Out of the North

May 15, 2015 | By | Reply More

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Cold spells on the bay are rare. San Francisco is kept warm in winter, as it is kept cool in summer, by the even-temperatured waters around it.

But the popular notion that the “Japanese Current” flows down the California coast – and is responsible for the warm winters – is a myth. The only warm current from Japan reaching the eastern Pacific never gets closer than five hundred miles from the coast.

What does flow down the coast is the California Current, a cold stream of water from the Aleutians. But warm and cold are relative terms; the winter  water temperature in the Golden Gate stays near fifty, cold compared to the sixty degree Japanese waters, but still warm enough to prevent bay area land temperatures from dropping very far.

San Francisco’s average temperature in January, the coldest month, is almost exactly the temperature of the water entering the bay on the flood tide.

Thus the bay acts as a natural thermostat; just as the ocean moderates the climate of the continent, so the entire bay cools the summers and warms the winters of the communities of its shores.

Sometimes, however, the movements of the weather take place on such a vast scale that the normal local influences are overwhelmed by far larger outside forces. It is as if a window in a thermostat controlled room were broken, allowing a sudden flow of air from the outside, hot in summer, cold in winter.

Just as occasional hot spells in summer result from the inrush of masses of dry continental air from inland valleys and desert areas, so a cold snap will result from similar causes. An imbalance in the normal front of polar and equatorial air over the continent will send icy winds across the snow-covered high plains of Oregon, Idaho, and Nevada, whistling down the Sacramento Valley and through the passes and canyons of the Coast Range, roaring out across the bay from the northeast, whipping it into a forth of whitecaps.

Again the storm warnings go up; this time the Telephone Building masts flies two flags – besides the red pennant, warning small craft, is a square red flag, storm signal for ships as well.

Fishermen’s fingers grow numb in the icy spray. Towboat pilots curse as the wind swings their barges out of control. Pilots of big freighters and liners try to avoid docking on the north sides of piers where the ships may batter the wharves as they wallow in the wind. In the streets of the bay’s cities hats go flying and pedestrians turn up coat collars and huddle in doorways against the cold blasts.

The northers may come at almost any time of year but are most probable in early spring, when the air over the continent is most turbulent.

They may blow for two or three days and die out in fits and gusts. Once more the air becomes calm, and the bay is placid before noon, ruffled in later afternoon by gentle ocean breezes.

Then early one morning you may hear, as you rouse to wakefulness, the bass chorus of the great horns. The earth has completed another cycle around the sun; the coastal waters are beginning to well up from the bottom; and a long white arm of fog is once more moving silently through the Golden Gate and over the face of the bay.

 

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Category: Fog City - City of Fog, San Francisco Bay

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