A standard rainbow is a common optical spectacle, formed when sunlight is refracted, internally reflected, and dispersed by spherical raindrops, separating white light into its constituent colors. The familiar arc requires the sun to be behind the observer and the rain to be in front, with the angle of light reflection precisely focused at 42 degrees. The atmosphere occasionally produces variations that are profoundly rare, requiring an unlikely convergence of atmospheric physics and precise observer positioning. These unusual events range from bows defined by their unique structure to those created by an unconventional light source or filtered light.
Identifying the Rarest Structural Rainbow
The rarest structural form is the twinned rainbow, which is distinct from the more common secondary or “double” rainbow. A twinned bow appears when the primary arc splits into two separate rainbows that emerge from a single base, running parallel for a short distance before merging again. This optical event is highly unusual because it requires two different sizes of raindrops to be falling simultaneously within the same part of the sky.
The explanation lies in the way air resistance deforms water droplets as they fall. Smaller raindrops, approximately one millimeter in diameter, maintain a nearly perfect spherical shape, which produces a typical, singular primary rainbow. Conversely, larger raindrops flatten into a disk shape due to air pressure.
When a rain shower contains a mixture of both spherical and flattened drops, each shape reflects and refracts the light at a slightly different angle. This dual mechanism results in two distinct, yet overlapping, primary bows that appear to split from a common point. Unlike a secondary rainbow where the colors are reversed, the twinned bow’s arcs display the colors in the same order.
Other Extremely Uncommon Varieties
Other bows gain their rarity from the light source or the specific composition of the air. The moonbow, or lunar rainbow, is an arc created by moonlight instead of direct sunlight. They are exceedingly rare because the moon must be nearly full to provide sufficient illumination, and the sky must be intensely dark, usually limiting visibility to a few nights per month.
Moonlight is significantly dimmer than sunlight, meaning the light energy reaching the observer is low, which prevents the color-sensitive cones in the human eye from activating. As a result, moonbows often appear to the naked eye as a pale white or ghostly arc, though long-exposure photography can reveal the full spectrum of color.
Fogbows
The fogbow, sometimes called a white rainbow, requires the presence of extremely tiny water droplets. The microscopic size of the water particles found in fog, typically less than 0.05 millimeters in diameter, causes light waves to behave differently. Instead of the clear separation of color seen in a regular rainbow, the light waves diffracted by these small droplets overlap extensively, leading to a wide, faint, and nearly colorless appearance.
Red Rainbows
A completely different mechanism creates the monochrome or red rainbow, defined by its lack of the usual color spectrum. This bow occurs only when the sun is very low on the horizon, either at sunrise or sunset. At these times, the sunlight must travel a much longer path through the Earth’s atmosphere to reach the raindrops. Along this extended path, atmospheric scattering removes the shorter wavelengths of light, such as blue, green, and violet, leaving primarily the longer red and orange wavelengths to reach the observer. The resulting arc is a vivid red or orange band, with the other colors filtered out.
Specific Atmospheric Conditions for Rarity
The extreme rarity of these bows is determined by the precise atmospheric conditions required for their formation. The mechanism of light reflection and refraction is highly sensitive to the size and shape of the water droplets. For the common rainbow, the water drops must be perfectly spherical to produce a uniform, bright arc, whereas the twinned bow requires a specific, simultaneous mixture of spherical and flattened drops.
The exact angular position of the light source is also a limiting factor. A terrestrial observer can only see a rainbow when the sun is no higher than 42 degrees above the horizon. For the red monochrome bow, the angle is even more restricted, requiring the sun to be practically on the horizon to achieve the necessary filtering of shorter wavelengths.
Atmospheric clarity plays a considerable role, as the light rays must travel unimpeded from the source to the droplets and then to the observer’s eye. Even minimal haze or pollution can scatter the already faint light of a moonbow or fogbow, preventing the arc from forming with enough intensity to be visible. The convergence of perfect droplet size, a precise light angle, and a clear atmosphere makes these rare bows so fleeting and difficult to observe.