Using the pyrheliometric measurements of the Smithsonian Astrophysical Observatory (APO), the relative atmospheric transmission is calculated at four locations during the period 1923--1957. Decreases in atmospheric transmission caused by three major volcanic eruptions are evident in the Mt. Montezuma, Chile, records. For those volcanic eruptions in the tropics the transmission also decreases at Table Mountain, California. Each volcanic eruption is widely separated from the others in time and easily distinguishable in the radiation records. Both stations appear to suffer from local anthropogenic aerosol pollution near the ends of their records. It is concluded that the post 1940 cooling is not likely to be caused by increased volcanic activity. The annual cycle in atmospheric transmission is caused by variations in total precipitable water, so the climatology of total precipitable water is also investigated. Although an annual cycle in total precipitable water and aerosol loading is clearly evident, no long-term trend in water vapor occurs at the APO sites. The large temporal and spatial variability of water vapor causes adjacent years to differ.