Ground based comparison of solar UV index in Kathmandu, Pokhara and Biratnagar

Abstract

This paper presents the ground based comparison of solar UV index at three major cities in Nepal (Kathmandu, Pokhara and Biratnagar). Kathmandu (27.72° N, 85.32° E), Pokhara (28.22° N, 83.22° E) and Biratnagar (26.45° N, 87.27° E) are located at an elevation of 1350 m, 800 m and 72 m respectively from the sea level. This comparison was performed from January to December 2009 for Kathmandu (KTM) and Pokhara (PKR) and from February to December 2009 for Biratnagar (BRT). The measurement instruments used are the NILU-UV irradiance meter and the Moderate Resolution Imaging Spectroradiometer (MODIS). The NILU-UV irradiance meter records the UV radiation whereas the MODIS estimates AOD. During this study, the monthly mean UV Index (UVI) in July were found to be 7.5 ± 2.5 in KTM, 8.6 ± 3.5 in PKR and 7.2 ± 3.1 in BRT. Meanwhile, the hourly mean UVI during monsoon at 7UT was found to be 5.7 ± 2.5 in KTM, 6.4 ± 2.6 in PKR and 5.8 ± 2.5 in BRT. The highest values of these hourly mean UVI was found at or around noon. The UVI values between KTM and PKR was slightly more correlated with its correlation coefficient being 0.73 than that between KTM and BRT whose correlation coefficient was 0.65. Similarly, the mean bias difference for KTM, BRT and KTM, PKR was found to be 4.8% and −38.1% respectively. This higher and lower mean bias difference indicates large and small deviation in UV data. The study also showed that besides solar elevation, the fluctuation in UVI was also influenced by the variation in ozone, aerosols and clouds. Furthermore, a year long comparison of UVI indicated that the UVI in Pokhara was higher than that in Kathmandu and Biratnagar. This analysis also showed that the extreme values of UVI were not found during the study period.