Analysis of atmospheric turbidity for daylight calculations: Fid-Bau Portal

Analysis of atmospheric turbidity for daylight calculations

Navvab, M. / Karayel, M. / Ne'eman, E. / Selkowitz, S.

AbstractA large set of illuminance and irradiance data has been collected for four years at 15-minute intervals in San Francisco. This data set has been used to investigate the impact of atmospheric turbidity on daylight calculations. Existing predictive formulae for Linke turbidity, $T l$ , provide moderate agreement to measured values of $T l$ when using nominal design values for the Ångström scattering coefficient, β, and precipitable water vapor, w. When average measured values for β and w are used, the agreement improves. We suggest the use of an illuminance turbidity, Til, to calculate direct normal illuminance directly. We derive a simple approximate solution, Til = 1 + 21.6 β. Til appears to be a better parameter to describe atmospheric conditions since, unlike $T l$ , it is insensitive to air mass and thus solar altitude or time of day. We present and compare plots of Til and $T l$ vs. solar altitude, time of day, and month. Finally, we examine and compare several alternative pathways to derive direct normal illuminance from irradiance and luminous efficacy (dependent on β and w), or directly from β.