Sky Oddities • Atmospheric Optics • Rainbow Phenomena
Rainbows and atmospheric arcs are among the most spectacular optical phenomena in the sky. From classic rainbows and double rainbows to full-circle rainbows and so-called fire rainbows, these colorful displays reveal how sunlight interacts with water droplets and ice crystals in Earth’s atmosphere.
TL;DR: What Causes Rainbows and Atmospheric Arcs?
Rainbows and atmospheric arcs form when sunlight is refracted, reflected and dispersed by water droplets or ice crystals. Different viewing angles, particle types and atmospheric conditions create classic rainbows, double rainbows, full-circle rainbows and colorful arc phenomena such as circumhorizontal arcs (“fire rainbows”).
Nature’s Most Famous Optical Illusion
Rainbows are among the most recognizable atmospheric phenomena on Earth, yet many people never learn how they actually form. The sky can produce not only ordinary rainbows but also double rainbows, complete circular rainbows, rainbow-like arcs in high clouds and rare optical displays that appear almost impossible.
While folklore often treats rainbows as symbols, bridges or treasures, atmospheric science reveals them as predictable consequences of how sunlight interacts with water droplets and ice crystals.
How Rainbows Form
A rainbow forms when sunlight enters a water droplet, bends as it passes into the droplet, reflects from the back surface and bends again as it exits.
This process separates sunlight into different colors because each wavelength bends by a slightly different amount.
- Red light bends less.
- Violet light bends more.
- Millions of droplets contribute to the visible arc.
- The observer’s position determines what rainbow is seen.
- No two people see exactly the same rainbow.
Rainbows are not physical objects in the sky. They are optical phenomena created by the geometry between sunlight, water droplets and the observer.
Explore Rainbow & Arc Phenomena
Fire Rainbows Explained
Learn why so-called fire rainbows are not true rainbows and how ice crystals create spectacular circumhorizontal arcs high in the sky.
Full-Circle Rainbows Explained
Discover why rainbows are actually complete circles and why most observers only see the upper arc from ground level.
Double Rainbows Explained
Understand how secondary reflections inside water droplets create a second rainbow with reversed colors.
Major Types of Rainbow Phenomena
Primary Rainbow
The primary rainbow forms from one internal reflection inside water droplets and displays the familiar color sequence from red on the outside to violet on the inside.
Double Rainbow
Double rainbows occur when light undergoes two internal reflections inside droplets. The secondary bow appears outside the primary bow and has reversed colors.
Full-Circle Rainbow
Every rainbow is actually a complete circle. Ground-based observers usually see only the upper portion because the horizon blocks the lower half.
Supernumerary Rainbows
Extra faint bands sometimes appear inside the primary rainbow. These are caused by wave interference and demonstrate the wave nature of light.
Fogbows
Fogbows resemble pale white rainbows and form when light interacts with tiny fog droplets rather than larger raindrops.
Circumhorizontal Arcs (“Fire Rainbows”)
These colorful displays are produced by sunlight passing through plate-shaped ice crystals rather than water droplets.
Rainbow Phenomena Comparison Guide
| Phenomenon | Main Cause | Appearance | Water or Ice? |
|---|---|---|---|
| Primary Rainbow | One internal reflection | Single rainbow | Water droplets |
| Double Rainbow | Two internal reflections | Secondary reversed rainbow | Water droplets |
| Full-Circle Rainbow | Standard rainbow geometry | Complete circular ring | Water droplets |
| Fogbow | Diffraction and scattering | Pale white bow | Fog droplets |
| Fire Rainbow | Ice crystal refraction | Bright colored arc | Ice crystals |
Why Rainbows Are Different for Every Observer
Rainbows do not occupy a fixed location in space. Every observer sees light coming from a unique collection of water droplets.
This means:
- You never see exactly the same rainbow as another person.
- The rainbow moves as you move.
- You can never physically reach the “end” of a rainbow.
- Aircraft passengers often see complete circular rainbows because there is no horizon blocking the lower half.
Why Rainbow Arcs Are Often Misidentified
Rare atmospheric arcs are frequently described as portals, sky rifts, alien clouds or mysterious energy phenomena. Circumhorizontal arcs, circumzenithal arcs and unusual rainbow displays often go viral because they appear too colorful or geometrically perfect to seem natural.
In reality, these displays are among the best-understood atmospheric optical phenomena and follow strict geometric rules controlled by sunlight, water droplets and ice crystals.
Best 301 Redirect Targets for Older Rainbow Articles
- Rainbow photos and sightings: redirect here.
- Fire rainbow articles: redirect to Fire Rainbows Explained.
- Circular rainbow stories: redirect to Full-Circle Rainbows Explained.
- Double rainbow posts: redirect to Double Rainbows Explained.
- General atmospheric optics content: redirect to Atmospheric Optics Explained.
Rainbows & Arc Phenomena FAQ
What causes a rainbow?
Rainbows form when sunlight is refracted, reflected and dispersed inside water droplets.
Why do double rainbows occur?
Double rainbows occur when light reflects twice inside water droplets before leaving them.
Are fire rainbows real rainbows?
No. Fire rainbows are circumhorizontal arcs produced by ice crystals, not water droplets.
Can rainbows be complete circles?
Yes. Every rainbow is a complete circle, although the horizon usually hides the lower portion.
Can you reach the end of a rainbow?
No. A rainbow is an optical phenomenon that changes position relative to the observer.
Are rainbow phenomena rare?
Ordinary rainbows are common, but full-circle rainbows, fire rainbows and certain atmospheric arcs are much less frequently observed.
