  We present Hubble Space Telescope optical and near-IR transmission spectra of
the transiting hot-Jupiter WASP-31b. The spectrum covers 0.3-1.7 $\mu$m at a
resolution $R\sim$70, which we combine with Spitzer photometry to cover the
full-optical to IR. The spectrum is dominated by a cloud-deck with a flat
transmission spectrum which is apparent at wavelengths $>0.52\mu$m. The cloud
deck is present at high altitudes and low pressures, as it covers the majority
of the expected optical Na line and near-IR H$_2$O features. While Na I
absorption is not clearly identified, the resulting spectrum does show a very
strong potassium feature detected at the 4.2-$\sigma$ confidence level.
Broadened alkali wings are not detected, indicating pressures below $\sim$10
mbar. The lack of Na and strong K is the first indication of a sub-solar Na/K
abundance ratio in a planetary atmosphere (ln[Na/K]$=-3.3\pm2.8$), which could
potentially be explained by Na condensation on the planet's night side, or
primordial abundance variations. A strong Rayleigh scattering signature is
detected at short wavelengths, with a 4-$\sigma$ significant slope. Two
distinct aerosol size populations can explain the spectra, with a smaller
sub-micron size grain population reaching high altitudes producing a blue
Rayleigh scattering signature on top of a larger, lower-lying population
responsible for the flat cloud deck at longer wavelengths. We estimate that the
atmospheric circulation is sufficiently strong to mix micron size particles
upward to the required 1-10 mbar pressures, necessary to explain the cloud
deck. These results further confirm the importance of clouds in hot-Jupiters,
which can potentially dominate the overall spectra and may alter the abundances
of key gaseous species.
