San Francisco fog

The prevalence of fog in the San Francisco Bay Area has decreased, and this trend is typically attributed to climate change.

Moisture evaporated from the ocean surface over hundreds, even thousands of miles of the open Pacific is carried to California from various directions.

Condensation nuclei in coastal fog are primarily composed of salt from surf and spray, with lesser amounts of iodine from kelp.

This significant 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.

[7] As the city of San Francisco lies adjacent to the Golden Gate, it is often subject to the fog and low clouds blowing in on the marine layer.

By late afternoon, the wind increases and begins to cool the onshore marine layer, allowing the fog and low clouds offshore to progress inland without evaporating.

As night falls and inland areas cool down, the winds usually decrease, but the fog and clouds remain wherever they have blown in until the following morning when the cycle repeats.

Such heat waves typically occur when an area of high atmospheric pressure orients itself so that the north to northeast gradient becomes dominant, driving the marine layer out to sea south and west of the California coast.

These spells typically end with what is called a southerly surge, when the northerly gradient relaxes, allowing the marine layer to "slosh back" up the coastline.

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 evaporate the clouds within it.

Writers, poets, and photographers have long been inspired by the fog, including Herb Caen, Jack Kerouac, August Kleinzahler, and Arthur Ollman.

[19] Many Alfred Hitchcock movies including Shadow of a Doubt, Vertigo, and The Birds were set in San Francisco, invoking the fog and its eeriness as a backdrop.

In 1901, 128 people died on the SS City of Rio de Janeiro after the boat hit the reef due low visibility from fog.

[30] In 2013, the Overseas Reymar, a 748-foot-long tanker, was proceeding to sea from an anchorage off San Francisco after discharging its cargo of crude oil at a Martinez refinery when it hit the base of the Echo tower of the Bay Bridge in fog.