Two weeks ago I went for a boat ride on Lake Superior out of Silver Bay, Minn. The reporter and I were doing a story on a new tour operation. Fog is common on the lake even during summer as warm, humid air blows across the 40-degree waters, gets chilled and condenses into low clouds.
This was a partly-foggy day with occasional clearings that brought welcome sunshine. While standing on the prow of the boat with the sun at my back, I soon became aware of a ghostly double-bow of light arcing from the water’s surface up and over the horizon. A fogbow!
Even though the fog between the prow and water’s surface was barely noticeable it was enough to create a gorgeous, color-fringed bow. That’s not all. Further down on the water directly opposite the sun I could make out my shadowed head surrounded by a multi-ringed halo of colored circles called a glory. It’s as close to angelic as I’ll ever be.
Like rainbows, fogbows are opposite the sun. To see one, the sun must be at your back and less than 30-40 degrees high (10 degrees is one fist held at arm’s length). Since I was standing on the front of the boat some 20 feet above the water, I could look down on the bow and see it below the horizon. On flat ground it would have never showed until later in the day when the sun’s altitude dropped to 40 degrees or less.
Unlike rainbows, which are formed by nice big raindrops and produce spectacular colors through reflection and refraction, fogbows are created by extremely tiny water droplets (0.0020 inches / 0.05 mm) that diffract sunlight. Sunlight enters the fog droplet and reflects off its backside just as it does in a rainbow, but instead of emerging as a neat spread of colors from red to violet, diffraction smears everything together to produce a diffuse, white band.
You do sometimes see a bit of color – a red outer edge and blue inner – but very rarely. The photo also shows a second bow below the primary fogbow called a supernumerary bow.
Supernumeraries form when light rays in the droplets overlap and interfere with each other. Imagine simultaneously dropping two stones a few feet apart in a still pond. When the waves meet, some will overlap and intensify (grow taller) while others cancel each other out. The same happens with light. Where wave crest meets crest, light’s intensified; where crest meets trough, the two cancel each other other and there’s darkness. The overall pattern looks something like a bullseye or series of bright and dark rings. The dual bows show the bullseye pattern well.
What about my (less than) glorious head? Glory formation is still not completely understood. Diffraction, reflection and backscattering of light by tiny droplets toward the light source (sun) all appear to be involved. Whatever the exact process, a glory is as beautiful as its name. If you’ve ever flown in a plane, you may have already seen one surrounding the plane’s shadow on the clouds. If not, watch for it on your next trip. And keep an eye out for fogbows on these cooler mornings as we transition from summer to fall.
Glories and fogbows can form anywhere there’s fog. That includes places like Mars, where water vapor occasionally condense as ground fog over low regions of the planet like the grand canyons of Valles Marineris. Just think of it – a future astronaut looks up from sample collecting to see a Martian fogbow and his helmeted head fringed by glorious rings of color!