Foggy at the coast. Sunny and warm 2 miles inland.

In celebration of the return of our beloved fog to the California coast after two weeks of weather extremes: 30 mph wind gusts for a week, followed by wiltingly-hot temperatures, let’s resurrect two old books for their descriptions of California’s sea fog, also known as “Advection Fog.” Advection fog forms when warm, moist sea air blows onshore, and is brought up short from its travels where the upwelling colder water runs along the coast. When the interior warms, a wind develops and blows the fog back to sea. This is why fog brings breezes whether it’s coming or going. Some people call advection fog, “Tule Fog,” but that is a particular type of inland valley fog; completely different in both its location and how it arises. (I think people like saying the word “tule” more than they like saying “advection,” so that is why the error persists.)

The first golden oldie resource is a little volume of essays published in 1915, Nature & Science on the Pacific Coast : a guide-book for scientific travelers in the West. In the essay, “Weather Conditions on the Pacific Coast” is a single entry describing, “one of the most climatic features of San Francisco is the prevalence of fog.”

The prevailing weather condition of the City could have seven different descriptions by the locals, as it is said the Eskimos have a variety of words for snow, and indeed, sometimes the fog crawls in like cat feet, but there is another fog that pushes first a wind, then blows dramatically into the City, swirling grey gusts around the hilltops and cable cars, making the town come alive.

“Comparing the percentage of possible sunshine at San Francisco and Mount Tamalpais, it is at once apparent that the summer afternoon sea fog shuts out 50 per cent or more of the possible sunshine between 3 and 7 p.m. during June, July, and August. There is also a curtailment of sunshine between 7 and 9 a.m. during May, June, July, August, and September.”

To simplify, the only clear, sunny and warm California coastal months are May and October. The rest of the months are either foggy or stormy.

Continuing the description of fog at the California coast, from Carey McWilliams’ 1946 classic, Southern California, An Island Upon The Land, “The fog banks, however, prevent the summers from being oppressive throughout the season. Forming beyond the islands, they can be seen moving in toward the coast, ‘in long attenuated streamers and banners, as night comes on, filling up the valleys of the coast with great tumultuous seas.’ With the morning sunlight, the mists obligingly roll out to sea. Throughout the summer, one can see this fog bank, about a thousand feet thick, lying offshore on the water. It has the strange feature, wrote Van Dyke, “of moving in against a breeze – the land breeze – and moving out against another – the sea breeze.”

“Doctors and amateur climatologists were put to work, at an early date, to help the newcomer to understand the nature of the fogs. For example, Dr. S.P. Ford divided Southern California fogs into two general categories: some were high and dry, others were low and wet. It was only with the latter type, frequently rare, that it became necessary ‘to have a little fire in the house.’ There were, he explained, five types of “foggy” days and it was grossly inaccurate, not to say misleading, to use a single term to characterize these variations.” (Note: I count only 4 variations given, for Dr. Ford repeats his “foggy and misty.” Nonetheless, the list:)

“1. “A foggy morning” was one in which the fog came up during the night and disappeared before seven o’clock.

2. If the fog continued until noon, the day should be characterized as “foggy and misty.”

3. ‘Cloudy foggy weather’ implied that it had been more or less foggy during a part of the day.

4. ‘Foggy and misty,’ on the other hand, implied that it had almost rained, while,

5. ‘dense fog in the night’ meant precisely what the term implied.

After a season or two, the tourists became somewhat sophisticated and told their relatives in the East that there were two seasons in Southern California: the wet season in which it may rain but seldom does, and the dry season in which it cannot rain but sometimes does. What is rain and what is fog remains, however, a moot question in the region.

Some years ago, one local weather bureau contended that .01 inch precipitation was merely ‘fog,’ while another bureau, with equal firmness, continued to report .01-inch precipitation as ‘rain.’ As I write these lines, I have before me, a local newspaper with the headline: ‘L.A. Wet Mist Will Stay.'”

Fog is California’s air conditioner. It’s nature’s fire retardant. Fog cools the coast with its grey overcast as well as inland by the winds winding their way through the coastal canyons into the interior. At the end of a hot spell, the fog advances onshore with its gusty grey vapor, grasping and overcoming houses and streetlights until the entire coastside is cloaked in a murky haze. Sounds soften in the gloom, and at dawn, one is awakened by drips falling from roof eaves, pattering onto the ground. Owls hoot more likely during foggy nights than clear ones.

Fog should be celebrated, not despaired over, unless it’s end of July and you haven’t seen sun all month. It’s ‘supposed’ to be summer. The fog’s been thick as rain with droplets watering your yard for days upon days, drowning your tomato crop.

By the way, Pacifica, a town known more for fogginess than a sunny seaside resort, hosts Fog Fest every September.
This post would not be complete without Carl Sandburg’s description of San Francisco Bay Area:

76. Fog

The fog comes
on little cat feet.

It sits looking
over harbor and city
on silent haunches
and then moves on.

And lastly, we must include, the line supposedly attributed to Mark Twain, “The coldest winter I ever spent was a summer in San Francisco.” The chill owing to, of course, advection fog.