Impact of atmospheric extinction coefficient on the solar spectrum at selective wavelengths at Helwan in Egypt

Abstract

Solar irradiance alterations due to the various atmospheric conditions have received great scientific concern. These alternations can be investigated by both monochromatic extinction coefficient and atmospheric transparency. In this work, continuous measurements of solar spectral irradiation using the Spectroradiometer for six selective wavelengths (350, 368, 500, 615, 780, and 870 nm) are carried out at the Helwan station, (NRIAG) in Egypt. Twelve clear-sky days of continuous observations have been monitored under different atmospheric conditions. The results have shown that using Langley method under cloudless sky for the total atmospheric extinction at the selective wavelengths was correlated. The extinction coefficient at the selected wavelengths shows obvious diurnal variations in each season. The perceptible effect of local climate has been obtained from the comparison of seasonal extinctions. The highest value of extinction coefficient was 0.59 km−1 at 615 nm in summer while the lowest one at 350 nm was 0.07 km−1 in winter. Moreover, it has been found that the afternoon values of transparency have a shift toward the longer wavelengths.