Measurements of spectral ultraviolet-B irradiance under optically thick clouds show strongly enhanced attenuation by molecular and particulate absorbers. The atmospheric photon path is enhanced owing to the presence of a highly scattering medium, leading to an amplification of absorption by tropospheric ozone and aerosol. Calculations with discrete ordinate and Monte Carlo models show that photon paths in realistic water clouds may be enhanced by factors of 10 and more compared to cloudless sky. Model calculations further show that UV spectra measured under thick clouds can be well simulated within 10%, indicating that the involved processes are quantitatively described by current models. These findings are of important consequence for all ground-based remote sensing applications which take advantage of measuring scattered radiation in order to infer atmospheric trace gas abundances. These algorithms, for example, the calculation of total ozone from global irradiance or zenith sky radiance, are subject to large errors, when neglecting the influence of cloud scattering on the derived data. In the present study, errors of more than 300 Dobson Units (DU) have been found, if such methods were applied without care in the presence of thick clouds. ¿ 1998 American Geophysical Union