A marine layer forms when cool air over cold ocean water is trapped under warmer air above it. Cooling can then make clouds and fog. Along the California coast the cold water comes from the California Current and from upwelling.
Half Moon Bay has a warm-summer Mediterranean climate with fog and overcast common in the night and morning hours, usually clearing offshore in the afternoon. Persistent sea breezes keep it mild. September is the warmest month, with an average high of 66.8 °F, and January the coolest, with an average high of 58.4 °F. Rain falls mostly from November to April, about 26 inches a year.
The same fog is part of what makes the area suit cool-weather crops such as Brussels sprouts, and it helped the rum runners of Prohibition.
More on this site: the beaches