission spectra across a wide range of temperatures (200-1000
K). Using condensation cloud and hydrocarbon haze models, we find that the
exo-Neptune population is best described by low cloud sedimentation efficiency
($\mathrm{f_{sed}}\sim0.1$) and high metallicity ($100\times$ Solar). There is
an intrinsic scatter of $\sim0.5$ scale height, perhaps evidence of
stochasticity in these planets' formation processes. Observers should expect
significant attenuation in transmission spectra of Neptune-size exoplanets, up
to 6 scale heights for equilibrium temperatures between 500 and 800 K. With
JWST's greater wavelength sensitivity, colder (<500 K) planets should be
high-priority targets given their clearer atmospheres, and the need to
distinguish between the "super-puffs" and more typical gas-dominated planets.
