Sunrise at the Golden Gate

Flown by Mark Johnson, below, through and above the fog! By Visual-aerials

Fog is a common and typical weather phenomenon in the San Francisco Bay Area as well as along the entire coastline of California. The frequency of fog and low-lying stratus clouds is due to a particular combination of factors peculiar 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 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.

A common local expression, "June Gloom", refers to a spell of fog and stratus ("overcast") that does not clear all the way back to the coast for several days. These extended periods of cloudiness are usually a consequence of a weak area of low pressure above the marine layer which increases its depth, making it more difficult for surface heating to break it up. In actuality, these extended spells of low cloudiness can occur at any time, not just June.

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