Yes, night rainbows, more accurately known as lunar rainbows or moonbows, do exist.
While less common and often fainter than rainbows caused by sunlight, lunar rainbows occur when moonlight refracts and reflects off water droplets in the air, similar to how sunlight creates daytime rainbows. The primary difference is the light source: the moon instead of the sun. Because moonlight is significantly weaker than sunlight, lunar rainbows are often difficult to see with the naked eye and may appear white or pale grey. Long exposure photography can often reveal the colors that are too faint for human vision to discern.
What Conditions Favor Lunar Rainbows?
Several factors contribute to the formation of lunar rainbows:
- A Full or Nearly Full Moon: A brighter moon provides more light to create a more visible lunar rainbow.
- Dark Skies: Away from city lights, the faint light of the moonbow is easier to see. Light pollution significantly reduces visibility.
- Water Droplets in the Air: Rain, mist, or spray from waterfalls can provide the necessary water droplets.
- Moon Position: The moon needs to be low in the sky (less than 42 degrees above the horizon) on the opposite side of the viewer from the rainfall.
Examples of Locations Where Lunar Rainbows Can Be Seen:
- Cumberland Falls, Kentucky, USA: Often referred to as the "Niagara of the South," Cumberland Falls is renowned for its frequent lunar rainbows.
- Yosemite National Park, California, USA: Waterfalls in Yosemite, particularly during peak runoff season, can produce lunar rainbows.
- Victoria Falls, Zimbabwe/Zambia: The massive spray cloud created by Victoria Falls makes it a prime location for lunar rainbows.
- Waimea, Hawaii: Rain combined with full moons create opportunities.
Why Are They Harder to See?
The relative dimness of moonlight compared to sunlight makes lunar rainbows more elusive. Human eyes are less sensitive to color in low light conditions, so lunar rainbows often appear as a whitish arc. It is only through long exposure photography that the colors are truly revealed.
