Increase in solar UV radiation with altitude

Since 1981, broad-band measurements have been made at Jungfraujoch, Switzerland (3576 m above sea-level (a.s.l.)) and Innsbruck, Austria (577 m a.s.l.), where daily totals of erythemal effective irradiance, UVA irradiance and total irradiance have been compared. Under clear sky conditions, the observed increases in irradiance with altitude (altitude effect) of the daily totals of global irradiance are 8% +/- 2% per 1000 m (total irradiance), 9% +/- 2% per 1000 m (UVA irradiance) and 18% +/- 2% per 1000 m (erythemal effective irradiance) during the summer. The altitude effect of the simultaneously measured erythemal effective irradiance between Innsbruck (577 m a. s.l.) and Hafelekar (2300 m a.s.l.), horizontally separated by 2.5 km, shows a slight dependence on the solar elevation: 15.1% +/- 1.8% per 1000 m at 60 degrees solar elevation and 18.6% +/- 2.9% per 1000 m at 20 degrees solar elevation. Simultaneously taken measurements of solar irradiance with high resolution spectrometers at Garrnisch-Partenkirchen, Germany (730 m a.s.l.) and Wank (1730 m a.s.l.), horizontally separated by 5 km, show a clear wavelength dependence of the altitude effect of the global irradiance: 9% per 1000 m at 370 nm increasing to 11% per 1000 m at 320 nm and 24% per 1000 m at 300 nm. The altitude effect of direct irradiance is considerably higher than that of global irradiance at all measured wavelengths. (C) Elsevier Science S.A.