Solar radiance is measured by an Apogee Instruments SP-100 pyranometer.
Today's solar radiance, read and logged every 1790 seconds, is graphed in red. Shown in black is the theoretical solar radiance. Places where the actual radiance dips below the theoretical are the result of cloud cover. Scant, passing clouds create sharp spikes, while persistant cloud cover will show as a more prominent dip. Also shown below are the minimum and maximum values for the past week, in blue for comparison.
Here the current solar radiance and theoretical values are compared to the minimum and maximum values on this date each year since 2012.
A webcam on the 9th floor of Millikan Library provides a live view of the skies over Caltech. Click to enlarge.