As far back as the time of Aristotle, it was known that both a bright Sun and a bright Moon could produce a rainbow effect.

Most moonbows – lunar rainbows – are seen in the spray produced by waterfalls

According to an article appearing on pages 3 – 5 of the Yosemite Association’s Spring 2007 quarterly journal, there are six conditions required for a moonbow to be readily visible.
  1. Clear sky
  2. Abundant mist and spray at the base of the fall
  3. Dark sky
  4. Bright moonlight
  5. Unobstructed moonlight
  6. Correct rainbow geometry
The best time of year to see moonbows in Yosemite National Park are in the evenings preceding a full moon during periods of peak snow melt – in April, May, and June.

The best place to see moonbows in YNP is from the paved area at the west end of the wooden bridge over Yosemite Creek near the base of Lower Yosemite Fall.

In order for the moonlight to strike the spray from Lower Yosemite Fall, the moon must rise above the mountains, domes, and cliffs.