Have you ever wondered why it gets so foggy in San Francisco in the summertime? Summer fog is common in the San Francisco Bay Area and on the coastline of California, and it's caused by a few different factors working together. Along the California coast, the cold ocean current flowing south from the North Pacific keeps ocean surface temperatures cool year-round. In the summer months, when the weather farther inland heats up, the hot air to the east rises and the heavier, colder ocean air from the west rushes in to replace it. This flow from the high- to the low-pressure zone pulls the fog through the Golden Gate passage and into the Bay.