The Fog That Defines Point Reyes
For much of the year, the fog at Point Reyes is not weather; it is the place itself.
The Point Reyes Lighthouse sits at the end of a stiff railed stairway of 308 steps cut into a rocky cliff. On most days between May and August, the stairway is in the fog. The lower landing cannot be seen from the upper gate. The foghorn, mounted on a shelf below the lighthouse, makes the place known before the eye can place it. The horn is not decorative. It is an aid to navigation, as much a part of the point as the rocks. In the fog, the ocean is a continuous low rumble; the horn is the only thing that marks time.
The point is the westernmost limit of Marin County, and the westernmost land of the Bay Area. It is not attached to the same crust as the mainland. The San Andreas Fault runs along the straight edge of Tomales Bay and through the valley behind the headland. The peninsula rests on the Pacific Plate and has been carried north past the mainland for millions of years. That geology is why the point is here: a piece of a different California, pushed against the edge of the continent.
How the Fog Forms
The fog is a marine layer. The California Current brings cold water down from the Gulf of Alaska. Along the Point Reyes coast, winds pull deep water upward in a process called upwelling. The surface temperature of the water offshore often sits in the low fifties, even in summer. The air on top of that water cools and condenses into a layer of stratus cloud. The cloud is usually between one and two thousand feet thick. The prevailing wind out of the northwest shoves the layer toward the coast.
Point Reyes is the first obstacle that the cloud encounters. The headland reaches roughly ten miles into the Pacific, an unusually far projection compared with the rest of the California coast. It catches the marine layer like a breakwater. This is why the fog is so persistent here: the open ocean supplies it, the cold water maintains it, and the land forces it upward toward the ridges, where it condenses again on the Bishop pines and the coast live oaks.
The fog has a diurnal rhythm. It usually builds at night and in the early morning, fills the valleys and the ocean side of the ridge by late morning, and then, when the interior of the Bay Area heats up, the pressure difference pulls the fog through the Tamalpais Gap toward the bays. This is the well-known summer pattern of the Bay Area. At Point Reyes, the pattern is compressed. There is no wide interior valley to draw the fog away. It often stays all day, blowing steadily from the sea. Visitors may reach the Bear Valley Visitor Center in clear sun and drive west into a wall of white by the time they approach the lighthouse parking lot.
The Lighthouse and the Horn
Shipwrecks encouraged the federal government to build the lighthouse. In the 1860s, maritime traffic around the point was heavy, and the coast had a reputation for sudden fog and strong currents. The lens was a first-order Fresnel, the largest type in common use, and was lit on December 1, 1870. The fog signal was originally a steam whistle. It was later replaced by a diaphone and then by the present electric horn. The light itself, after more than a century of service, was replaced by an automated beacon in the 1970s. The original lens still stands in the lantern room, but it is not lit.
The wind matters as much as the fog. The lighthouse catwalk is sometimes closed when gusts exceed forty miles per hour. The winter storm season can bring winds above a hundred miles per hour to the point. In summer, the wind is lighter, but it is constant, blowing out of the northwest. In this wind, the fog does not hang still; it moves as a white wall, atomizes, and wets everything. Visitors to the lighthouse know to wear rain gear in July, not because it is raining, but because the fog is a cloud with its water in the air.
Fog as Water
The fog is the source of the gray-green landscape. Along the ridge, Douglas-fir and Bishop pine gather fog directly from the air. Droplets accumulate on needles and fall to the ground as drip. In the summer dry months, fog drip can amount to a hundred or more millimeters of water for the forest understory. That is more water than the summer rains ever provide. The grass and the coastal scrub also intercept fog. The result is that the peninsula can be dry and golden in the open fields while the shaded slopes and gullies stay damp.
The dairy ranches of Point Reyes existed because of this fog-kept pasture. Since the middle of the nineteenth century, the peninsula has supported cattle and creameries. The fog does not let the grass dry to straw in July and August, the way it does in the inland valleys. This is still the case within the national seashore, where the National Park Service has continued to lease several ranches along the esters and the flat drainages.
The fog is not uniform across the peninsula. The eastern side of Inverness Ridge, toward Tomales Bay, is often clear while the western side is covered. This is because the marine layer spills over the ridge and evaporates on the lee side. At Drakes Beach, at the southern edge, the fog is less common than at the lighthouse, but it still appears on many summer days. No part of the point is entirely out of reach of the marine layer.
