A partial rainbow is not a unique optical phenomenon, but simply a visible portion of a larger, full circle. A rainbow is created when sunlight enters water droplets, refracts, reflects internally, and then refracts again upon exiting, dispersing the light into its constituent colors. The colored arc you observe is visually centered on the antisolar point, which is the point directly opposite the sun from your perspective. Because the full light effect is a complete 360-degree circle, the limited section visible from the ground is most accurately called a rainbow arc or a rainbow segment.
Defining the Common Terms for Rainbow Segments
The most common, non-technical term for what observers see is a rainbow segment or a rainbow fragment. These names correctly identify the visible display as only a piece of the primary or secondary rainbow arc. The segment’s bright colors are produced by the same physics as a full arc, namely the angular separation of light that occurs inside raindrops.
The popular phrase “pot of gold” at the end of the rainbow contributes to the idea of a “partial” rainbow. This happens when the lower portion of the arc appears to touch the ground. The arc only appears to “end” because the necessary illuminated water droplets are no longer present below a certain height, or the view is obstructed by the landscape.
The term “rainbow segment” emphasizes that the phenomenon itself is complete, but the observer’s view is incomplete due to environmental factors. Meteorologists typically refer to the visible manifestation as a primary arc or secondary arc, often with qualifiers like “incomplete” or “fragmentary” if only a small portion is visible.
The Geometry of Incomplete Visibility
The reason a rainbow is typically seen as a partial arc is purely a matter of geometry and perspective. A rainbow is always a full circle, but from the ground, the horizon cuts off the lower half of this circle. The center of the rainbow circle, known as the antisolar point, is always exactly opposite the sun and on the line extending from the sun through the observer’s head shadow.
For the primary rainbow to be visible at all, the sun must be positioned at a low angle, specifically less than 42 degrees above the horizon. The maximum possible arc seen from the ground is a semicircle, which occurs only when the sun is right on the horizon line. As the sun rises higher, the visible arc sinks closer to the horizon, and less of the segment is visible.
Any obstruction in the viewing area, such as trees, buildings, or mountains, will further block the lower portion of the arc, resulting in a smaller, more fragmented segment. To see the full circular rainbow, an observer must be at a high vantage point, like an airplane or a tall building, looking down onto the illuminated water droplets below. This elevated position allows the full 360-degree phenomenon to be visible since the horizon no longer blocks the lower half of the circle.
Distinguishing Separate Atmospheric Optical Phenomena
Many colorful atmospheric displays are mistakenly identified as partial rainbows, but they are entirely separate optical events. One such phenomenon is the Sun Dog, or Parhelion, which appears as a bright spot of light on one or both sides of the sun. Sun Dogs are caused by the refraction of sunlight through plate-shaped hexagonal ice crystals suspended in high-altitude cirrus clouds, rather than liquid water droplets. They remain at a fixed angular distance of approximately 22 degrees from the sun.
Fogbows, sometimes called white rainbows, appear as arcs caused by the diffraction of light through extremely small water droplets found in fog or mist. Because these droplets are much smaller than raindrops, the light is smeared out, preventing the clear separation of colors seen in a traditional rainbow. This results in an arc that is mostly white or very faintly colored, making it visually distinct from a rainbow segment.
Another group of phenomena often confused with partial rainbows are the Circumzenithal Arc and the Circumhorizontal Arc. These colorful displays are halo phenomena caused by ice crystals, similar to Sun Dogs. The Circumzenithal Arc appears high in the sky, well above the sun, and is often described as an “upside-down rainbow.” The Circumhorizontal Arc appears low and parallel to the horizon, but both require specific sun angles and correctly oriented ice crystals, separating their formation entirely from the water droplet-based rainbow segment.