Ring up the sun

A halo around the sun is an unusual weather phenomenon caused by ice crystals in high cirrus clouds. — Photo by Steve Myrick

A halo circled the sun at high noon over Martha’s Vineyard Friday. It’s an unusual weather phenomenon, essentially a round rainbow high in the sky.

It happens when the sky is clear, except for thin cirrus clouds, very high in the atmosphere, usually about 20,000 feet.

Ice crystals in the cirrus clouds both refract and reflect the sun’s rays. The ice crystals are like a prism bending the light, and a mirror bouncing the light, at the same time.

EarthSky has a more detailed scientific explanation.