Information on column ozone and the earth's reflectivity obtained from satellite-based data allows estimates of the long-term behavior in erythemal irradiance at any location, including the attenuation provided by clouds. Year-to-year changes in cloudiness over specific geographic regions make a major contribution to interannual variability in irradiance at the ground. Although the general decline in column ozone amounts acted to increase erythemal irradiance over the period November 1978 to April 1993, these changes tend to be obscured by the erratic variability associated with cloudiness. The noise introduced into a time series of erythemal irradiance by clouds effectively widens the error bars assigned to derived trends. This behavior could complicate attempts to establish links between any observed changes in the biosphere and measured changes in the ozone layer.