What is a 22 degree halo?

There's a pretty cool phenomenon that can occur when high, thin clouds interact with our moon. On mostly clear nights when the moon is most visible in the sky, sometimes there is a halo that appears around the moon. This is caused by a sheet of high, thin clouds called cirrostratus clouds. Ice crystals suspended in the cloud refract the moon's light on its way to our eyes, making it appear as a ring or halo.

In the old days, it was referred to as a sign or omen that bad weather is on the way. This can be true at times but it doesn't always work out that way. High, thin clouds often are out ahead of an incoming low pressure system with rain, but they can form in other ways as well.

There's even a chance we could see one tonight! The moon sets around 1 AM Thursday morning so be sure to look up at the sky tonight, towards the west as the moon sets. You might be able to see some cirrostratus clouds moving in front of the moon, creating that halo effect.