What could be more picturesque than a rainbow?
How about a Moonbow? Or more correctly; a lunar rainbow!
As many of you will know, a rainbow forms as a result of the refraction of sunlight as it passes through water droplets in the sky. The refractive properties of the droplets cause sunlight to be split into a band of colours, more specifically the light spectrum.
For a Moonbow, the principles behind its formation are quite the same. The only exception is the light source is no longer the Sun, but the Moon.
While most of us will have seen a rainbow at least once in our life time, a much fewer number will have had the pleasure to encounter the lunar equivalent. This is because, as the Moon is more variable than the sun, many conditions need to be just right.
1- Moonbows are best seen when the Moon is conspicuous in the sky, so the period around the full moon, and more particularly before and after the gibbous phase is particularly favourable.
2- Of course, the brightest Moon thinkable will not cause a moonbow on its own accord. Moisture in the air also a crucial component. The best nights to see a moonbow is when there is rain coupled with intermittent clear skies.
3- The last main consideration is the position of the Moon in the night sky. Acredited to the atmospheric optical parameters of water droplets; a low Moon altitude is desired. If the Moon is greater than 42 degrees above the horizon, no bow will form.
These three things are the basic necessities for the formation of a lunar rainbow. While they may sound like reasonable considerations individually, it is quite a different situation when you need all three of these to work together at the same time. As a result, this phenomenon is really quite rare.
The best places to witness this spectacle are said to be the Scottish Highlands and The Hawaiian Islands as well as Ireland and the United Kingdom.
So, if you ever think it is a Goldilocks night, in which all the conditions are right- get outside and see if you can catch a glimpse of this amazing phenomenon.
-Jean
Photo courtesy of Rob Ratkowski of Hawaii.
Photo link: http://all-that-is-interesting.com/post/8779470889/seven-bizarre-and-beautiful-natural-phenomena/2
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