Do You See A Moon Halo Or A Moonbow?
Quick Reference
- Moon halo: A 22-degree ring of light around the Moon, formed by moonlight refracting through ice crystals.
- Moonbow: A faint nighttime rainbow created by moonlight reflecting through water mist.
- Folklore: A halo around the Moon means rain or storms within 24 hours.
- Best moonbow spots: Niagara Falls (NY), Yosemite Falls (CA), Cumberland Falls (KY).
- Best timing: 1-2 hours after sunset on a near-full Moon, dark sky, no trees blocking.
It is late at night, light wispy clouds float gently by as the nearly full Moon shines down. But then you notice a faint, ghostly glow in the sky. It looks like a ring around the Moon, a delicate circle of off-white light with the Moon sitting perfectly in its center. This rare event is called a Moon halo. It happens when moonlight bends through lots of six-sided ice crystals in the Earth’s atmosphere to create a huge, perfect circle of light.
Moon halos may occur all year round but are slightly more noticeable during colder months in the Northern Hemisphere when more ice crystals can form at high altitudes. Legends say that a Moon halo is a warning that bad weather is coming soon. Although it may not be true every time, meteorologists have documented that high cirrus clouds that help form Moon halos often come before storm systems.
The radius of a Moon halo is always the same, about 22 degrees from the Moon. You can use your hand at arm’s length to measure that angle in the sky. (See our illustration here.) Spread out your fingers, cover the Moon with your thumb, and your pinkie should reach the edge of the ring around the Moon.
How About A Moonbow (Night Rainbow)?
Experiencing a rainbow shining majestically before a bank of dark clouds is one of the most amazing sights on Earth. Sunlight is passing through water vapor in the atmosphere which shines in a colorful arc. But can the Moon be bright enough to cast a similar shape during the nighttime, a moonbow? It can, but only in very select places and under perfect conditions.
First you need water vapor, lots of it, off which the moonlight can reflect. This could be kicked up by large waterfalls or forest mist that spread over a large area.
Then you need an almost full Moon. Only the light of the Moon near its brightest phase casts enough light on the water vapor to create a suitable moonbow (night rainbow) your eye can detect.
Also you need a location with no trees between the mist and the Moon. And just like a rainbow that appears on the opposite side of the sky from the Sun, the mist must be on the opposite side of the sky from the Moon.
Other things that help is a location far from city lights where the sky is naturally darker. A moonbow can also only form if there are no clouds and the Moon is low in the sky. This means the best viewing times are usually 1-2 hours after sunset, which is when a full Moon rises.
There are only three places in the United States where moonbows are regularly seen: Niagara Falls, New York, Yosemite Falls, California, and Cumberland Falls, Kentucky. Moonbows have been spotted through mists and light rain in forests of Hawaii and Costa Rica, but they are harder to predict.
Even under perfect conditions from the perfect location, a moonbow is still incredibly faint. Long exposure photography can bring out all the usual colors from red to violet. To your naked eye, however, the arc of light from a moonbow appears mostly white with very little color evident.
Check out the dates of the next full moons and take a pilgrimage to a waterfall on a clear evening. See if you can catch a nighttime rainbow in the mist.
Moonbow Watch Calendar
The three U.S. moonbow sites publish their own moonbow-watch nights based on the full Moon and the rise time of moonlight relative to sunset.
- Cumberland Falls, Kentucky. Two nights before, the night of, and one night after each full Moon. Park rangers run guided viewings.
- Niagara Falls, New York. Same window. Best viewed from the Observation Tower or Cave of the Winds platform.
- Yosemite Falls, California. Late April through early June only, when meltwater volume is highest. Look from Cook’s Meadow on full-Moon nights.
The National Weather Service publishes moonrise and moonset tables for every U.S. station, which is the calendar you need to time a moonbow chase.
Frequently Asked Questions
What is a moon halo?
A 22-degree circle of light around the Moon, formed when moonlight refracts through hexagonal ice crystals in cirrus clouds.
What is a moonbow?
A faint nighttime rainbow created when bright moonlight reflects through water vapor or mist. Almost always pale or white to the naked eye, with full color visible only in long-exposure photography.
Does a moon halo predict bad weather?
Often. The cirrus clouds that produce the halo also lead the leading edge of an approaching warm front. Rain or snow typically follows within 24 hours.
Where can I see a moonbow in the United States?
Three sites regularly produce them: Niagara Falls (NY), Yosemite Falls (CA, late April to early June), and Cumberland Falls (KY). Hawaii and Costa Rica have spotted moonbows in forest mist.
When is the best time to see a moonbow?
One to two hours after sunset on a clear, nearly full-Moon night. The Moon must be low and bright, and the mist must be on the opposite side of the sky.
Why do moonbows look white?
Because human cone cells (which detect color) need more light than rod cells (which detect dim brightness). The reflected moonlight is too faint to trigger color vision, so the eye sees a pale arc.
How do I measure 22 degrees from the Moon?
Hold your hand at arm’s length, fingers spread. Cover the Moon with your thumb. Your pinkie should reach the edge of the halo. The classic 22-degree halo always sits at that distance.
Join The Discussion
Have you ever seen a ring around the Moon or a moonbow (night rainbow)?
Share your experience (and photos) with your community here.
What questions do you have? Let us know in the comments below.
Related
A Halo Around The Sun (Sundog)
Dean Regas
Dean Regas is an astronomer and author of seven books including 100 Things to See in the Night Sky and How to Teach Grown-Ups About Pluto and host of the popular astronomy podcast Looking Up with Dean Regas. He can be reached at: www.astrodean.com
- Dean Regas
- Dean Regas
- Dean Regas
- Dean Regas
Moon halo from DC area, circle within the larger circle and a green alien too🤔
Moon balobin full effect 2 nights in a row here in NW Arkansas!
A moon halo central NJ early morning of 9/24/24 , it was magical!
CLEAR VIEW ON PASSOVER -23/04/2024
ACCRA-GHANA
Last night 12/26/23 in San Diego CA
I saw a halo around the moon quite recently. All the colors of the rainbow were there. It was not the first time. But my husband said he did not see it. How could I see it and he not see it??
I saw a moonbow long before either the internet nor camera phones existed. I was a teen, in Felton, Delaware. And while riding in the front seat of a car, I looked through the windshield and directly ahead in the sky was a silver, shimmering cord of reflected light with barley any color visible but silvery shimmering white. It was INCREDIBLE. Majestic. Awe inspiring. Magic. Never saw one before or since, but I’ve never forgotten it. It’s an experience everyone should have at least once. Once had, you’ll never forget it.
That does indeed sound incredible. Thank you for sharing it with us as well, and sometimes those memories that aren’t physically documented are the most treasured ones.
SCIENCE ROCKS!!!!!!!!!!!!!!!!!!!
science ricks!
ur mom is gay bc of science