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Moonbows are formed by light from the moon and showers or storms at night. Double moonbows are possible when moonlight and moisture is abundant. Colors are less vibrant at night because there is less light available.
Most of us have seen rainbows or even double rainbows illuminating the muggy sky after a thunderstorm, but there's a good chance you haven't seen a moonbow.
The way we see rainbows or moonbows depends on how light travels from the sun to the moon through individual raindrops to our eyes.
The sun is not the only source of light that can create these bows.
We see "rainbows" quite often, whether an actual arch of colored raindrops or the reflection off of your prismatic bike, glass of water, phone or car.
Lunar rainbows, or moonbows, form in the very same way as the "ROY G. BIV" rainbows we are taught about in school – except the light comes from the moon. Rainbows and moonbows are all about location.
The moon can create rainbows if the light reflected is bright enough and there's sufficient moisture in the right spot in our atmosphere.
Our moon must be nearly full in order to provide enough light for moonbows to form. Even as a full moon, our natural satellite doesn't provide nearly as much light as the sun.
The light that is sent from the moon toward our atmosphere refracts, or bounces, through droplets of rain. This light is separated at different angles in the raindrop, like a prism into multiple colors, where warmer colors like red and orange correspond to longer wavelengths and cooler colors such as blue and purple correspond to shorter ones.
If the droplets are too small, like in fog or mist, the colors are less able to separate completely and a white or gray fogbow will form. Fogbows can form overnight if there's enough moonlight and moisture in the air.
What about double rainbows, which occur when light bounces twice inside a raindrop? With additional bounces, this light comes out of the raindrop in the opposite array compared to the first bow and at a slightly higher angle in the sky.
In theory, if light refracts three or four times, you could get triple or quadruple rainbows with the colors reversed each time. This is far less likely in moonbows because the intensity of light needed for even a double moonbow isn't strong enough in most cases. Light is lost from the raindrop after each moonbow is created, so light for the second or third moonbow is only the light left over from the first bow.
As a result, colors exuded in a moonbow are not as vivid as those seen in a rainbow. The colors and sharpness may also be more diffuse, especially for waning or waxing moon phases.
You also have to be standing in the right spot to see it.
If you're on the ground, the moon (or the sun, for rainbows) must be 42 degrees above the ground, or the moonbow will be below the horizon.
According to Live Science, if you stick your fist out to arm's length, it's about 10 degrees in height. In other words, the moon must be within four fist-heights of the horizon in order to see a moonbow. The second moonbow is always located at about 52 degrees, or five fists high.
If you're trying to observe a rainbow or moonbow from a skyscraper, airplane or mountain, you might actually be able to see the whole rainbow, which is actually more circular than bow-shaped at higher elevations.
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