If true fog alone be considered-that is, suspended water particles which reduce horizontal visibility at the earth's surface to 1,000 meters (or½ mile) or less-then fog at San Diego occurs nearly four times as frequently during the winter period, from October through April, as during the remaining months of the year (11, p. 34)....
Leipper, Dale F.. 1948. "Fog development at San Diego, California." Journal of Marine Research 7, (3). https://elischolar.library.yale.edu/journal_of_marine_research/673