The typical image of a rainbow is a majestic arc of color stretching across the sky. This familiar half-circle suggests the phenomenon is two-dimensional, but this perception is a consequence of our position on the ground. In reality, the optical event is always a full, three-dimensional circle. Being earthbound prevents us from seeing the complete shape of this atmospheric spectacle.
The Definition of a Full-Circle Rainbow
The complete form of this optical display is correctly referred to as a full-circle rainbow or a 360-degree rainbow. It is the exact same phenomenon as the familiar arc, resulting from sunlight being refracted, reflected, and dispersed by spherical water droplets. Seeing the full circle requires an observer to be elevated high enough so the horizon does not block the lower portion of the light cone.
It is important to distinguish this true meteorological event from other circular atmospheric displays sometimes mistakenly given the same name. A glory, for example, is a smaller, multi-colored ring that appears around the shadow of an observer, often seen from an airplane. The glory has an angular diameter of only about 5 to 20 degrees, and its colors are caused by light wave interference and diffraction.
Another distinct display is a halo, a large circle of light centered on the sun or moon, not the anti-solar point. Halos are formed by the refraction and reflection of light through hexagonal ice crystals in high-altitude clouds, not liquid water droplets. The full-circle rainbow is characterized by its large angular diameter (approximately 84 degrees across) and always has the observer’s shadow directly at its center.
The Geometry That Makes Rainbows Look Like Arcs
The circular shape of a rainbow is a result of precise geometry dictated by the physics of light interaction within water droplets. Sunlight enters a spherical raindrop, refracts, reflects off the back inner surface, and then refracts again upon exiting the droplet. This process separates the white light into its constituent colors.
The most intense light returns to the observer’s eye at a specific angle relative to the incoming sunlight, creating a cone of light. For the primary rainbow, this angle is consistently about 42 degrees. Every raindrop positioned at this exact 42-degree angle relative to the observer’s eye and the sun contributes to the visible bow.
Because the sun’s rays are parallel, the water droplets reflecting light back at the 42-degree angle form a perfect circle around the anti-solar point. This point is directly opposite the sun in the sky and always coincides with the shadow of the observer’s head. When a person stands on the ground, the horizon acts as a flat plane that cuts the cone of light, obscuring the lower portion of the circle. The higher the sun is in the sky, the lower the anti-solar point sinks toward the horizon, meaning less of the circle is visible. If the sun is higher than 42 degrees above the horizon, the entire rainbow circle is below the ground and cannot be seen.
Observing the Complete 360-Degree Phenomenon
To see the full 360-degree circle, the observer must be positioned above the layer of rain or mist, eliminating the ground as an obstruction. The most common way to view a full-circle rainbow is from an airplane flying above a cloud layer or a rain shower. From this elevated perspective, the observer looks down onto the water droplets, allowing the entire circular cone of light to be visible.
Passengers flying at typical cruising altitudes have an optimal vantage point, provided the sun is behind the aircraft and low enough in the sky. The center of the full circle aligns precisely with the shadow of the airplane cast onto the clouds below. This alignment confirms that the rainbow is an observer-centric phenomenon, meaning its position is unique to the viewer.
While aircraft offer the clearest view, the phenomenon can also be observed from very tall buildings, high mountain peaks, or deep canyons. The observer must be looking down on a mist or rain layer with the sun at their back. A full-circle rainbow can also be seen in localized, high-pressure water sprays, such as the mist generated by large waterfalls or industrial irrigation systems, where the observer is elevated relative to the spray source.