Cloudbows

A cloudbow, also known as a fogbow or white rainbow, is an optical phenomenon that occurs when sunlight refracts through water droplets in a cloud or fog. Unlike a traditional rainbow, which is formed by raindrops, cloudbows are seen in fog or mist. The water droplets in fog are much smaller than raindrops, typically less than 0.05 millimeters in diameter. This small size causes diffraction to become a more significant factor in how light is scattered.

The small size of fog droplets means that the different wavelengths of light are not separated as distinctly as they are in a rainbow. Consequently, the colors in a cloudbow are much fainter and often indistinguishable, leading to the appearance of a white or pale arc. The light is scattered forward by the droplets, meaning the observer must be between the sun and the fog.

Cloudbows are most commonly observed when the sun is low in the sky, such as during sunrise or sunset, and when there is a dense fog or mist. They can appear around aircraft during flight, especially when flying through clouds or fog. The phenomenon is similar to a rainbow, but the lack of distinct color separation is the key characteristic. Mariners have also reported seeing cloudbows when sailing through fog banks. The underlying physics involves the interaction of light with small water particles, leading to a diffuse and often colorless arc in the sky.