Halos around the sun or moon come from light passing through tiny ice crystals.

The longer version

Halos are produced by millions of tiny six-sided ice crystals drifting in high cloud. Light entering one face of a crystal and leaving through another is bent by a fixed angle, so the light is concentrated into a ring rather than spread evenly across the sky.

The most common halo forms a circle roughly a hand's width away from the sun or moon. Since the crystals must be below freezing, halos signal ice in the upper air and often appear ahead of a change in the weather. Bright spots beside the sun come from the same crystals.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.