Abstract
A physically based method of estimating the photosynthetically active radiation (PAR) incident at the earth's surface is described. Ultraviolet reflectivity, inferred from the Total Ozone Monitoring Spectrometer (TOMS) onboard the polar orbiting Nimbus-7 satellite, is used to account for the effect of cloud albedo on the attenuation of PAR incident at the surface. The clear sky incident radiation for the PAR wavelengths (400-700 nm) is computed from the spectral model of Goldberg and Klein (1980). Monthly averages of incident PAR at the surface estimated from the satellite method differed from estimates of PAR from ground pyranometers by less than 6%. This method of PAR estimation can be applied globally over snow and ice free areas.