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What Is A Moon Halo And How Is It Different To A Moonbow?

Moonlight can create some pretty cool optical phenomena.

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Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockDec 2 2022, 12:50 UTC
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The moon is visble through the trees and a ring of ligth surrounds it.

A Moon halo always has the same size. Image Credit: cihanyuce/Shutterstock.com

Have you ever looked at the Moon and seen a halo around it? This is not a hallucination but an atmospheric phenomenon not dissimilar to rainbows, or in this case Moonbows. There are certainly important differences between the two that are worth exploring.

Moonbows form just like rainbows although they are rarer. You need to have water droplets in the air opposite the Moon, of course. But given that the Moon is almost 400,000 times dimmer than the Sun, you also need to have an exceptionally dark sky for them to be visible. This requirement reduces the possible times a Moonbow can form quite significantly. A stunning Moonbow was snapped a few years back in the UK.

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A Moon halo, however, doesn’t come with all the caveats. While there are still requirements for its formation, they are not as demanding. And they can be equally as spectacular. 

the water droplets created by the waterfalls are the perfect setup for the formation of a Moon bow
A moonbow across Victoria Falls, Zambia, Africa. Image Credit: KITAMU/Shutterstock.com


What Is A Moon Halo?

Instead of water droplets, a Moon halo requires ice crystals. But not just anywhere. They need to be in high-altitude clouds known as cirrus clouds, as well as cirrostratus clouds. Present at a height of 6,000 meters (20,000 feet), cirrus clouds are short hair-like clouds. During the day, when they appear, they are extremely white.

Ice crystals usually take a hexagonal form and that’s very important. It guarantees that Moon halos are always the same, no matter what: a faint ring of light 22 degrees away from the Moon. If the conditions are right, a second ring will appear at 44 degrees, but more often than not, it will just be the arches of this second ring.

In many cultural traditions, a Moon halo heralds bad weather ahead. And this idea, borne by observations of the phenomenon, can be correct. Cirrus clouds tend to form ahead of warm fronts, preceding them by one or two days. This usually means rain and storms. That said, isolated cirrus clouds can also form without storms, so this is not a perfectly reliable forecasting method. 

The sun is centers as it rises over a snowy landscape. on its left and right two reflections known as sun dogs are present on a very faint ring of light.
The two sundogs and a hint or the 22 degree halo around the sun on a winter morning. Image Credit: BILD LLC/Shutterstock.com


Why Does It Occur Around The Sun Too?

As with rainbows versus moonbows, halos can also happen around the Sun or Moon. Thanks to ice crystals and the brightness of the Sun, Sun halos deliver more peculiar phenomena alongside the 22-degree halo.

The most interesting feature, just by name alone, is the sundogs. These are two bright features that happen to the left and right of the Sun on the halo. Their etymology remains mysterious but their scientific explanation is not. They can be seen anywhere in the world and at any time, but they are most visible when the Sun is low on the horizon.


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