Seas of fog around San Francisco

From KomoNews

With its perch right on the chilly Pacific Ocean, San Francisco has quite the foggy reputation and several foggy days have been immortalized in postcards and print.
But I'll bet you've never seen the fog quite like this.
Photographer Simon Christen just released his video "Adrift," culminating a two year project that he says "is a love letter to the fog of the San Francisco Bay Area."
His project aimed to "capture the magical interaction between the soft mist, the ridges of the California coast and the iconic Golden Gate Bridge.
 In the meantime, he got quite a lesson on Bay Area meteorology.
"The weather conditions have to be just right for the fog to glide over the hills and under the bridge," Christen wrote on his Vimeo page "I developed a system for trying to guess when to make the drive out to shoot, which involved checking the weather forecast, satellite images and webcams multiple times a day."
If the weather looked promising, he'd make the 45-minute drive to the Marin Headlands.
While many trips ended up fruitless because the fog bank was too high, too low, or already gone, "once in a while the conditions would be perfect and I was able to capture something really special."
Mission accomplished.

photo Guilain Grenier

Why is San Francisco so foggy?

The loud drone of fog horns is a near daily occurrence around San Francisco, thanks to their proximity to the chilly waters of the Pacific Ocean, which varies between 52 degrees in the late winter to around 60 in the late summer.
That's due to a process called "upwelling", which occurs when winds along the surface blow ocean water away from the coastline.
That process pulls up deeper water to the surface to replace the "missing" water that's being blown away.
That deep water is much colder since it doesn't get the surface warming from the sunshine, and thus the surface temperature of the ocean where upwelling is occurring is much cooler.
In our case, It's helped by the easterly trade winds out in the central Pacific Ocean that help pull the water away from the West Coast -- sort of like when you roll over and pull the blanket off your spouse.
Here is a recent chart of sea surface temperatures off the California coast -- notice how chilly it is along the coast.

When you get warmer air moving in off the Pacific and it hits those cooler waters along the shorelines, the air near the surface will cool and condense into a thick fog.
Then as the ground in the inland Bay Area heats up each day, the hot air rises, creating lower pressure near the surface.
The fog just offshore is then drawn inland to replace that rising air -- a "marine push".
A similar effect occurs here in the Seattle area, only the fog has a lot farther to travel, so it takes a moderate to strong push to bring the marine clouds into Seattle, while even a very light push will bring fog in through the San Francisco Bay, allowing for those dramatic shots Christen found.

P.S. if you like that top video, you might like Christen's earlier project on Bay Area fog:

The Unseen Sea from Simon Christen