​​What is the moon halo, and what does it mean​

There’s something magical about looking up at the night sky and spotting a glowing ring encircling the moon. It feels completely out of the world, almost like a secret message from the universe. For centuries, people around the world have gazed at this halo in awe, created stories, superstitions, and also weather predictions around its appearance in the night sky. But what exactly causes this beautiful phenomenon?

Despite its mystical feel, the moon’s halo isn’t magic, but pure atmospheric science. This beautiful glow is actually a natural effect of light interacting with tiny ice crystals floating high up in the atmosphere. While it is rare, this phenomenon is common, especially during colder months, mostly at higher altitudes or when certain cloud types drift into the sky.
Here we will know what a lunar halo is, how it forms, and what it means for the weather predictions.

A Moon halo is created when moonlight passes through ice crystals in cirrus or cirrostratus clouds that remain high in the sky, usually 20,000 feet or more. These ice crystals act like a huge natural lens, bending and reflecting light to create a ring around the moon. Because most ice crystals are hexagonal, the light always refracts at roughly 22 degrees, forming a 22-degree halo. This gives the ring a consistent size, no matter where or when you spot it.

Though moon halos often appear white or faint, they’re actually formed by the same prism effect that causes rainbows. According to NASA, “These ice crystals also demonstrate a prism effect that separates white light… into various individual colours just like the atmospheric effect that creates a rainbow.” The halo may appear tinted with red on the inside and blue on the outside, though moonlight is often too dim for the full spectrum to show clearly to the naked eye.

Moon halos are quite common and can happen at any time of year, but they are more likely to appear during winter. The Farmers' Almanac says, “There is a good chance of spotting one, as long as you are willing to brave cold and possibly wet weather.” Thin cirrus clouds covering large parts of the sky are the most ideal for the rings to be formed. Surprisingly, some of the best chances to see halos happen when cloud cover is just enough to reflect light, but not so thick that it hides the moon.

Lunar halos have long been seen as signs of coming weather changes. As Farmers’ Almanac explains, cirrus clouds, which cause halos, can mean a warm front and bad weather are on the way. One old English proverb quoted in George Latimer Apperson’s Dictionary of Proverbs says,
“If the moon show a silver shield,
Be not afraid to reap your field;
But if she rises haloed round,
Soon we’ll tread on deluged ground.”
While it is poetic, this method isn't always reliable as cirrus clouds don’t always signal a storm.

Lunar halos are just one type of atmospheric optical illusion. Similar effects include coronas, smaller rings caused by water droplets and sun dogs, which are bright spots that appear beside the sun. NASA has even noted a rare quadruple lunar halo in 2012 over Madrid, saying that it was “extremely rare, especially for the moon.” These phenomena happen when different ice crystal shapes and orientations bend light in difficult ways.