Daylight Availability Data for San Francisco

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.