fogbow

Fogbow

A fogbow is a pale, rainbow-like arc that forms when sunlight is scattered by tiny water droplets in fog or mist. It appears opposite the sun, as the observer looks into the fog with the sun behind them.

Formation and appearance: Light is refracted, reflected inside each droplet, and refracted again on exit. When the droplets are very small (on the order of a few micrometers to a few tens of micrometers), the dispersion of wavelengths is limited, causing the colors of a rainbow to be washed out. The result is a broad, faint bow that is usually white or only lightly tinted. The angular radius is roughly the same as a rainbow’s, about 40 to 42 degrees, but the inner edge is diffuse and the overall contrast is weak. In favorable conditions, a fogbow can form a near-circular arc around the antisolar point.

Conditions and occurrence: Fogbows typically occur in fog or mist with some sunlight, often when the sun is relatively low in the sky (early morning or late afternoon). They are commonly observed in coastal regions, valleys, and other places where fog or low-lying mist is present near bright light.

Differences from a rainbow: The primary distinction is colorfulness. Rainbows are bright and vividly colored due to dispersion of light in larger raindrops, whereas fogbows are faint and largely colorless because of the small droplet size. The two phenomena share a similar geometry and arise from the same basic optical process, but the fogbow’s diffuse appearance makes it harder to detect.

See also: Rainbow, Halo, Glory, Atmospheric optics.