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There’s a good chance that you’ve already seen a rainbow in real life. The colorful result of sunlight being reflected and scattered after hitting water droplets, this prismatic phenomenon takes place during or after daytime showers. Interestingly enough, rainbows can show up at night as well. They’re rarer, though, and the conditions have to be just right for you to see them.

Lunar rainbows (or moonbows) form when light from the Moonโ€”which is reflected sunlightโ€”hits droplets of nighttime rain. The water droplets act as prisms that separate the moonlight at different angles. They form the same ROYGBIV (red-orange-yellow-green-blue-indigo-violet) configuration we see in rainbows.

How to spot a moonbow

Moonbows are harder to spot because they require specific meteorological and astronomical conditions.

First, the Moon has to be low in the sky (about 42 degrees above the ground) from where you’re standing.

The Moon must also be in its full phase (or as close to full as possible) to provide sufficient light for a moonbow.

There must be water droplets in the air, in the opposite direction of the Moon. Additionally, they must be big enough to separate the colors completely. When moisture is especially abundant in the atmosphere, a double moonbow may form.

Lastly, because there’s less light at night, colors are less vibrant. Thus, the sky has to be really dark for you to see a moonbow. This is also why moonbows typically appear white to us: The colors aren’t bright enough for our eyes’ receptors to perceive. You can, however, get a better, more colorful look at a moonbow with long exposure photography.

Tropical areas that experience nighttime showers tend to be where moonbows show up. They’re also more common in places with waterfalls, due to the increased presence of droplets in the air.


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Cover photo:ย Calvin Bradshaw (calvinbradshaw.com)

References

  • https://science.howstuffworks.com/nature/climate-weather/atmospheric/10-rainbow-myths6.htm
  • https://www.farmersalmanac.com/what-is-a-moonbow-34842
  • https://weather.com/science/weather-explainers/news/science-behind-lunar-rainbow-moonbow
  • https://www.timeanddate.com/astronomy/moonbows.html