Solar radiation for plant growth.
Analysis of theoretical calculations and experimental measurements showed that the photosynthetically useful fraction of the solar spectrum for total (direct + diffuse) radiation was almost independent of atmospheric conditions and was nearly constant at 50 plus or minus 3%. For direct-beam radiation in very clean air in winter, it increased from 35 to 45% as solar elevation increased from 10 to 45 deg ; in summer the range was smaller, 41 to 49% for the same range of solar elevations. The figure for blue sky was 75%. It was concluded that climatological measurements of solar radiation could confidently be used for estimating efficiencies of photosynthesis and crop growth in the field.