Observations and estimations of PAR and solar visible radiation in North China

Integrated observations were made of solar irradiance (including photosynthetically active radiation (PAR), visible radiation (VIS), global radiation (G), etc.), and meteorological parameters at 4 stations in North China between September, 2004 and October, 2006. The ratios of hourly sum of PAR to G (H _PAR/H), VIS to G (H _VIS/H), and PAR to VIS (H _PAR/H _VIS) varied smoothly but exhibited significant diurnal, daily, seasonal variations, and were influenced by several factors, such as water vapor, cloud, haze, etc. The objective of this paper is to develop empirical formulas for estimation of hourly and daily sums of PAR and VIS and their relationship under all sky conditions in North China. The calculated PAR and VIS were in agreement with measured values, the relative biases were 13.0?% and 12.4?% for hourly sums, 9.9?% and 9.0?% for daily values, respectively.