The coastal waters of Northern California are cold. What this means is that the cool temperature over the ocean in the summertime causes moisture in the air to condensate. The cold waters are caused by two factors: the Californian Current bringing cold surface water from the north and also upwelling that brings cold water from the depths to the surface. Hence the fog. When it winds from the west, it can get foggy within some distance in the inland from the sea as well.
Summertime fog and cloudy skies are not unusual in the south, either. But I'm guessing the shape of the coast makes Southern California less susceptible to the effect of the ocean currents. Also, the waters aren't quite as cold.
You got that right to a T, you are %100 right about that. I used to swim in the ocean every year at the end of August the water would start to warm up from the water drifting from Hawaii
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Yeah. what happens is that during late summer (of the northern hemisphere) the entire planetary wind system begins to shift southward. Northern California gradually enters the zone where westerly winds prevail. That puts a stop to the upwelling of cold water. Paradoxically, the weakening of the power of the sun coincides with the surface waters getting warmer.