San Francisco fog

Fog is a common weather phenomenon in the San Francisco Bay Area as well as along the entire coastline of California extending south to the northwest coast of the Baja Peninsula. The frequency of fog and low-lying stratus clouds is due to a combination of factors particular to the region. These factors are especially favored in the summer. Another type of fog, tule fog, can occur during the winter. There are occasions when both types can occur simultaneously in the Bay Area.

The Pacific Ocean contributes to the frequency of fog by providing atmospheric moisture and by its temperature. It is also the major source of nuclei for the condensation of moisture from vapor into cloud droplets. Moisture evaporated from the ocean surface over hundreds, even thousands of miles of the open Pacific is carried to California from various directions. This water vapor contributes to the development of what is called a marine layer near the surface of the ocean.

Along the California coast, the prevailing current flows from the northwest and is cool owing to its origin in the North Pacific. Additional cooling occurs due to strong upwelling of cooler subsurface waters, especially along the immediate coastline and near various promontories. Sea surface temperatures along the coast are generally 52–58 °F (11–14 °C) year-round.

When the marine layer encounters the colder waters along the California coast, it cools to its dewpoint, and if small particles called condensation nuclei are present, liquid water drops will form. Condensation nuclei in coastal fog are mostly composed of salt from surf and spray, with lesser amounts of iodine from kelp. These nuclei are so effective that condensation can occur even before the dewpoint is reached.

The prevailing wind along the California coast is from the northwest owing to the normal location of the North Pacific High, a large area of high atmospheric pressure. As the coastline is oriented from northwest to southeast, the marine layer and any clouds present within it would be confined to the coast and adjacent offshore waters, and often are, but for the large difference in temperature between the coastal waters and the inland valleys, especially the Central Valley. In the summer, inland temperatures can soar as high as 100 °F (38 °C), and often higher. This large difference creates a strong pressure gradient that turns the prevailing northwest flow to a westerly and even southwesterly direction near the coastline, driving the marine layer and its clouds onshore and through any gaps in the Coast Ranges.

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