Very common. It means it will rain in the next 72 hrs. What causes the halo? I haven't a clue. You don't believe me,or think I'm blowing smoke...Ask a farmer,they will tell you the same. Also when the leafs on a bay tree turn upside down to where the leafs look white...Rain is a coming. Around 5:00 am this morning as I was leaving I noticed the halo,& yes we have terrible weather coming to the Gulf Coast in the next 48-72 hrs.