This paper analyzes solar radiation and daylight measurements taken during a four-year period in San Francisco, California. Horizontal and vertical surface measurements were taken by nine sensors at 15-minute intervals under all sun and sky conditions. The data base from which results were derived exceeds 400,000 measurements.

Equations are derived for clear sky global, direct, and diffuse illuminance and irradiance on a horizontal surface as a function of solar altitude, and for overcast sky horizontal illuminance and irradiance. We present the standard deviations for all parameters in our equations to show the scatter in our data. The average illuminance on horizontal and vertical surfaces by hour and by month are presented as isolux contour plots. These data are also displayed as probability distributions, showing the percent of time in a year that a given irradiance or illuminance value will be exceeded. Monthly average values of sunshine probability are determined and compared to long-term NOAA data.

%B Energy and Buildings %V 6 %P 273-281 %G eng %L LBL-15623 %1Windows and Daylighting Group

%2 LBL-15623 %0 Journal Article %J Energy and Buildings %D 1983 %T Analysis of Atmospheric Turbidity for Daylight Calculations %A Mojtaba Navvab %A Mahmut Karayel %A Eliyahu Neeman %A Stephen E. Selkowitz %XA 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, TL, provide moderate agreement to measured values of TL when using nominal design values for the Angstrom scattering coefficient, B, and precipitable water vapor, w. When average measured values for B 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 B. Til appears to be a better parameter to describe atmospheric conditions since, unlike TL, it is insensitive to air mass and thus solar altitude or time of day. We present and compare plots of Ti, and TL 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 B and w), or directly from B.

%B Energy and Buildings %V 6 %P 293-303 %G eng %L LBL-17727 %1Windows and Daylighting Group

%2 LBL-17727 %0 Journal Article %J Energy and Buildings %D 1983 %T Zenith Luminance for Daylighting Calculations %A Mahmut Karayel %A Mojtaba Navvab %A Eliyahu Neeman %A Stephen E. Selkowitz %XWe derive an equation for zenith luminance as a function of turbidity and solar altitude based on analysis of large quantities of luminance and illuminance data measured in San Francisco, California, between September 1979 and August 1982. Using only average turbidity values to predict hourly zenith luminance as a function of solar altitude can produce large errors. We compare the equation derived from our data, which is valid over a wide range of turbidity and solar altitude, to other published models. We also compare the relationship between horizontal illuminance and zenith luminance from the clear sky and conclude that, when ideal clear days are compared, this relationship is similar to earlier work based on measurements in European climates. Finally, we compare our sky luminance distribution measurements to previous published luminance distributions using the diffusion indicatrix. Our results are intended to help improve daylight availability prediction techniques and define additional requirements for data collection.

%B Energy and Buildings %V 6 %P 283-291 %G eng %L LBL-15622 %1Windows and Daylighting Group

%2 LBL-15622