PHOTOSYNTHETICALLY-ACTIVE RADIATION - SKY RADIANCE DISTRIBUTIONS UNDER CLEAR AND OVERCAST CONDITIONS

The photosynthetically active radiation (PAR), defined as the waveleng th band of 0.400 mu m to 0.700 mu m, represents most of the visible so lar radiation. Although the proportion of global irradiance that origi nates from diffuse sky radiation is higher for PAR than for all solar shortwave radiation, it is often assumed that the PAR diffuse sky radi ation is distributed identically to that of all shortwave solar radiat ion. This assumption has not been tested. PAR sky radiance measurement s were made in a rural area over a wide range of solar zenith angles. The distribution of PAR sky radiance was modeled using physically-base d, non-linear equations. For clear skies, the normalized sky radiance distribution (N) was best modeled using the scattering angle (Psi) and the zenith position in the sky (Theta) as N(Theta, psi) = 0.0361[6.3 + (1 + Cos(2) psi)/(1 - cos psi)][1 - e(-0.31sec Theta)]. The angle Ps i is defined by cos psi = cos Theta cos Theta + sin Theta sin Theta* cos Phi, where solar zenith angle is Theta and the difference in azim uth between the sun and the position in the sky is Phi. Modeling of th e overcast sky depended on the visibility of the solar disk. The trans lucent middle/high cloud overcast conditions (cloud base greater than 300 m above ground level) were best modeled as: N(Theta, psi) = 0.149 + 0.084 Theta + 1.305e(-2.5 psi) while the translucent low cloud ove rcast conditions (cloud base less than 300 m above ground level) were best modeled as: N(Theta, psi) = 0.080 + 0.058 Theta* + 0.652e(-2.1 p si). The obscured overcast sky condition (solar disk obscured) was bes t modeled as: N(Theta) = 0.441[1 + 4.6cos Theta]/[1 + 4.6]. The unit o f N for all equations is pi Sr-1, so that integration of each function over the sky hemisphere yields 1.0. These equations can be applied di rectly to the sky diffuse irradiance on the horizontal, I-diff, to pro vide radiance distributions for the sky. Estimates of actual sky radia nce distribution can be estimated from N-a(Theta, psi) = IdiffN(Theta, Phi).