ATMOSPHERIC TRANSPARENCY AT FLAGSTAFF, ARIZONA, 1972-1996 - BASE-LINES AND VOLCANIC EPISODES COMPARED

We present a summary of measurements of the atmospheric transparency o ver Flagstaff, Arizona, USA, made from 1972 to 1996. This period inclu des two major volcanic eruptions and two periods we believe are free f rom volcanic influence. The two eruptive periods show general similari ty in history, although some differences are noted. Comparison of the two ''clear'' periods shows some deterioration in the tranparency over Lowell Observatory on a time scale of a decade.