Most of the bay area is chaparral. The Central Valley is generally ten to twenty degrees hotter than the coastal areas during the summer. The hot air literally

Most of the bay area is chaparral. The Central Valley is generally ten to twenty degrees hotter than the coastal areas during the summer. The hot air literally sucks the cold wet marine air inland right across SF during most summer days. That is why the best SF weather is early spring and late fall when the temperature difference between the Central Valley and the SF Bay Area is minimal, and their is much less fog. When the summer sun goes down, the fog evaporates in the dry air and the temperature drops.