Lunar rainbow also called the moonbow is produced by moonlight. There are only five known waterfalls in the world that are congenial for the formation of a lunar rainbow with three of them being in the US and one each in Africa and Europe. The formation of a moonbow is a rare and unique event that happens at night and cannot actually be identified through naked eyes.
photo tip: It is all about chances as you keep rolling the camera throughout the night and click odd images in time-lapse mode.
Among the factors that determine the occurence of a lunar rainbow at a waterfall include a bright full moon, the amount of spray mist generated from the waterfall and the alignment.