TEMPORAL CHANGES IN SURFACE ULTRAVIOLET-RADIATION - A STUDY OF THE ROBERTSON-BERGER METER AND DOBSON DATA RECORDS

Ultraviolet radiation data sets obtained by Robertson-Berger meters located at Bismarck, ND and Tallahassee, FL show variations over the time period 1974-1985 which we interpret in terms of clouds and ozone. Cloudiness is a major source of variance in the irradiance measurements. When this variance is minimized, the monthly mean Robertson-Berger meter record contains trends which are in good agreement with irradiance calculations based on the Dobson ozone measurements in spring, summer and early autumn. Despite the agreement among trends, predictions based on the ozone data explain 40% or less of the variance in the monthly mean radiation values over the 11-year period. The radiation measurements contain negative trends in winter which are contrary to expectations based on the behavior of ozone alone. These trends remain when we minimize the effects of cloudiness. Based on the information available in this study, it is not possible to determine whether the wintertime trends have an instrumental or environmental origin.