Knowledge of the spectral attenuation associated with clouds is important for accurate estimates of natural irradiance at the Earth's surface. We compare spectral measurements of visible downwelling irradiance, under varying sky conditions at Halifax, Nova Scotia, Canada, with results from a clear-sky model. The spectral effect of clouds is estimated by taking the ratio of the measurements to the modeled irradiances and removing spectrally consistent instrumental effects and errors in the model. Empirical relationships derived between the spectral cloud effect and both CF, the cloud factor (the ratio of measured to modeled irradiances at 490 nm), and f, the fraction of sky covered by cloud, were found to follow a wavelength (λ) dependence of the form a(CF or f )+b(CF or f )(λ/490)-4 in the 412--700 nm wavelength range. Both this relationship and a previously published linear relationship were found to be inadequate for describing cloudy irradiance data from the Bering Sea, indicating that the spectral effect of clouds can vary with cloud type and location. We show here that the spectral cloud effect can be mimicked by using a clear-sky model and changing the magnitude of the sky reflectivity or the spectral shape and magnitude of the ground albedo within the model. An investigation of the effects of cloud-dependent changes in irradiance spectra on calculations of bio-optical properties is also presented. Estimates of chlorophyll concentration from near-surface radiances are found to vary by up to 30%, whereas the effects on estimates of photosynthetically available and usable radiation at the sea surface are negligible. ¿ 1998 American Geophysical Union