Extraordinary weather phenomenon: the white rainbow

The appearance of a rainbow lights up the sky but often also our mood. However, there is another type of arch, with a mysterious, even ghostly appearance: a more or less transparent whitish arch, which appears in rather bad weather conditions. This is the white rainbow, also called mist bow (bow of mist) by the Anglo-Saxons, or ghost rainbow (ghost rainbow)!

White arcs form in the same way as regular rainbows: they are generated by the reflection of sunlight at an angle of 40-42° within raindrops. Rainbows can also be of solar or lunar origin, they always appear in the opposite direction to the Sun or the Moon. The only difference between classic rainbows and white ribbons is the size of the water droplets. While the multi-colored arches are formed by quite large drops of water, the white arches are formed by very small droplets: fog or mist.

The tiny water droplets that make up fog are too fine (less than 0.1mm in diameter) to display the colors of the rainbow, so the result is a white arc. Compared to raindrops that give rise to a colored arc, fog drops that give rise to white arcs are 10 to 1,000 times smaller. The water droplets that make up the fog are then so fine and so close together that light cannot pass through. That is why the bow is not colored, or very weakly. The white stripes are bordered on the outside by a thin red stripe and on the inside by a barely noticeable thin blue stripe. White bows are usually similar in length to rainbow bows, but much wider.

White arches are more common in cool, humid regions

Without being really rare, white arches are also not very common. They can occur in all temperate and cold regions of the world, but are seen more in often gray and humid countries: the British Isles (Scotland and Ireland in particular) are among the areas where they are most frequently seen, but also the north . from France, the Scandinavian countries, the northeastern United States (Maine, Massachusetts, New York State…), New Zealand and Australia.

As for the weather, as well as the type of relief, the best conditions to see them are:

  • we have a light fog ahead, not too thick and quite low with a sunny space above;
  • being at the time of sunrise which is more prone to white arches than sunset;
  • being on a beach, the presence of the sea or the ocean providing the necessary humidity;
  • being in the mountains, overlooking a valley in the mist;
  • being in a cold or even icy region, such as the Nordic countries, because the air is usually saturated with suspended moisture, which does not freeze.

White arcs tend to be very short-lived, much longer than classic rainbows: they usually dissipate after a minute or two.

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