A combination of published and archival observations from the Smithsonian Institution's Astrophysical Observatory (APO) is presented and analyzed. This paper concentrates on the data from the two primary sites—Mount Montezuma, Chile, and Table Mountain, California, from 1923 to 1957. Baseline values and their variations are presented for solar aureole brightness, precipitable water vapor, pyrheliometry, corrected pyrheliometry, and spectrobolometry. In addition to clouds and water vapor, possible causes for the observed variations in atmospheric transmission are volcanic eruptions, nuclear weapons testing, and aerosols from plants. Physical theories may not be sufficient to explain climate change, and considerations of biological processes may be required.

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