Stunning Rainbows and Their Less-Known Optical Cousins
Explore the science and artistry behind rainbows, halos, moonbows, and more, revealing nature’s most colorful optical spectacles.
Stunning Images of Rainbows — and Their Less Famous Cousins
Nature’s most vibrant spectacles often appear fleetingly and unexpectedly, enchanting observers with arcs of color and mysterious optical effects. The rainbow, so iconic it has become a universal symbol of hope and beauty, is merely the most famous member of a wider family of atmospheric wonders. Many of these phenomena — from moonbows and fogbows to halos, glories, and fire rainbows — are seen only under just the right conditions, making them both rare and even more magical. This article explores the science, artistry, and diversity of these gems in the sky, inviting you to discover them anew.
What Exactly Is a Rainbow?
The rainbow is an arc of spectral colors that appears in the sky when sunlight interacts with water droplets in the atmosphere. The classic rainbow forms through:
- Refraction: Light bends as it enters a raindrop.
- Reflection: Light bounces internally off the back of the drop.
- Dispersion: Different colors spread apart due to varying wavelengths.
This combination results in the visible bands of red, orange, yellow, green, blue, indigo, and violet. Rainbows typically appear when sun shines onto rain clouds or mist in the late afternoon or early morning, when the sun is low in the sky.
The Diversity of Rainbow Relatives
Beyond the classic rainbow, nature regularly conjures up an array of lesser-known but equally mesmerizing optical effects. Each is rooted in the interplay between sunlight, water, ice, or atmospheric particles, and the unique angles at which they interact with our eyes.
1. Double and Supernumerary Rainbows
A double rainbow forms when sunlight reflects twice inside a raindrop before leaving and reaching the observer. This leads to a secondary arc outside the primary rainbow, featuring reversed colors — red on the inside and violet on the outer edge.
A supernumerary rainbow may sometimes appear as thin, pastel-colored bands just inside the main rainbow. These arise from interference rather than simple refraction, and are possible only when raindrops are particularly small and uniform.
| Phenomenon | Location | Key Feature |
|---|---|---|
| Double Rainbow | Worldwide | Secondary, reversed-color arc appears above the main rainbow |
| Supernumerary Rainbow | Worldwide | Extra fainter, pastel bands inside main arc |
2. Moonbows: Rainbows by Moonlight
The moonbow (or lunar rainbow) appears at night when a bright moon takes the place of the sun, illuminating raindrops much as sunlight does. Human eyes are less sensitive to color in low light, so moonbows often appear white or faintly colored to the naked eye, though long exposure photography can capture their full spectrum.
- Conditions needed: A bright, nearly full moon; rain in the opposite part of the sky; a dark, clear night.
- Most commonly seen: In places with frequent mist or waterfalls, such as Yosemite National Park.
3. Fogbows: Ghostly White Arcs
A fogbow resembles a rainbow but is much paler, often appearing as a white or faintly colored arc. These appear when sunlight shines through tiny water droplets in fog or mist. Because the droplets are smaller than those in rain, diffraction dominates over refraction, spreading the colors out so much they become nearly invisible.
What others are reading
- Also called: “White rainbows” or “ghost rainbows.”
- Often seen: Early morning fog, coastal regions, and mountains.
4. Glories: Colorful Rings Wrapped Around Shadows
Glories are brilliant colored rings that appear around the shadow of the observer’s head, often visible from airplanes when looking down on clouds, or on mountain tops peering over a sea of fog. Caused by the interaction of light with minute water droplets, glories display concentric circles of color centered around the shadow, sometimes called the “Brocken spectre.”
- Most visible: From high elevations, with the sun behind the observer.
- Distinct from halos: Glories are formed through complex processes involving both diffraction and backscattering.
Ice Crystal Phenomena: Halos, Sun Dogs, and Light Pillars
While rainbows require liquid water droplets, a different class of optical phenomenon arises from ice crystals in the upper atmosphere. These crystal wonders are most often seen in cold climates or high altitudes.
1. Halos: Symmetrical Rings Around the Sun or Moon
Halos are luminous circles that often appear around the sun or moon, caused by the refraction and reflection of light through millions of tiny, hexagonal ice crystals suspended in cirrus clouds high above the earth.
- 22° halo: The most common, forms a circle with a radius of approximately 22 degrees from the sun or moon.
- Occasionally: Halos may overlap or display colorful edges, with reds on the inside and blues outside.
2. Sun Dogs (Parhelia): Dazzling Twin Spots
Sun dogs or parhelia are bright spots that appear on either side of the sun, typically at the same elevation. Sun dogs are formed as sunlight passes through horizontally-aligned ice crystal plates, bending the light to focus it into concentrated spots of color. The effect is most pronounced when the sun is low.
- Colors: Often display vivid reds inside, fading to blue outside.
- Location: Best seen in cold regions or during winter in temperate areas.
3. Light Pillars: Towers of Light in the Sky
Light pillars appear as vertical beams of light rising above or below a luminous object, such as the sun, moon, or streetlights. They are created when flat, plate-like ice crystals reflect light vertically. While most often associated with the sun at sunrise or sunset, they can also accompany bright artificial lights on cold winter nights.
4. Circumhorizontal Arcs (Fire Rainbows): Bands of Color ‘on Fire’
Circumhorizontal arcs, sometimes called “fire rainbows,” are actually not rainbows at all, but sunlight refracted through horizontal ice crystals in cirrus clouds. These arcs run parallel to the horizon, well below the sun, and require precise alignment of high, wispy clouds and a high sun.
- Visible: Only when the sun is higher than 58 degrees above the horizon.
- Appearance: Intense, spectrum-colored bands; they do not form a bow.
5. Circumzenithal Arcs: The ‘Smiling Rainbow’
Circumzenithal arcs are among the most vivid and colorful of ice crystal effects, appearing as an upside-down, rainbow smile high in the sky above the sun. Requiring sunlight passing through flat, horizontally-oriented ice crystals, they fade quickly as the sun rises higher.
- Intensity: Can outshine real rainbows in vibrancy.
- Transience: Often fleeting, and easily missed.
Lesser-Known Wonders: Other Optical Phenomena
1. Coronae: Tiny, Colorful Halos Around the Moon and Sun
Coronae are small, colorful circles often seen tight around the sun or moon, produced by diffraction of light around small water droplets or tiny dust particles. Unlike halos, coronae display vivid rainbow-like colors focused closely about the celestial body.
2. Kelvin-Helmholtz Clouds: Waves in the Sky
While not a true optical phenomenon, Kelvin-Helmholtz clouds deserve mention for their visual spectacle. These rare clouds appear as rolling, breaking waves in the sky, reflecting different patterns of light and shadow to create a dynamic, painterly effect. They result from wind shear causing two air layers to travel at different speeds.
How to Spot These Atmospheric Marvels
Many of these phenomena share certain preconditions that make them more likely to appear:
- Low angle sunlight, either soon after sunrise or before sunset.
- Moisture in theair, whether as fog, rain, or high-altitude cirrus clouds.
- Clear viewing area with minimal artificial light or obstruction.
- In cold regions, look for halos, sun dogs, or pillars when clouds are wispy and temperatures dip below freezing.
Photographers and skygazers can improve their chances by consulting weather reports for rain showers or ice cloud formation and positioning themselves with the sun at their back or side, depending on the desired effect. Long exposure photography at night is essential for capturing moonbows and the subtle tints of fogbows.
Frequently Asked Questions (FAQs)
Q: Why do we rarely see the full circle of a rainbow?
A: Rainbows are actually full circles, but the ground usually obstructs the bottom half. From high vantage points, such as in an airplane, full circular rainbows are sometimes visible.
Q: What causes the reversed color order in double rainbows?
A: The secondary arc is formed as light reflects twice inside the raindrop, reversing the usual color sequence.
Q: Are halos exclusive to icy conditions?
A: Yes, halos are a direct result of ice crystals in cirrus or cirrostratus clouds refracting light, making them most common in cold weather or at high altitudes.
Q: Can fogbows show colors like rainbows?
A: Fogbows can show very faint colors, but they mostly appear white due to the small size of the droplets responsible for diffusing the light.
Q: Where is the best place to see fire rainbows or circumhorizontal arcs?
A: Fire rainbows are a misnomer for circumhorizontal arcs, which require the sun to be high and cirrus clouds present. They are most common at latitudes below 55º in the summer months.
Appreciating the Science and Beauty
The next time an arc of color appears against the sky, consider the intricate processes and rare alignments that make these displays possible. From the commonplace to the rare, the world of rainbows and their optical siblings reveals a tapestry woven from light, moisture, and crystal. Whether you’re gazing at a fogbow on an early morning hike or catching a fleeting circumzenithal arc, you’re witnessing one of nature’s most exquisite collaborations of physics and art.
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