A fogbow is a rainbow that forms in fog instead of rain. It has the same arched shape as a rainbow, but it looks white or only faintly colored, like a pale ghost of the real thing. The Bay Area is a good place to look for one, because the summer marine layer fog often sits just offshore while the coastal hills stand in sunshine. When you stand in that sunshine with the sun behind you and fog in front of you, a broad white arc can appear in the fog. The best places to try are high coastal viewpoints such as the Marin Headlands, along with the bluffs above Ocean Beach and the hilltops of San Francisco.
How a Fogbow Forms
A rainbow forms when sunlight enters a raindrop, bounces off the back of the drop and comes back out toward the viewer. Raindrops are large enough to bend each color of light by a slightly different amount, so the colors fan out into separate bands. That is why a rainbow shows a clean stripe of red, orange, yellow, green, blue and violet.
Fog droplets are far smaller than raindrops, often less than a tenth of a millimeter across. At that size, light does not just bend through the droplet. It also spreads around the droplet's edges, an effect called diffraction. Diffraction smears each color across a wide range of angles, so the colors overlap and blend back toward white. The result is a broad, pale arc instead of a thin, bright one.
Some fogbows still show a hint of color, with a faint reddish tinge on the outer edge and a bluish tinge on the inner edge. The smaller the droplets are, the wider and whiter the bow becomes. A fogbow is also slightly smaller than a rainbow, with a radius of roughly 35 to 40 degrees instead of about 42.
The viewing geometry is the same as for a rainbow. The sun must be behind you, and the fog must be in front of you and lit by the sun. The bow is centered on the antisolar point, which is the spot directly opposite the sun, where the shadow of your own head falls. When the sun is low, that point sits near the horizon, so the arc rises above the fog in front of you. Standing on a hill above the fog lets you see more of the circle.
Where to See Fogbows in the Bay Area
The Marin Headlands are the most convenient place to look. The bluffs rise several hundred feet above the ocean, and on many summer mornings they poke out above a fog bank that sits over the water. With the sun rising in the east behind you, the best direction to face is west over the Pacific. Facing southeast toward the Golden Gate puts the morning sun in front of you, which will not work.
Point Reyes offers the same chance on a larger scale. The ridges and bluffs above the outer coast look west over open ocean, which is exactly the direction you want on a sunny morning. The catch is that the Point Reyes headland is often buried in fog itself. Your best odds come on a morning when the fog is thinning or pulling back from the land and you can stand in the sun at its edge.
Within San Francisco, the summits of Twin Peaks and Bernal Heights sometimes rise above a shallow marine layer. On those mornings you can look west across a sea of fog covering the Sunset and Richmond districts, with the sun behind you to the east. Ocean Beach can work too, when the fog lies just offshore and the sand is already in sunshine.
Fogbow Variations and Related Phenomena
Two related light effects show up in the same conditions. A glory is a small set of colored rings centered right on the antisolar point. When you look down into fog, it appears around the shadow of your own head. Many air travelers have seen one as a ring of color around the plane's shadow on the clouds below. From a Bay Area hilltop above the fog, a glory can appear around your shadow, sometimes inside a larger fogbow.
A corona is a ring of light around the sun or moon, caused by diffraction through thin cloud or fog. It appears around the light source you are facing, while a fogbow appears on the opposite side of the sky. On nights when the marine layer is thin enough to let moonlight through, you may see a pale corona around the moon.
Fogbows are also common at sea, and sailors have described them for centuries. Along the Northern California coast, a boat running along the edge of the marine layer on a sunny morning is in a good position to see one. On a ferry or a whale watching trip on a partly foggy morning, watch the fog on the side of the boat away from the sun.
Deep Dive: Why Is a Fogbow White Instead of Colored?
A fogbow is white because light behaves as a wave as well as a ray. In big raindrops, the wave effects are small and each color leaves the drop at its own angle. In tiny fog droplets, the wave effects spread each color over a wide band, and the bands overlap until the colors blend.
Why Do Wave Effects Matter for Small Droplets?
Simple ray optics, which bends each color by a set amount, explains the bright, clean rainbow seen in rain. In 1838, the British astronomer George Airy showed that this picture breaks down for smaller drops. His theory treated light as a wave passing through the drop, and it predicted that the bow widens as drops shrink.
By our rough calculation, light diffracting around a fog droplet 20 micrometers across spreads over about 1.6 degrees. That is close to the 2 degrees or so that separate red from violet in an ordinary rainbow. The colors smear into each other, and what reaches your eye is a broad, pale arc.
Simulations of Airy theory on the Atmospheric Optics website show the bow growing wider and whiter as the drops get smaller. Droplets smaller than about 0.05 millimeters leave only faint color, usually a reddish outer edge and a bluish inner edge. That size range is typical of Bay Area fog.
What Do the Faint Inner Bands Reveal?
Some fogbows show one or more faint bands just inside the main arc. These supernumerary bows come from light waves passing through a droplet by slightly different paths and interfering with each other. Where the waves line up, they brighten, and where they cancel, they darken.
Supernumeraries appear only when the droplets are nearly all the same size. If the sizes vary widely, each size puts its bands in a slightly different place and the pattern blurs away. A clear set of inner bands therefore means the fog formed evenly, with droplets that grew together under the same conditions.
French physicist Auguste Bravais studied fogbows as early as 1847, and later researchers refined the theory with more complete wave calculations. The basic lesson has held up. A fogbow is a measuring tool as well as a pretty sight, because its width and color tell you the size of the droplets around you.
Best Conditions to See a Bay Area Fogbow
The ideal setup is a summer morning when the marine layer is present but thinning. You want to stand in sunshine, at or above the top of the fog, with a fog bank in front of you and the sun behind you. A low sun works best, so the first two or three hours after sunrise give the best chances. As the sun climbs, the bow sinks toward the ground and soon disappears below your feet.
June through August mornings at the Marin Headlands, before about 9 a.m., are a reliable starting point. Before you go, check whether the fog is sitting offshore while the hills are clear. Once you are there, find your shadow on the fog and look for a broad, pale arc roughly 35 to 40 degrees away from it. The arc is faint, so give your eyes a moment to pick it out against the bright fog. If you want a photo, a wide-angle lens helps, because the whole bow spans about 80 degrees of sky. Phone cameras often wash out the faint arc, so lowering the exposure a little can bring it back.
