A white rainbow is a rare atmospheric optical phenomenon, technically known as a fogbow or a cloudbow. This ethereal arc appears similar in structure and geometry to a common rainbow but lacks the vibrant spectrum of colors. A fogbow is predominantly white, sometimes displaying only very faint, muted colors along its edges. The unusual appearance is entirely dependent on the specific size of the water droplets that create the phenomenon. It is an interaction of sunlight and moisture in the atmosphere.
The Role of Fog and Mist in Formation
The existence of a fogbow depends on the presence of fog, mist, or a low-lying cloud. Unlike the familiar rainbow, which requires large, distinct raindrops to form, the white rainbow relies on extremely small, suspended water droplets. These minute droplets are characteristic of ground-level fog and mist, where the water remains airborne.
The overall geometry of the phenomenon remains consistent with a traditional rainbow, requiring the observer to have the sun at their back. The white arc will appear in the direction opposite the sun, centered on the antisolar point. For a fogbow to be visible, the light must pass through a sufficiently dense and extensive bank of fog or mist.
Why Droplet Size Eliminates Color
The reason a fogbow appears white is rooted in the physics of light interacting with tiny water particles. A standard rainbow is formed when sunlight enters large raindrops, is internally reflected, and then exits, with the water acting like a prism that separates the light into its distinct color wavelengths through refraction. This process is highly dependent on the droplet size, which must be around 1 millimeter or larger for a well-defined color separation.
Fog droplets are substantially smaller, typically less than 0.05 millimeters (50 micrometers) in diameter. When light encounters droplets this small, the primary physical mechanism shifts away from refraction and internal reflection toward diffraction. Diffraction is the bending and spreading of light waves as they pass around the edges of an obstruction.
With these minute droplets, the light waves spread out significantly, causing the distinct color bands to overlap considerably. This blurring recombination of all visible wavelengths results in the appearance of white or a pale, diffuse arc. In the best viewing conditions, faint traces of red may be visible on the outer edge and blue on the inner edge of the white arc.
Best Times and Places for Observation
Spotting a fogbow requires a precise combination of sunlight and moisture, often making it a fleeting and localized event. The most favorable viewing times are typically early morning or late afternoon when the sun is low in the sky, generally less than 30 to 40 degrees above the horizon. A low sun angle ensures the arc of the bow is high enough to be seen against the fog bank.
Geographical locations prone to consistent fog or mist provide the best opportunities for observation. Coastal regions, mountain valleys, and areas near large bodies of water are ideal settings. Look for instances when bright sunshine is just beginning to break through a thick layer of fog. The fogbow will be most visible when the sun is shining brightly from behind the observer.