The Relative Flux Density of Photosynthetically Active Radiation

Measurements of global radiation using pairs of thermopile pyranometers, one of which was filtered with an RG8 hemisphere, were made under high insolation conditions in Israel and U.S.A. For half-hourly periods, when solar elevation was above 10?, the relative flux density of photosynthetically active radiation ii, was nearly constant at 0 49 ? 0 05. Daily total values measured throughout one summer gave i = 0 47 + 0 07 and monthly values of iy measured at three stations gave ii = 0 49 + 0-02. The results confirm those previously reported for Europe and suggest that, within the accuracy with which it can be defined and measured, ,7 can be taken as 0 5 for periods of 30 min or longer when the solar elevation is greater than 10?.