Ocean
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 in the mid to upper 50s F, 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.
Read more about this topic: San Francisco Fog
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