The rainbow-like light phenomenon observed around the sun, often appearing as a bright, circular, or arc-shaped cloud with colors, is commonly known as a sundog or parhelion.
Understanding Sundogs and Parhelia
A sundog, scientifically termed a parhelion (plural: parhelia), is a captivating atmospheric optical phenomenon that creates bright, often colorful patches of light in the sky, typically seen to the left or right of the sun. While often described as a "circular rainbow around the sun," sundogs are usually specific, luminous spots or arcs, often at the same altitude as the sun, appearing as if there are "mock suns" alongside the actual sun.
How Do These "Rainbow Clouds" Form?
The formation of sundogs is a fascinating interplay of light and ice:
- Ice Crystals: This phenomenon occurs due to the presence of countless tiny, hexagonal ice crystals suspended in the atmosphere. These crystals are typically found in high-altitude, wispy clouds like cirrus or cirrostratus clouds.
- Sunlight Interaction: As sunlight passes through these ice crystals, it undergoes a process called refraction. This means the light bends as it enters and exits the crystals.
- Reflection and Dispersion: In addition to refraction, the light also experiences internal reflection within the crystals and dispersion, which separates white sunlight into its constituent colors, much like a prism. This is why sundogs display rainbow-like hues, with red typically closest to the sun.
- Crystal Orientation: For a sundog to be visible, the hexagonal plate-shaped ice crystals must be oriented horizontally as they fall through the air. This specific alignment directs the refracted sunlight towards the observer's eyes, creating the bright patches.
Key Characteristics of Sundogs
- Location: Sundogs typically appear about 22 degrees to the left or right (or both) of the sun, at the same elevation as the sun.
- Color: They often exhibit iridescent colors, usually brightest on the side closest to the sun (reddish) and fading to blue or white further out.
- Shape: They appear as bright, sometimes elongated, patches or "mock suns."
- Conditions: They are most commonly observed when the sun is low in the sky, during sunrise or sunset, and when high cirrus clouds are present. They can occur in any season but are particularly common in colder climates where ice crystals are prevalent.
Sundogs vs. Solar Halos: A Quick Comparison
While the term "circular rainbow around the sun" might also bring to mind a full solar halo, sundogs are distinct phenomena that are often observed as bright spots on or adjacent to a larger 22-degree halo.
| Feature | Sundog (Parhelion) | 22-Degree Solar Halo |
|---|---|---|
| Appearance | Bright, often colored patches/spots to the sides of the sun | A complete, faint, rainbow-colored circle around the sun |
| Location | 22 degrees left/right of the sun, at the same altitude | A full circle 22 degrees from the sun |
| Primary Cause | Refraction through horizontally oriented plate ice crystals | Refraction through randomly oriented hexagonal column ice crystals |
| Visibility | Common, especially when sun is low | Common, can be seen any time of day with cirrus clouds |
Observing the Phenomenon
To best observe a sundog, look towards the sun when it is low on the horizon, especially if thin, wispy clouds are in the sky. Always exercise caution and protect your eyes when looking near the sun, using appropriate sun-filtering glasses or by blocking the sun with your hand or a building.
