What causes a sun halo?

MADISON, Wis. (WMTV) Many of you snapped photos of an ominous disk around the sun earlier today.

This is called a sun halo! The million dollar question is how do they form? Well recall that the atmosphere is filled with different types of gasses such as oxygen, nitrogen, and water vapor. If water vapor is at a high enough altitude it freezes into tiny ice crystals. In the case of today, we had high cirrus clouds (around 30,000 feet), and as the sunlight passed through those ice crystals it was forced to bend (refraction). When sunlight is forced to bend it causes a ring to appear around the sun causing a sun halo!