Abstract. Solar ultraviolet-B (280-320 nm) spectral irradiance and total shortwave radiation were measured along an arctic-alpine life zone gradient from the Arctic (70°N) at sea level to equatorial latitudes at high elevations above sea level. This series of measurements portrays the radiant flux to be anticipated during periods of maximum annual solar radiation. Along this arctic-alpine gradient, maximum daily total shortwave irradiance varies only by a factor of 1.6 and total daily shortwave radiation by less than 15% for cloudless conditions. In contrast, the maximum integrated effective UV-B irradiance can vary by a full order of magnitude and by seven-fold for total daily effective radiation. The steep UV-B radiation gradient is the result of a natural latitudinal gradient in total atmospheric ozone column thickness, prevailing solar angles at different latitudes, elevation above sea level, and an optical amplification effect, which results from a combination of highly wavelength-dependent radiation attenuation in the atmosphere and the pronounced wavelength dependence of biological action spectra. Forty to 70% of the solar UV-B flux is in the form of scattered radiation. Because of the large proportion of diffuse UV-B radiation, steeply inclined foliage would still receive a sizable fraction of the global UV-B irradiance in most situations. Snow cover can add substantially to the radiation load.

This research was supported by grants from the National Science Foundation (DEB-7622381) and the National Aeronautics and Space Administration (NAS-9-14871).