Science Explains Formation of White Rainbows

White rainbows, also known as fogbows, form when sunlight interacts with extremely small water droplets in the air [1, 2].

These phenomena provide insight into how light behaves differently depending on the size of particles in the atmosphere. Understanding the distinction between a standard rainbow and a fogbow helps meteorologists and physicists study the properties of mist and fog.

A standard rainbow appears when sunlight is refracted through larger raindrops, which separates the light into distinct colors. In contrast, a fogbow occurs when the water droplets are so small that diffraction blurs the individual colors [1, 2]. This process causes the colors to overlap and blend, resulting in a single white arc instead of a multicolored spectrum [1, 2].

Specific environmental conditions are required for a fogbow to appear. The Sun must be low in the sky, typically during the early morning or late afternoon [1, 2]. Additionally, the air must contain mist or fog [1, 2].

These events are most common in humid or misty environments. Examples include foggy valleys or areas over bodies of water, such as the Elterwater riverbank in North West England ^[1]. Because the droplets are so minute, the resulting white band is the primary visual characteristic of the event ^[1].

While a typical rainbow is a common sight after rain, the fogbow is a specialized version of the same optical process. The lack of color is not due to a lack of light, but rather the physical size of the droplets causing the light to scatter in a way that merges the spectrum [1, 2].