Spectral (direct) solar irradiance at Pune

Abstract

Direct Solar Irradiation (DSI) measurements are made with pyrheliometers. When used with selected filters, the spectral distribution can be worked out. Measurements at Pune indicate that large scale attenuation takes place in blue-green region of solar spectrum likely due to aerosol particles injected by industrial advancement over the region. consequent rapid urbanisation and resultant changed land-use patterns. Values show evidences on the role played by moisture field on the transmission characteristics of the Pune skies. The afternoon irradiances are seen to be generally higher than the forenoon values. However, IR and yellow-orange wavelengths have higher irradiances in the forenoons during the pre-monsoon months and occasionally even in March. Over a year 774 Wm^-2 of irradiance can be expected on any cloudless occasion around noon time, the individual spectral values vary depending on the actual sky and atmospheric conditions. From the comparative study of the irradiances in the different spectral regions, inferences could be drawn on the changes that take place in the size distribution of aerosol particles during a day.