Effects of the Mount St. Helens Volcanic Cloud on Turbidity at Ann Arbor, Michigan

Abstract

Measurements of turbidity were made at the University of Michigan irradiance and meteorological measurement facility just prior to, during and after the passage of the volcanic cloud from the 18 May 1980 eruption of Mount St. Helens. They were made with a Volz sunphotometer at wavelengths of 500 and 880 nm. The volcanic cloud gradually moved over the measurement facility on the afternoon of 20 May and remained over it until 2300 GMT 21 May. Its effects on the turbidity coefficient and wavelength exponent in Ångström's turbidity equation are evaluated by comparing results for 20 and 22 May, both of which were cloudless days, with those on 21 May. The volcanic cloud increased the turbidity coefficient by a factor of about 8 and decreased the wavelength exponent by a factor of about 3 compared to average values for 20 and 22 May. Some uncertainty in the results is caused by cirriform clouds in addition to the volcanic cloud on 21 May. These results and effects of the volcanic cloud on recordings of global, direct normal and diffuse solar irradiances are described.