Ask Ellen: Why is there a halo around the moon?

Ask Ellen

GRAND RAPIDS, Mich. (WOOD) — On a mostly clear spring night, 13-year-old Kam Richardson took this shot of a halo around the moon in Grand Haven. He wanted to know what makes that happen.

The technical term for the effect is a 22-degree halo.

While the phenomenon isn’t rare, it still brings a bit of wonder to the sky. Halos can be spotted all over the world and often can even occur around the moon.

When spotting it, be sure to never look directly at the sun.

A 22-degree halo forms on quiet days or nights when a layer of thin, high clouds streams in. This layer of clouds must be high enough to contain only ice crystals.

Smaller halos around the sun are called coronas and are formed when water droplets are present instead of ice.

Example of a corona around the moon with colors emanating from the center

22-degree halos will only form if the ice crystals in the high cloud layer are all uniform in size and shape. When this happens, all the ice crystals will deflect light twice within their structure, creating a perfect 22-degree halo around the sun.

The color red will always be on the inside of the halo, closest to the sun or moon. The blue and purple colors from the halo will often blend in with the rest of the sky on the outer edges of the halo.

Ice crystals that form the halo are always hexagonal, or, six-sided, which is why the halo will always be the same size around the sun or moon no matter where it sits in the sky, or the time of year that you spot it.

Courtesy of Atmospheric Optics

United Kingdom-based Atmospheric Optics explains that “rays passing through two prism side faces inclined 60 degrees to each other are deflected through angles from 22 degrees up to 50 degrees.” This means the halos will always start at 22d egrees and end when the angle hits 50 degrees.

Halos used to be though to be an old wives’ tale forecasting tool, indicating rain is on the way. While high clouds can frequently precede warm fronts and rain a day or two later, a halo is not a surefire sign that it will soon rain.

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