Spectral ultraviolet‐B radiation fluxes at the Earth's surface: Long‐term variations at 39°N, 77°W

Abstract

Precision Measurements of UVB global irradiance in 5‐nm‐wide spectral bands centered from 290 to 320 nm were made continuously from September 1975 through December 1990. The spectral radiometer was calibrated at roughly monthly intervals. Annual maximum monthly means for total daily radiation, radiation at solar noon, and clear sky radiation at small secants all were observed to be significantly higher during the period from 1983 through 1989. The maximum monthly mean of integrated 295–320 nm total daily radiation in 1986 was 29% above the long‐term mean, 47% above the maximum in 1982, and 54% above the mean maxima for 1979–1982. These interannual changes were largest for the shorter wavelengths but were significant for all spectral bands. Interannual variations in clear sky radiation were largest at small secants. For maximum monthly means of integrated 295–320 nm flux, the secant 1.2 maximum in 1989 was 19% above the long‐term mean and the average of the maxima during 1983–1989 was 13% above the long‐term mean.