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Currently, 19 transiting exoplanets have published transmission spectraobtained with the Hubble/WFC3 G141 near-IR grism. Using this sample, we haveundertaken a uniform analysis incorporating measurement-error debiasing of thespectral modulation due to H$_{2}$O, measured in terms of the estimatedatmospheric scale height, ${H_s}$. For those planets with a reported H$_{2}$Odetection (10 out of 19), the spectral modulation due to H$_{2}$O ranges from0.9 to 2.9~${H_s}$ with a mean value of 1.8~$\pm$~0.5~${H_s}$. This spectralmodulation is significantly less than predicted for clear atmospheres. For thegroup of planets in which H$_{2}$O has been detected, we find the individualspectra can be coherently averaged to produce a characteristic spectrum inwhich the shape, together with the spectral modulation of the sample, areconsistent with a range of H$_{2}$O mixing ratios and cloud-top pressures, witha minimum H$_{2}$O mixing ratio of 17~$^{+12}_{-6}$~ppm corresponding to thecloud-free case. Using this lower limit, we show that clouds or aerosols mustblock at least half of the atmospheric column that would otherwise be sampledby transmission spectroscopy in the case of a cloud-free atmosphere. Weconclude that terminator-region clouds, with sufficient opacity to be opaque inslant-viewing geometry, are common in hot Jupiters.

A Characteristic Transmission Spectrum dominated by H$_{2}$O applies to the majority of HST/WFC3 exoplanet observations