A moonbow, also known as a lunar rainbow, is a rainbow produced by moonlight rather than direct sunlight. Other than the difference in the light source, its formation is the same as for a solar rainbow.
It is caused by the refraction of light in many water droplets, such as a rain shower or a waterfall. Moonbows are much fainter than solar rainbows, due to the smaller amount of light reflected from the surface of the moon. As a result, a moonbow often appears to be white.
Moonbows are most easily viewed when the moon is at or nearest to its brightest phase of full moon. For moonbows to have the greatest prospect of appearing, the moon must be low in the sky. This means they can only be observed two to three hours before sunrise or two to three hours after sunset. This combination of requirements makes moonbows much rarer than rainbows produced by the sun.