A straight rainbow, more accurately known as a circumhorizontal arc, appears due to the way light interacts with ice crystals. This is different from how regular rainbows form.
What is a Circumhorizontal Arc?
A circumhorizontal arc is an optical phenomenon that manifests as a bright, colorful band, often seen parallel to the horizon. It's commonly referred to as a "horizontal rainbow," even though its formation process differs significantly from that of a regular rainbow.
How Does a Circumhorizontal Arc Form?
Unlike a regular rainbow, which results from light refracting and reflecting within raindrops, a circumhorizontal arc arises from:
- Light passing through ice crystals: These ice crystals must be plate-shaped and oriented horizontally.
- Specific Sun angle: The sun must be high in the sky, at an altitude of 58 degrees or greater.
- Refraction: Sunlight enters a vertical side face of an ice crystal and exits from the flat bottom face. This refraction separates the light into its component colors, creating the arc.
Key Differences from a Regular Rainbow
| Feature | Circumhorizontal Arc | Regular Rainbow |
|---|---|---|
| Formation | Light through ice crystals | Light through raindrops |
| Orientation | Horizontal, parallel to horizon | Arced, opposite of the sun |
| Sun Position | High in the sky | Lower in the sky |
| Color Order | Red on top, blue on bottom | Red on outside, blue on inside |
Practical Implications
- Observation Timing: Circumhorizontal arcs are more common during summer when the sun is high.
- Atmospheric Conditions: They require high-altitude cirrus clouds composed of ice crystals.
- Rarity: Because of their specific formation conditions, circumhorizontal arcs are considered less common than regular rainbows.
In Summary
A "straight rainbow," or circumhorizontal arc, is not a rainbow formed by raindrops but rather an optical phenomenon created when sunlight passes through horizontally oriented ice crystals in high-altitude clouds. This process refracts the light, splitting it into the colors of the spectrum.
