Exploring Rainbows and Moonbows

Caitlin Dempsey

Updated:

A rainbow is an optical phenomenon when light is refracts through moisture in the air. If you have ever shone light through a prism to view a rainbow, you are seeing the same phenomenon that happens when light hits droplets on water in the air.

A rainbow, therefore, is a product of the interaction of sunlight with water moisture in the air. As light from the Sun passes through a water drop or a prism, it is refracted as a color spectrum, producing a rainbow. There are seven colors in the spectrum of a rainbow: red, orange, yellow, green, blue, indigo, violet.

As light passes through a prism (left) or a drop of water (right), the light is refracted as a color spectrum.  Image: USGS, public domain.
As light passes through a prism (left) or a drop of water (right), the light is refracted as a color spectrum. Image: USGS, public domain.

While we typically see rainbows as arcs, they are actually full circles. The ground usually obstructs the bottom half, but from high vantage points like airplanes or tall buildings, it’s possible to see the complete circle.

Rainbow in Denali National Park.  Photo: NPS, public domain.
Rainbow in Denali National Park. Photo: NPS, public domain.

Rainbows always appear opposite the sun in the sky. Rainbows can appear anytime there are water droplets in the air, not just after a storm. Rainbows can appear near waterfalls where the presence of water spray combined with sunlight produces a rainbow.


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A rainbow appears in the spray from a waterfall.  A view of American Falls from the United States side of Niagara Falls. Photo: Alex Demas, USGS. Public domain.
A rainbow appears in the spray from a waterfall. A view of American Falls from the United States side of Niagara Falls. Photo: Alex Demas, USGS. Public domain.

What is a Double Rainbow?

A double rainbow happens when a second fainter rainbow appears offset from the first stronger rainbow. Double rainbows are caused by light being refracted twice inside each raindrop.

The colors in the second rainbow are in reverse order to the colors in the first rainbow. This means the red band will be in on the inside of the rainbow and end with violet on the outside.

Double rainbow seen from Lower Mammoth in Yellowstone National Park.  Photo:  Dan Hottle/NPS, public domain.
Double rainbow seen from Lower Mammoth in Yellowstone National Park. Photo: Dan Hottle/NPS, public domain.

The Sky Beneath a Rainbow is Lighter

Rainbows also refracts light into the area beneath the arc, making it lighter compared to the sky above the rainbow.

The sky between a double rainbow, however, is darker than the sky outside that band. The section of the sky between a double rainbow is known as Alexander’s band. This band is named after Alexander of Aphrodisias, a philosopher who was born in present day Turkey. Alexander first described this band in 200 C.E.

A rainbow arches over the Owyhee Canyonlands wilderness area.  Photo: ohn Wirt, USGS. Public domain.
A rainbow arches over the Owyhee Canyonlands wilderness area. Photo: John Wirt, USGS. Public domain.

Occasionally, you can observe additional faint arcs inside the primary rainbow. These are called supernumerary rainbows and are created by the interference of light waves.

Moonbows Happen at Night

While rainbows are the results of direct sunlight hitting water droplets in the air, moonbows (or lunar rainbows) are caused when sunlight reflecting off the moon is refracted by water droplets in the sky.

The only difference between a rainbow and a moonbow is the source of light. For a rainbow the source is direct sunlight. For a moonbow, the source is indirect sunlight radiated from the surface of the moon.

A moonbow is also known as a lunar rainbow, white rainbow, lunar bow, or space rainbow. Moonbows tend to be fainter than rainbows due to the weaker strength of the light source from the moon (compared to direct sunlight). The human eye struggles to discern color in low light, so moon bows can appear as white arcs. However, the colors can be captured in long-exposure photographs, revealing a spectrum similar to that of a daylight rainbow.

Moonbows always occur opposite the sky from the moon.

The conditions required for a moon bow to occur are quite specific:

  1. The Moon’s Phase: The moon needs to be near its full phase to provide enough light.
  2. Moon’s Position: The moon must be low in the sky, typically less than 42 degrees from the horizon.
  3. Weather Conditions: There must be rain or a mist opposite the moon from the observer’s perspective.
  4. Dark Skies: Light pollution can make it difficult to observe a moon bow, as it needs a dark night sky for contrast.

Due to these requirements, moon bows are quite rare and are usually only seen in places with clear skies and where water droplets are present in the air, such as near waterfalls or in certain meteorological conditions.

Moonbows in Yosemite National Park

Moon bows are often seen in specific locations where the conditions are just right. For example, Yosemite National Park in the United States and Victoria Falls on the border of Zambia and Zimbabwe are known for their occasional moon bows, thanks to the combination of waterfalls (which produce mist), clear skies, and minimal light pollution.

Moonbow at Yosemite Falls.  Photo: Screenshot from video produced by Yosemite Conservancy, CC BY 4.0
Moonbow at Yosemite Falls. Photo: Screenshot from video produced by Yosemite Conservancy, CC BY 4.0

During the spring and summer in Yosemite National Park when the sky is clear and it’s a full moon, moonbows can be seen in the mist from some of the park’s iconic waterfalls.

Watch: Rainbows and Moonbows

This article was originally written on January 10, 2021 and has since been updated.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.