Summer fog

Climate--from "Encyclopedia Britannica"

Winter in San Francisco is rainy and mild, spring sunny and temperate, summer foggy and cool, and autumn sunny and warm. The average minimum temperature is 51 °F (11 °C), and the average maximum is 63 °F (17 °C). The mean rainfall, almost all of which occurs between November and April, is about 21 inches (533 mm). There is sunshine during two-thirds of the possible daylight hours. The most characteristic feature of the weather, however, is the summer fog, which lies low over the city until midday, creating consternation among shivering tourists. This fog is a phenomenon of temperature contrasts, created when warm, moist ocean air comes in contact with cold water welling up from the ocean bottom along the coast.