Photosynthetically active radiation in Zambia

Global solar radiation (K↓) and photosynthetically active radiation (PAR) were measured at Lusaka, Zambia from April to December 2000. Daily K↓ and PAR followed a similar trend, and the seasonal cycle was primarily determined by extraterrestrial radiation and cloudiness. The daily PAR/K↓ ratio also exhibited a distinct seasonal variation. The mean daily ratio was 1.992 mol MJ^-1; the maximum daily ratio was 2.414 mol MJ^-1; and the minimum daily ratio was 1.778 mol MJ^-1. Generally, high PAR/K↓ ratios were associated with low clearness indices (ratio of solar to extraterrestrial radiation), and low ratios with high clearness indices. The maximum daily ratio occurred in December, when dense cloud associated with the Intertropical Convergence Zone prevailed. The minimum daily ratio occurred in September when predominantly cloudless conditions, associated with subtropical highs, occurred. These minimums also coincided with widespread regional biomass burning throughout southern Africa and the corresponding enhancements to atmospheric turbidity.