  Currently, 19 transiting exoplanets have published transmission spectra
obtained with the Hubble/WFC3 G141 near-IR grism. Using this sample, we have
undertaken a uniform analysis incorporating measurement-error debiasing of the
spectral modulation due to H$_{2}$O, measured in terms of the estimated
atmospheric scale height, ${H_s}$. For those planets with a reported H$_{2}$O
detection (10 out of 19), the spectral modulation due to H$_{2}$O ranges from
0.9 to 2.9~${H_s}$ with a mean value of 1.8~$\pm$~0.5~${H_s}$. This spectral
modulation is significantly less than predicted for clear atmospheres. For the
group of planets in which H$_{2}$O has been detected, we find the individual
spectra can be coherently averaged to produce a characteristic spectrum in
which the shape, together with the spectral modulation of the sample, are
consistent with a range of H$_{2}$O mixing ratios and cloud-top pressures, with
a minimum H$_{2}$O mixing ratio of 17~$^{+12}_{-6}$~ppm corresponding to the
cloud-free case. Using this lower limit, we show that clouds or aerosols must
block at least half of the atmospheric column that would otherwise be sampled
by transmission spectroscopy in the case of a cloud-free atmosphere. We
conclude that terminator-region clouds, with sufficient opacity to be opaque in
slant-viewing geometry, are common in hot Jupiters.
