Exploring Rainbows and Moonbows

| |

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.

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.

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.

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.

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.

Rainbows can appear near waterfalls where the presence of water spray combined with sunlight produces a rainbow.

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 arch, 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.

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 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).

Moonbows always occur opposite the sky from the moon.

Moonbows in Yosemite National Park

A popular place to spot moonbows is in Yosemite National Park. During the spring and summer 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.

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

Watch: Rainbows and Moonbows

Other Geography Phenomenon

Related

Share:


Enter your email to receive the Geography Realm newsletter: