We generally think of June and July as summer and expect everything to be pretty hot, the ocean changes temperature very slowly, so it doesn't reach its maximum temperature until late summer or even early fall. In June and July the ocean is still fairly cold, especially since the California current carries water down from farther north. This cold water makes for a cold, moist marine layer in the air over the ocean, which is blown onto the shore by sea breezes and a wind pattern called a Catalina eddy, which is strongest in May and June.

The cold, moist air condenses into clouds and fog that are held in place by a layer of warm air sitting over top of them. Generally, the air is warmer near the ground, so we call this cold air on the bottom and warm air on top an inversion.

In the winter, we don't have as much warm air to hold the clouds in place, and storms are more likely to break up a warm layer if it did form. Later in the summer, the ocean is warmer, so we don't get the same marine layer.