Breakdown: Why the moon can have a halo around it

Why a halo can form around the moon

MEMPHIS, Tenn. (WMC) - Have you ever looked up into the night sky and noticed a ring around the moon? What you were seeing is an optical illusion, caused by reflections of ice crystals in the upper atmosphere from cirrus or cirrostratus clouds. This is called the “halo effect,” or a Lunar Halo, and it is caused by light rays diffracting around the moon.

Twenty thousand feet above your head are thin wispy cirrus clouds. You may or may not be able to see them, but you will when the light from the moon reflects on those tiny hexagon-shaped ice crystals. The light rays coming from the sun that are then reflected on the moon are bent by the crystals 22° away from the moon. This creates a halo that has a 22° radius (44-degree diameter).

This phenomenon is most common during winter months due to the frequency of systems in the cool season but can occur in any season. This is applicable to tropical and hurricane systems too, because there are high thin clouds surrounding the outskirts of those systems.

These halos can even resemble pale rainbows, with red color on the inside, and a blue outside. The full process is due to refraction, reflection, and dispersion. They are caused by the same effect, because the ice crystals are 6-sided prisms.

So how does this predict precipitation? One weather folklore says a ring around the moon means precipitation soon because high cirrus clouds often come before a storm. So there is some truth to the statement. Cirrus and cirrostratus clouds are often precursors for storm clouds behind them, because they are most often the first cloud layer seen when a weather system approaches.

In rare circumstances you could see a double halo, caused by less-than-perfect crystals. These crystals wouldn’t have the hexagon shape and would rather take on a different form (such as a triangle), causing a different sized halo.

If you spot a halo, keep in mind that rings around the moon are mostly colorless, but you may notice more red on the inside and more blue on the outside of the halo. These colors are more noticeable in halos around the sun. If you do see a halo around the moon or sun, notice that the inner edge is sharp, while the outer edge is not as sharp.

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