Al Gore & the Fog

05Sep07

Dashiell Hammett concocted the hardboiled detective, framing his stories in the stuff. Newfoundland, Argentina and Washington’s Point Disappointment have even more of it than us—but who celebrates disappointment? London’s pea soup turned out to be pollution. There are at least 12 identifiable types of it.

We’re not talking Foggy Bottom, nor the Fog of War—nor memory lapses by administration officials testifying before Congress. We’re talking our fog, the world’s finest fog—that stuff pouring into the SF Bay Area at those exquisite moments when all points east suffer triple digit temperatures. It’s the world’s best air conditioning system.

Poor souls condemned to California’s Central Valley suffer stifling heat, frigid cold and oatmeal-thick Tule fog. Our fog, by contrast, is mythic, romantic, generally welcomed and often gorgeous to behold. It comes in lacy wisps, billows up like cumulus cotton, caressing and cascading down our hillsides. Our fog floats in, spreading out in wraith-like fingers. Its arrival is heralded by a raw symphony of barnacled fog horns bellowing out from bay and coast, alike.

Today’s question is: Where does it come from—and will we lose it to global warming?

“What residents know for sure is that the San Francisco Bay Area has three seasons: winter, summer and fog.”
—Carl Nolte

Labor Day’s done. The kids are back in school. Halloween’s already goblin-ing up shop windows. For the rest of the country summer’s officially, if not technically, kaput. But here in the Bay Area summer weather is just cranking up. In fact, it’s getting damn hot. Why? Because fog pouring through the Golden Gate and sliding between North and South Bay mountain chinks is receding.

What’s beginning to change now is:

The Pacific High
Each spring we inch toward the sun. A warm high pressure system near the equator pushes north over the Pacific Ocean towards the Arctic.

“Some of it cools off and sinks to the ocean surface again several thousand miles to the north as the Pacific High—a ‘mountain’ of cool air weighing heavily on the water.”
—Harold Gilliam

The Sun’s rays evaporate surface water, pumping moisture into winds heading our way. The earth’s rotation spins northern hemispheric winds clockwise, so those winds hit us at about a 45-degree angle from the northwest. This phenomenon is called the Coriolis Force. As this moist air approaches the Bay Area cold up-rushing water replaces warmer surface water along our coast. Salt sprayed into the air captures minute water particles as moisture-laden winds meet cold coastal waters, condensing and producing “the great fog bank.” From late spring through summer this mass of fog hugs our coastline. It’s often a hundred miles thick and a half mile from the water up into the sky.

‘This… is a point upon a map of fog.”
—Ambrose Bierce

Meanwhile, as spring wends toward summer California’s Central Valley turns infernal. One hundred-plus degree temperatures become the summer norm. With Sierra peaks blocking it from heading east, the Central Valley’s hot rising air sucks fog into the Bay Area, like goose down into a vacuum. Fog sneaks through land-breaks like the Russian River Valley, Petaluma Gap’s Estero Lowland and the Nicasio and Muir Woods Gaps to the north; and the San Bruno and Crystal Springs Gaps to the south. Of course, the heavy fog-hitter is the Golden Gate, providing both unhindered access to the interior, and plenty of cool water to juice up the in-rushing fog.

But our fog is not an equal opportunity weather provider, even here in the Bay Area. I’ve left San Francisco’s Richmond district covered with fog in the upper 50’s, driven across the Golden Gate Bridge a few miles to Sausalito, where it’s sunny mid-90s. On days like that you can’t even see San Francisco for the fog bank overwhelming it. I’ve driven from the Alexander Valley south to the City, monitoring temperatures rising and falling three or four times through Sonoma and Marin Counties, the fog coming and going, until the temperature bottoms out in foggy San Francisco. Then consider the many microclimates within the 47 square miles comprising San Francisco itself. It’s even possible to experience a range of different types of fog on the very same day when traveling from one Bay Area destination to the next.

Cycles & Breakdown
The fog comes and goes in cycles—daily, weekly and such, from spring through summer. We’re all familiar with these cycles. But in September and into October, as the Central Valley temperatures begin to dip and the Pacific High heads south, the whole system breaks down. That’s why we often experience our clearest, hottest days this time of year.

Global Warming & The Fog
No one knows for certain how ongoing world climate change will affect our fog. While we’ve heard dire predictions concerning the toasty fate of Wine Country vineyards, I was pleasantly surprised to read Harold Gilliam, the dean of Bay Area environmental journalists’ opine on this subject in the updated edition of his 1962 classic Weather of the San Francisco Bay Region. Gilliam contends that as the earth warms we may actually experience more fog than at present. But he sure doesn’t guarantee it. Earth-wide weather patterns, greenhouse gasses, melting ice caps, the jet stream and our localized carbon emissions might combine to alter or dissipate the fog entirely, though it seems unlikely to happen in our lifetime.

Of the many things our Bay Area is famous for, summer fog ranks right up there with wine, computers, cable cars and sourdough. Can you imagine life here without any one of them?

Filed under: Ambrose Bierce, Coriolis Force, Harold Gilliam, North Bay, Pacific High, Russian River, San Francisco Bay Area, climate change, fog, global warming, wine country   |  1 Comment