What is a Rainbow? | How Rainbows Form | Almanac.com

What is a Rainbow? | How Rainbows Form

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James J. Garriss
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What is a rainbow? Most everyone has seen a rainbow sometime in their life, but do you know how rainbows are actually formed? Here's an explanation.

What is a Rainbow?

To put it plainly, rainbows are reflections of sunlight through raindrops. As the light is reflected, it is refracted, which means that the direction of the light wave is changed. Different wavelengths of light, which we see as colors, bend at different angles and produce a rainbow's signature color banding, as seen in the photo below.

Light being refracted through a raindrop. Source: Wikipedia

Since sunlight is made of different wavelengths of light, we see the white light broken into an array of colors—the rainbow. The more the light bounces around, reflecting and refracting, the more types of rainbows there are.

Types of Rainbows

On December 17, 2015, rainbow scientist Jean Ricard concluded that there are 12 definitive types of rainbows.

Some of the most interesting types include the following:

  • Double rainbows: Two concentric rainbows appearing in the sky, with the higher rainbow's colors in reverse order of the lower rainbow's.
  • Twinned rainbows: Two rainbows appearing to stem from the same point—both presenting the typical ROYGBIV color ordering.
  • Circular rainbow: A fully circular rainbow, usually only seen from high vantage points such as skyscrapers or airplanes.
  • Monochrome rainbow: A rainbow that occurs when the sun is lower in the sky—such as at sunrise or sunset—and reflects more of one or two wavelengths than the others, making it appear monochrome.
  • Moonbow: A rainbow caused by the light of the Moon, rather than the Sun. These are typically quite dim and may even appear white in color.
  • Fogbows: A faint rainbow occurring within fog, usually over a body of water. 

The big debate is why rainbows are so different. In general, the scientists are divided between the “fatty” camp and the “low-life” camp. Most scientists have suggested that the size of the raindrops shape how they reflect light and what the rainbow will look like. Others have said that it depends where the raindrops are—since a low-lying haze of water will reflect at a different angle than a high shower of drops.

A rare winter rainbow.

What Ricard has shown with his research is that while both factors are important, what matters most is where the drops are—high or low in the sky. That’s why you see rainbows change as the raindrops fall. They can fade, brighten, split into double or multiple bows, be full circles or low arches.

If rainbows that form are too low, the thickness of the air makes it impossible to see the shorter waves of light—the purples and blues. The most low-lying droplets that are filtered through haze and smog finally filter out all but the long waves of red, producing a monochrome rainbow.

Monochrome rainbows are missing colors and may even be solid red. Source: Wikipedia

Rainbows become rare in winter, as ice scatters light instead of refracting it. So, as you sit through cold winters, dream of the all the types of rainbows that sparkle in summer skies!

Wondering where to see a rainbow? Here are the best places to look for a rainbows.