Sunshine duration is declining in Nepal across the period from 1987 to 2010

抄録

　We analyzed temporal changes of sunshine duration (SSD) and number of rainy days (NRD) in Nepal across its three physiographic regions: plains, low-hills, and high-hills and mountains for the period from 1987 to 2010 from records at 13 meteorological stations. We found declining trends in SSD (i.e., solar dimming) across Nepal at a rate of -0.20% per year, with the highest decline occurring during the post-monsoon season (-0.33% per year), followed by the pre-monsoon season (-0.24% per year). A close look at individual stations indicated that declines in pre- and post-monsoon seasons are common regional phenomena. By region, dimming was pronounced (-0.56% per year) in the plains at 0-300 m above sea level and gradually diminished as elevation rose. The NRD for the same 13 stations showed a significant declining trend (-0.20% per year), which suggests that the change in NRD is not a major driver of the decreased SSD in Nepal. We argue that the decline in SSD might be influenced by transboundary air pollution from the Indian subcontinent and biomass burning across the region as suggested by other studies.