Major volcanic eruptions occur every few years, but most have little effect on solar radiation or climate. However, in the last ten years two volcanoes have decreased solar radiation and influenced weather at a level that might be expected at the frequency of about once a century. The Mexican volcano El Chichon and the Philippine volcano Mount Pinatubo put 6 and 20 million metric tons of SO₂ in the stratosphere, respectively. SO₂ is converted into H₂SO₄, which mixes with water to produce aerosol. Since there is no weather in the stratosphere and the aerosol is small, these aerosol particles remain suspended until coagulation and sedimentation bring them to the troposphere where they are removed by normal wet and dry deposition processes. The extinction in the direct solar irradiance from El Chichon was found to peak during the winter of 1983 at about 11% for northern, mid latitudes. Mount Pinatubo`s peak extinction during 1992 was about 15%. Data from four northern, mid-latitude sites are examined to compare the direct consequences of the volcano`s eruption on the performance of concentrating solar energy systems and the indirect effects that may be associated with Mount Pinatubo`s perturbation of the weather.