The Mexican volcano El Chichon and the Philippine volcano Mount Pinatubo injected approximately 6 and 20 million metric tons of SO2 into the stratosphere, respectively. This, subsequently, produced H2SO4/H2O aerosol of twice the initial mass of SO2. El Chichon aerosol was confined largely to the northern hemisphere, while Mount Pinatubo aerosol was more evenly divided between hemispheres. Although the Pinatubo aerosol mass was about three times that of El Chichon, it produced less than a triple enhancement in aerosol in the northern hemisphere (the relative increase was 50%) because of this more uniform global spreading. Mount Pinatubo aerosol is shown to have a 15–20% peak extinction effect on direct beam radiation at northern hemisphere sites at 32°N, 40°N, and 46°N. This provides rather convincing evidence that the effect is large at all latitudes in this range and, specifically, that the SEGS performance at 35°N was similarly affected. Volcanic perturbations as large as Mount Pinatubo may produce weather changes that could further impact energy production. Good evidence is presented that weather affected one of the two sites investigated. Based on the close correlation between El Chichon and Mount Pinatubo, we predict the Mount Pinatubo aerosol layer decay pattern over the next year and a half.