ROCHESTER, N.Y. — We are looking at another aspect of optics in the atmosphere and the “cool” stuff you can sometimes see in the winter sky.

Recently, we received a weather picture from Karen from Webster. It appears to show the sun (check the video in this report), but actually it is the full moon which moved across the Rochester sky during the last few nights. The picture also shows some color around the moon along with some high cirrus clouds. This effect is called a lunar halo or a moonbow which is visually similar to a rainbow.

There are several ingredients that are needed for this optical effect to happen. First, there must be a nearly clear night sky. However, in addition ice crystals are needed to be present high in the atmosphere. This is provided by cirrus clouds passing between you and the moon. Secondly, the visible light coming from the moon needs to interact with the ice crystals in order to bend and refract the visible light. This effect is similar to a prism which you may have used back in your high school science class. This rainbow effect is more faint and not as bright as a standard rainbow, but can be just as amazing to see.

In the old days, before the invention of the radar, a sailor would see these moonbows or halos and they believed it was a harbinger of bad weather. There is a bit of truth to this as the high clouds usually precede the big, tall cumulonimbus clouds which can produce severe weather. So, there is some truth to this particular weather folklore.