It occurred to me that there might very will be a metric out there that measures and quantifies "daily brightness". Meaning, how bright any given day is, taking weather conditions and cloud coverage into account.
A general brightness metric is lumens, but I can't find anything on this subject with respect to meteorology.
And, if such a metric does exist, what's its range, 0 - 100? What weather/ambient conditions are represented at this range's extrema?