White misty rainbow: how a rare phenomenon arises

Recently, Melvin Nicholson, a British photographer, stumbled upon a snow-white rainbow hovering over the swampy Scottish plain of Rannoch Moore. As he himself says in an interview with ABC News, “When the sun rose and the fog cleared, I saw a dazzling halo in the sky. “I haven’t seen anything like it in the 10 years that I have been photographing landscapes around the world.”

White rainbows, which most often look like an ordinary but discolored rainbow, appear due to fog that is thin enough so that the visible part of the spectrum of sunlight can pass through it. Often, the observer can see a red or blue tint at the edges, while the center remains clean. Why is that? The fact is that any rainbow appears after light falls on drops of water hanging in the atmosphere. “These are droplets of fog that are much smaller than regular raindrops, ” explains Brian Jackson, a meteorologist at the National Meteorological Service. A rainbow is formed due to the fact that these drops refract the rays of light, dividing the beam into different colors of the spectrum. Part of the light is reflected from the back wall of the drop in the direction of our gaze, which is why we see different colors.

Visual diagram of how a beam of sunlight is reflected from the back wall of a drop of water

The rays of the violet spectrum bend most of all, and the red ones remain practically unchanged. This means that the rays of red light, which are at a sharper angle to the ground and refracted by the topmost layer of fog, are most likely to get into your eyes. Therefore, red is always located on the outside of the rainbow stripes. The arcuate shape of the rainbow is also a consequence of the fact that at this angle a person sees the most rays. But in the case of foggy rainbows, the colors just blur. Drops of fog are about 100 times smaller than rain drops, and therefore most of the light is scattered or refracted, but only a small part remains in the visible spectrum. “Due to the small size of the droplets, light waves spend too little time inside them and do not have time to completely refract, " Jackson says. Thus, in most cases, the colors overlap, which is why a white glow appears.

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