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Have you ever noticed a halo around the moon? Here's why that happens!

It's because of cirrus and cirrostratus clouds high up in the atmosphere.

Why is there sometimes a rainbow or halo around the moon?

Credit: Catherine Couyos

If it’s been as gloomy where you live as it is where I am, then you might have noticed something interesting at night: a sort of halo around the moon. Sometimes, it might even look like a rainbow.

This is called a lunar halo, and you may have heard an old wives’ tale that it means rain or snow is on the way.

This kind of ring occurs when ice crystals in the Earth’s upper atmosphere, located in thin cirrus and cirrostratus clouds, refract and reflect the moon’s light. Let’s break down what that means.

Credit: Simon Fitall

Cirrus are wispy, thin clouds that are patchy and look kind of fibrous, located very high up in the Earth’s atmosphere, between 15,000 and 30,000 feet above the ground. They’re made up of ice crystals.

Cirrostratus clouds are similar, also in the high atmosphere and made of ice crystals, but they’re more veil-like. It looks like a sheet over the sky, like a veil of fog over the sun or moon.

These clouds often form when a mass of warm air rises and over cold air. As the warm air rises high enough into the atmosphere, cirrus clouds form. The more that form and overlap, the more likely it is that cirrostratus clouds will form.

Credit: Tyler Olson

The light is basically bent at a 22 degree angle (which is why scientists often refer to this phenomenon as a “22 degree halo”). How distinctive the halo is depends on the amount of cirrus clouds there are and the amount of light passing through them to Earth — which is why you usually see a halo around a gibbous moon or a full moon, because that’s when the moon reflects the most light towards the Earth.

Usually lunar halos are white, whereas halos around the sun are more colorful, because the sun is a lot brighter in the sky than the moon. But you might notice sometimes the moon’s halo has a little bit of red and blue if the moon is full.

And what about that old wives’ tale — is it true that a lunar halo means bad weather is on the way? Well, not always, but it can. High cirrus clouds do contribute to storms, so they appear before rain or snow.

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