You are barreling down the highway on a gray day at an apparently reasonable speed of about 60 mph, able to see the road a good distance ahead, when suddenly visibility goes to zero.

Blinded by fog, you slam on the brakes. Within the next second or two, the driver behind you jams on his brakes -- but not soon enough. He plows into your rear with a crash. In another two seconds, the car behind him slams into him.

Within 10 seconds, cars are crumpling each other in succession like a row of dominoes until there are several dozen wrecks, many injuries and perhaps fatalities.

It happens nearly every winter. Last Tuesday, two crashes involving 87 vehicles killed two people and injured more than 25 on Highway 99 just south of Fresno.

Blame bad driving; drivers blame the fog, which every winter lies intermittently like a blanket over many inland valleys.

Unlike California's summer fog, which is formed over the ocean and moves inland, the winter fog begins in the inner valleys and may migrate toward the coast. The summer fog tends to rise as it moves in from the ocean, forming a gray ceiling, but the winter variety hugs the ground.

It is not only perilous to drivers but maddening to fliers when it blacks out airports. On Jan. 3, fogs delayed arriving flights at San Francisco International Airport by more than six hours. Airport devices to dissipate the mists seem to be of little use against the white aerial tides that cover the land and the bay.


The Bay Area's winter fogs are distinctive owing to the unique configuration of local geography, a combination of hill-and-valley topography with vast areas of marshes and open water. Once, in the fields near Mount Diablo, I saw the backs of cattle that were instinctively grazing on invisible grass with their heads buried in the fog.

Diablo is at the edge of the Sacramento-San Joaquin River Delta with its maze of islands, sloughs and marshes. Because the winter fog often originates in marshy areas where the tules grow, it's commonly called "tule fog." ("Tule, " which rhymes with "truly," is an Aztec-Spanish word meaning bullrushes.)

Scientists know the winter fog as "radiation fog." But not to worry: You won't be in danger from X-rays or nuclear radiation. This type of radiation -- quite benign, except for people who drive too fast -- is related to the process that forms fog and clouds of all kinds.

Nearly all air contains moisture, usually in the form of invisible water vapor. When the air cools, the vapor condenses into invisible drops that collectively create clouds or fog. Weather reports often predict "fog and low clouds," as if there were a distinction, but basically fog is simply a cloud on or near the ground; a cloud is fog in the sky. They take on different shapes, but they are made of the same stuff.

The summer fogs off the California coast are created when the ocean air is cooled by frigid ocean currents upwelled from the sunless bottom. The cooling condenses the vapor that the air has picked up in its transit over the Pacific.

The result is the great summer fog bank that hangs intermittently offshore and moves inland, producing the Bay Area's famed natural air conditioning system.


The winter fogs in the Bay Area come into being when air is cooled by a different process. After the first major storms have soaked the soil and moved on, and the air is clear, cold and still, the stage is set for a major atmospheric change.

During the long winter nights, the land and the air quickly lose the sun's heat they have absorbed during the day. The heat radiates upward. The ground, and the air in contact with it, become steadily colder. The moisture absorbed into the air from the damp earth condenses into radiation fog.

Because some parts of the ground are colder than others, the fog may form in separate masses and drift unpredictably, merging and reforming -- the process that causes motorists to drive suddenly from clear air into areas of zero visibility.

The fog reaches its greatest density in the frigid hours just before sunrise and will seldom diminish before the sun is high enough to begin burning it off. Most automobile accidents occur during those morning hours.

As we have seen, the tule fog tends to form inland. The reason is that the Pacific acts as a giant thermostat. Its temperature varies only a few degrees through the seasons. Consequently, the ocean is colder than the land in summer and warmer in winter.

So the coastal regions are warmer in winter than the inland valleys. When the tule fog forms in the cold inner areas, the coast is likely to remain clear. In the days before supersized jets, planes unable to land at San Francisco International Airport owing to the winter fog were diverted to the sunny airport at coastal Half Moon Bay.

At such times the Central Valley may be totally fogbound, with human activities as immobilized as during a heavy East Coast snowfall. Farmwork is at a standstill, most traffic slows to a near halt and schools are closed.

When the cold, calm weather persists for weeks, the tule fog building up in the Delta and other Central Valley areas may grow thicker each night until it piles up in snowlike drifts east of the Berkeley hills. Because air tends to move from a cold (high pressure) to a warmer (low pressure) area it may begin to flow through Carquinez Strait and other gaps in the hills to the bay.

Sometimes it will migrate westward across the bay, inundate San Francisco, and pour outward through the Golden Gate to the ocean, in a reversal of the summer fog pattern. Districts of the city that are normally fogbound in summer,

primarily the Sunset and the Richmond, will be clear and sunny while downtown and the Mission are enveloped in gray gloom. However, this winter situation is far more rare than the comparable period in summer when the fog covers the areas west of Twin Peaks.


Historically, tule fogs have been responsible for most shipping accidents in the bay, including the 1901 sinking of the liner Rio de Janeiro, which ran aground in the fog near Fort Point, now the southern anchorage of the Golden Gate Bridge. About 130 lives were lost in that disaster.

When the winter fog is at a nighttime maximum, it tends to burn off the next morning from the top. A personal experience of my own illustrates that phenomenon. I was asked to appear on the Arthur Godfrey TV show (once as popular as today's "Oprah") to point out the sights of the city from the top of Coit Tower, where the cameras were set up.

The producers couldn't believe my earlier warning that fog might obscure the view. The sun was shining then at the Palace Hotel, where they were staying, and they failed to reckon with this city's microclimates.

When I arrived at Coit Tower at 7 a.m. for the broadcast, visibility was zero -- and continued that way for the show's full two hours. Never had there occurred a more dreary two hours on television, as we tried to fill the time with chatter, awaiting the sun. Ten minutes after the show was over, the tops of the hotels on Nob Hill and the skyscrapers of the Financial District began to appear. The white curtain slowly lowered until only a few pools remained on lower Market.

Out on the bay, however, the gauzy veil remained -- only a few feet deep. The funnels and superstructures of ships appeared before the hulls became visible, and the white mists slowly evaporated. Had the cameras remained a little longer, they would have captured a natural spectacle more dazzling than any show that had ever been broadcast. And I might have had a promising career in television.


Tule fog, named for the bullrushes found throughout the Central Valley, occurs on calm, winter nights after a period of rainfall.

1) On a clear day after a rain, the sun warms the moist ground.

2) With nightfall, the ground and air radiate heat upward. Air near the ground becomes colder, causing its moisture to condense.

3) The condensation creates a low-lying fog that becomes very dense as the night grows colder.