As an atmospheric optics enthusiast, I find sunbows fascinating. While we love the vivid colors of rainbows set against rain, there’s something mysteriously alluring about those fleeting fragments of colored light occasionally seen around the sun. What secrets lie hidden in those delicate bands of hue set against the blue sky? Let‘s uncover the optics and mythology behind the optical wonder that is the sunbow.
From Sea Spray to Solar Halos
Sunbows arise through the same principles of optics that give us rainbows – sunlight entering water droplets, refracted and dispersed into spectral colors. But instead of vertical rain, sunbows are born within the mist, spray and fog produced naturally near waterfalls, fountains and the sea, or artificially via sprinklers and fog machines.
The key difference is the random orientation of the droplets in mist or spray, as compared to the uniform vertical alignment in rain. This irregular dispersion affects the intensity and clarity of the colors produced. While rainbows stand out vividly against the sky, sunbow bands are typically more diffuse and overlap into each other. But occasionally, the combination of bright sun and dense uniform mist can result in spectacular sunbows to rival even the most intense rainbows.
More commonly though, we observe sunbows and their spectral magic as part of solar halos – rings of light encircling the sun. Produced by ice crystals high up in wispy cirrostratus clouds, these luminous circles are formed by refraction of sunlight through these hexagonal prisms of ice.
The 22° Halo and Sundogs
The 22° radius halo is the most frequent type that occurs around the sun (or moon). As sunlight enters the vertically aligned ice crystals, it is bent by 22° due to refraction governed by Snell‘s law. This phenomenon produces a circular band of light with the sun at its center.
Often within this 22° halo we observe an additional optical effect – bright spots of light on either side of the sun called sundogs. Also known as parhelia or mock suns, these are formed by refracted sunlight passing through the preferentially horizontally aligned ice crystals. We may also see vertical sun pillars above and below the sun when the crystals are oriented vertically.
Beyond the 22° Halo
While the iconic 22° halo and sundogs are most common, cirrostratus clouds can produce a whole family of halos around the sun. We occasionally see:
- 46° halo – caused by refraction through randomly oriented ice crystals
- Circumscribed halo – passing through sun at an angle of 46°
- Parhelic circle – horizontal band passing through sun
- Supralateral and infralateral arcs
Moon halos can also occur, following the same principles but with much fainter colors due to the lower brightness of moonlight. Even more rarely, we may observe colorful optical companions to the moon like paraselenae or mock moons.
The Colors of Sunbows
When visible in solar halos, sunbow colors follow the same sequence we see in rainbows and other displays – red on the inside graduating through orange, yellow, green, blue, indigo to violet on the outside. Sometimes supernumerary arcs are also seen blending extra spectral hues.
In a vivid sunbow matching a rainbow, we observe the complete color spectrum. But more commonly, only fragments are visible like reddish on the inside, with orange and yellow towards the outside. The exact hue also depends on sun elevation and viewing angle.
Sundogs and Parhelia
Sundogs showcase another sunbow revelation – under certain conditions, we observe just a splash of spectral color. Parhelia are the colored spots on either side of the sun that produce sundogs. The redder zone is closest to the sun, with blue and violet farther out due to the principles of chromatic dispersion.
This separation of colors is based on wavelength – red light is refracted least and violet most. We can relate this to how prisms disperse white light into the visible spectrum, and rainbows showcase the spectral sequence.
Beyond the Optical Spectrum
Looking beyond visible light, sun-related optical effects extend into infrared and ultraviolet. But our eyes can only perceive the visible color bands. Advanced imaging techniques reveal that halos and sundogs stretch farther out with infrared inside the red and ultraviolet beyond the violet.
There are also effects like infrared glory and ultralumen that only occur in infrared or UV. This just shows how the optical magic of sunbows is only a partial glimpse into the full spectacle of how sunlight interacts with water and ice in the atmosphere.
Rare Spectacles
While 22° solar halos with traces of color are common, fully formed vivid sunbows are rare and ephemeral. It takes the perfect alignment of bright sunlight, optically dense mist or spray, and ideal viewing angle. But sometimes, nature reveals a magical glimpse of what’s possible.
Some of the most brilliant sunbow photographs showcase doubled or multiple rainbow rings around the sun. These rainbow-like solar halos are formed when conditions allow sunlight to be refracted through two overlapping sets of water droplets or ice crystals. Each produces its own circular sunbow, with red on the inside. Even more elusive are the fabled twinned or double sundogs gracing either side of the sun.
Hunting for Sunbows
The erratic nature of sunbows makes them challenging to observe. But some locations offer better odds, like mountain peaks and lookouts above low-lying fog and mist, or near waterfalls and fountains on bright sunny days. Specific weather conditions known to generate halos are also ideal for spotting colorful sunbow fragments.
Your best chance is around an hour after sunrise or before sunset, with the sun at least 15 degrees above the horizon. Position yourself with sunlight on your back and look directly opposite the sun. Exploring optical phenomena is an exercise in patience, persistence and optimism. But just the possibility of witnessing one brilliant sunbow makes all the effort worthwhile.
Norse Bifrost and Other Myths
Cultures worldwide have mythologized the fleeting magic of sunbows and halos. In Norse legend, the glowing ring around the sun was Bifrost, the bridge connecting earth and the heavens. Native Americans considered the colors a living symbol of dreams and the great mystery. Greeks saw the goddess Hera‘s veil adorned with sunbows on her wedding day.
African, Asian, Biblical, Celtic and Far Eastern cultures also imbued sunbow mythos with symbolism tied to deities, afterlife and divinity. The transitory nature of these optical wonders inspired imaginative stories aimed at making sense of their ephemeral beauty and mystery.
Studying Sunbows
Scientific study of atmospheric optical effects dates back millennia, with accounts by Aristotle in 340 BCE describing two mock suns flanking a halo. Rene Descartes‘ 1637 treatise on optics helped explain the 22° halo, with water droplet and ice crystal theories following centuries later.
In 1808, French physicist Etienne Louis Malus studied parhelia and noted their polarization. George Airy anatomized the pure spectral colors of sunbows. Other pioneers include Joseph von Fraunhofer, John Tyndall and Lord Rayleigh. Aristotelian ray tracing evolved into Mie and Rayleigh scattering explanations.
Today, sunbows and halos are well-documented atmospheric phenomena. But their fleeting, elusive beauty means we keep gazing skywards, hoping to witness nature‘s majestic optical magic in action. When the clouds and sunlight align just right, the atmosphere becomes a canvas for nature‘s paintbrush.
Conclusion
While less vivid than rainbows, sunbows and related optical effects like halos, sundogs and glories showcase the wondrous interactions of light and water. Nature‘s canvas may only reveal misty brushstrokes of color, but they help illuminate the principles of optics, light and perception that have fascinated scientists for ages.
And as symbols in culture worldwide, transient sunbows remind us of the fine balance between light and darkness, dreams and reality. Their ephemeral beauty inspires a sense of awe and wonder. No matter how much science explains their creation, sunbows remain magical gifts from above.
