When a rainbow is seen from the ground, it appears as a multicolored arc across the sky. This spectacle results from sunlight refracting, reflecting, and dispersing within atmospheric water droplets. The light separates into its constituent colors—red, orange, yellow, green, blue, indigo, and violet—creating the visible spectrum. Seeing only a partial bow is due to the observer’s low vantage point relative to the horizon. The view from a high altitude, such as from an airplane, changes the perspective dramatically, revealing the full geometric nature of the phenomenon.
The Geometry of the 360-Degree View
The fundamental physics of a rainbow dictates that it is always a complete circle, not an arc. A ground observer only sees an arc because the Earth’s surface obstructs the view of the lower portion. From an airplane, the elevated perspective eliminates this obstruction, making it possible to view the entire 360-degree circular phenomenon.
The center of the rainbow always lies on the antisolar point, the point directly opposite the sun from the observer. This point aligns with the shadow of the aircraft, which often appears on the clouds below. The primary rainbow is generated by water droplets at a consistent angular radius of approximately 42 degrees away from this antisolar point.
The complete circle is a consequence of this fixed 42-degree angle of deviation for red light after one internal reflection within the water droplets. This geometry means that any water droplet positioned 42 degrees from the antisolar point will send the dispersed red light back toward the viewer’s eye. Since this angular relationship is symmetrical in all directions, the resulting shape is a perfect circle when the view is unobstructed.
Necessary Atmospheric and Light Conditions
To witness a circular rainbow from an aircraft, a specific alignment of atmospheric and solar conditions must occur. The sun must be behind the viewer and relatively low in the sky for the full circle to be visible above the horizon. If the sun’s elevation is higher than 42 degrees, the entire circular rainbow will be below the plane’s horizon and out of sight.
The most effective time for viewing is when the sun is below 30 to 40 degrees in the sky, during the earlier or later parts of the day. Water droplets are necessary and must be positioned below the aircraft, often as rain or a thick layer of clouds or mist. Light from the sun must penetrate these droplets and reflect back toward the observer at the specific 42-degree angle.
A full circular rainbow formed by raindrops is distinct from a “glory,” which is another common aerial optical phenomenon. A glory is a much smaller, multicolored ring that appears directly around the shadow of the aircraft on a cloud layer. Glories are caused by the backscattering of light by much smaller cloud droplets, while a true circular rainbow is caused by refraction and internal reflection in larger raindrops or mist.
Practical Tips for Spotting an Aerial Rainbow
Observing a full circular rainbow from an airplane is an infrequent event because the required conditions of sun angle, plane position, and localized moisture must align perfectly. Passengers seeking this sight should select a window seat on the side of the plane opposite the sun, ensuring the sun is behind them. The phenomenon appears in the direction of the aircraft’s shadow, usually cast upon a lower layer of clouds or falling rain.
The best chances often occur during the descent or climb phases of the flight, when the aircraft is flying through or near layers of rain or mist. Sometimes, instead of a vibrant circular rainbow, an observer might see a “fog bow” or “cloud bow.” This is a similar phenomenon but with fainter or ghostly white colors. This happens when the water droplets are much smaller, leading to greater light diffraction and the overlapping of colors.