A moonbow forms exactly like a daytime rainbow, but the light source is the Moon rather than the Sun. Moonlight — itself reflected sunlight — is refracted and internally reflected inside falling raindrops or waterfall spray, splitting into a spectrum and bouncing back at roughly 42° from the point opposite the Moon. Because the arc sits opposite the Moon, it is best around a bright full Moon when rain or spray hangs in the opposite part of the sky.
Worth knowing
A moonbow nearly always looks white or pale grey to the naked eye — not because it lacks colour, but because the light is too faint to trigger the eye's colour-sensing cone cells. A long-exposure camera gathers enough light to reveal the full rainbow spectrum the eye cannot.
Note
Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.
