The light-scattering coefficients of aerosols as a function of relative humidity (RH) and wavelength were measured in flights off the mid-Atlantic coast of the United States during July 1996. At a wavelength of 550 nm the ratio of the total light-scattering coefficient at RH=80% to that at RH=30% in westerly airflows had a mean value of 2.30 with a standard deviation of 0.24; in northerly and southerly airflows the mean value was 1.81 with a standard deviation of 0.37. This ratio generally decreased with increasing altitude and with increasing mass ratio of aerosol carbon to sulfate, and it increased with increasing wavelength. The aerosol hemispheric backscatter ratio decreased by 30 to 40% as the RH increased from 30 to 80%. Direct radiative forcing by aerosols advected off the mid-Atlantic coast of the United States is strongly dependent on RH. At a RH of 80%, direct radiative forcing by the aerosol is roughly twice that of the dry aerosol. ¿ 1999 American Geophysical Union