Erythemally weighted UV variations at two high‐altitude locations

A field campaign, with the purpose of measuring the rate of integrated erythemally weighted ultraviolet radiation (UVR) with altitude, was carried out at two high‐altitude locations near La Paz, Bolivia. The horizontal distance between the two sites is 25 km and they differ in height by 2 km. Irradiance data were obtained only on clear days and were analyzed using three different methods: (1) by comparison of integrated erythemal solar UV irradiances (daily doses) at both locations; (2) by calculation of the slope of the plot of the irradiance at the higher location against the irradiance at the lower location at the same time; and (3) by the asymptotic ratio, based on Beer's law, of the irradiances. The results show that erythemally weighted UV irradiance increases with altitude at an approximate rate of 7% per kilometer. The experimental data were compared with the output of a well‐known radiative model for local conditions. A sensitivity study of the nonmeasured input parameters indicated that the modeled rates of increase are slightly lower than the measured ones. Even more, these rates are not very sensitive to reasonable variations of the unknown parameters.