Evaluation of Clearness and Diffuse Index at a Semi-Arid Station (Anantapur) using Estimated Global and Diffuse Solar Radiation

In the present study, collocated and simultaneous measurements on meteorological parameters and solar radiation on horizontal surface were obtained from different meteorological sensors and pyranometer sensor over a semi-arid station, Anantapur (14.62° N, 77.65° E and 331 m asl) during January–December 2013. The temperature is usually lower at midnight, decreasing in the early hours of the morning around 08:00 LT (Local Time) (24.71 ± 2.7 °C), and then increasing rapidly until just after midday at 15:00 LT (31.88 ± 3.2 °C). It decreases to 26.70 ± 2.9 °C during the night around 23:00 LT. The relative humidity ranges from 30–76%, 20–65%, 46–80% and 48–90% during winter, summer, monsoon, and post-monsoon seasons respectively. The diurnal variation of soil moisture exhibits slight variations at different depths over the region. The diurnal and monthly variations of solar radiation as well as clearness index (k t ), diffusion index (k d ) were studied in the present study. In diurnal variation, it shows a steady rise in solar radiation received at the surface after 7:30 LT and attains a maximum solar radiation between 12:00 – 13:00 LT. The amount of solar radiation also varies depending on the time of day and the season. The annual mean of daily global, extraterrestrial and diffuse radiation at Anantapur is found to be 202.43 ± 40.45, 408.15 ± 61.63 and 49.33 ± 11.26 W/m 2 . The high global and diffuse solar radiations were observed during March-May (257.67 ± 34.18 W/m 2 , 65.07 ± 11.20 W/m 2 ) whereas low global and diffuse solar radiations during monsoon months (137.66 ± 12.41, 33.47 ± 7.44 W/m 2 ). In the case of extraterrestrial solar radiation was high in the month of December (472.92 ± 0.67 W/m 2 ) and low in the month of June (312.87 ± 2.58 W/m 2 ).