The effect of altitude upon the solar UV-B and UV-A irradiance in the tropical Chilean Andes

Abstract

Using two Grasnick filter photometers measurements of the horizontal components of solar UV irradiance between 313 and 374 nm (mainly UV-A) and between 308 and 319 nm (mainly UV-B) were performed in the tropical Chilean Andes along the Tropic of Capricorn at different altitudes and cloudless skies in January and February 1992. A linear (but solar elevation dependent) increase (decrease) with altitude of the global and direct (diffuse) components in both the UV-A and UV-B ranges has been observed for altitudes up to 5500 m above mean sea level. At solar elevations between 20° and 90° the increase of the global irradiance was about 8–10%/1000 m for UV-B and changed from 15 to 7%/1000 m for UV-A. Thus, these changes in the comparatively clear and dry atmosphere over the Andes which show Linke turbidity factors between 1.06 and 1.70 are significantly smaller than in the Alps which are typical humid mountains. The increase of the direct component with altitude changes from 60 to 20%/1000 m for solar elevations between 20° and 90°. In contrast, the diffuse UV-A and UV-B irradiation show a decrease of 2–4%/1000 m between solar elevations of about 30–90°. The direct/diffuse ratio was found to increase with solar elevation and altitude, reaching values as high as about 5 for UV-A and about 4 for UV-B.