The evolution of atmospheric extinction at La Silla derived from measurements in the Stromgren photometric system

We describe atmospheric extinction data obtained at the La Silla Observatory in the Stromgren photometric system. Our measurements, covering a time interval of almost fifteen years, show three episodes of increased extinction due to airborne aerosols of volcanic origin. A small-amplitude seasonal variation and a short-timescale pseudo-periodic oscillation are also revealed. Implications on the observing procedures and photometric reduction schemes are discussed.