The rhyme:

"When halo rings the moon or sun, rain's approaching on the run".

The reason:

This is another solid indication that active weather is on the way. A halo around the sun or moon is caused by the refraction of that body's light by ice crystals at high altitude.

In winter, a halo around the sun or moon is evidence that cold air is here above the surface. These halos tend to evolve into a milky, toned sky where the typical blue is washed out.