Extraordinary meteorological phenomenon: the white rainbow

The appearance of a rainbow illuminates the sky but often also our mood. However, there is another type of arc, with a mysterious, even spectral aspect: a more or less transparent whitish arc, which appears in rather unfavorable weather conditions. This is the white rainbow, also called arc of fog (arc of fog) by the Anglo-Saxons, or ghost rainbow (ghost rainbow)!

White arcs are formed in the same way as normal rainbows: they are generated by the reflection of sunlight at an angle of 40-42 ° inside the 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 flakes is the size of the water droplets. While multicolored arcs are formed with fairly large water droplets, white arcs are formed with very small droplets – fog or mist.

The small water droplets that make up the 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, those of fog that give rise to white arcs are 10 to 1,000 times smaller. The water droplets that make up the fog are therefore so fine and so close to each other that light cannot pass. This is why the bow is not colored, or very faintly. The white stripes are bordered on the outside by a thin red fringe and on the inside by a barely noticeable thin blue fringe. White arches are often similar in length to rainbows, but much wider.

White arches are more common in cold and humid regions

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

In terms of weather, as well as the type of relief, the best conditions to see them are:

  • have a light fog in front, not too thick, and low enough with a sunny space above;
  • being at the time of dawn that is more prone to white arches than at sunset;
  • being on a beach, the presence of the sea or ocean that provides the necessary humidity;
  • being in the mountains, looking out over a valley in the fog;
  • being in a cold or even frozen region, such as the Nordic countries, because the air is often saturated with suspended humidity, which does not freeze.

White arches often have a very short life, much longer than classic rainbows: they usually dissipate after a minute or two.

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