Long-term trends in, and the spatial variation of, global irradiance in Israel

An analysis of 130 station-years of global irradiance measurements made in Israel during the last 40 years showed a significant decrease of 20%. In absolute terms, the reduction, averaging — 0.88 W m^-2 per year, was the same at Bet Dagan in the central coastal plain as at Jerusalem in the central hill region. The decline in irradiance was greater in winter than in summer, in the northern Jordan Rift Valley than in the southern Negev and under overcast compared with clear sky conditions. The cause of the decline could not be attributed to an increase in cloud cover, as analysis of 4 long-term series of daytime observations showed that small reductions in cloud cover had occurred. Possible reasons for the reduction in atmospheric transmissivity are discussed and the increase in emissions of anthropogenic aerosols within Israel and from the upwind regions of SE Europe is considered the most probable.