Why is there a circle outside the moon in today's sky?

This is a precursor to the weather change. On the ground, although we can't feel the change of air in the air, in this case, because of the relative flow of air in the air, air in the air has been formed. Moonlight is equivalent to coming from a point on the moon and shining on media with different air densities. Due to the refraction and reflection of light, part of the light is completely reflected on the surface of the medium where the air flows (no light penetrates the atmosphere), and then a dark tunnel is formed, and the light inside or outside the dark tunnel is completely penetrated into the atmosphere, thus forming a relative halo. At this time, the light reflected by the moon is thought to be emitted by many points, so a halo can be formed.