DIRECTIONAL SKY LUMINANCE VERSUS CLOUD COVER AND SOLAR POSITION

Measurements of sky luminance at 121 equally spaced points (θ, φ) over the sky dome under clear, partly cloudy and overcast skies have led to the following analytical expression for normalized sky luminance Lv(θ,φ,θ*,C) = CLcvc(θ,φ,θ,*) + (1 - C) Lcvc(θ,φ,θ*)Lvvc(θ,φ,θ,*) = 0.40 + 0.21φ* + 0.27 cos φ + 1.45e-2.41ψLcvc(θ,φ,θ*) = [1.28 + 147e-11.1ψ + 4.28 cos2ψ cos θ*]*[1 - e-0.42secθ]*[1 - e-0.67secθ*] where θ = sky point zenith angle, φ = sky point azimuth angle, θ* = solar zenith angle, ↓ = scattering angle between sky, and sun directions and C = opaque cloud cover (θ* and ↓ in radians). © 1990.