We present a summary of measurements of the atmospheric transparency over Flagstaff, Arizona, USA, made from 1972 to 1996. This period includes two major volcanic eruptions and two periods we believe are free from volcanic influence. The two eruptive periods show general similarity in history, although some differences are noted. Comparison of the two ''clear'' periods shows some deterioration in the transparency over Lowell Observatory on a time scale of a decade. ¿ American Geophysical Union 1996