Last Sunday morning a number of Vineyarders spotted an extraordinary atmospheric event in the sky, a peculiar high altitude rainbow. This was not the usual rainbow we are all familiar with following a summer shower, where rain clouds are nearby. This was different.

What our friends saw on Sunday was what meteorologists call a circumhorizontal arc, not a very attractive name for a pretty event, created by sunlight passing through high altitude ice crystals.

A circumhorizontal arc is far more related and a cousin to a halo and also a sun dog. Though all three are rare, the rarest of them all is the circumhorizontal arc. This is an event that takes place way up high in the neighborhood of cirrus clouds, well over 20,000 feet above ground.

Our changing atmosphere is a giant part in the creation of Sunday’s show. The air around us was dry and stable, while high aloft there was change, an approaching cold front. High in the sky, there was bitter cold air interacting with moisture creating ice crystals.

One more ingredient in this phenomena involves the relative angle between us, the crystals and the sun.

Sun dogs, halos and circumhorizontal arcs were used to predict the weather. They were a sign of an advancing change in the air. Last Sunday, the cold front passed over the Vineyard and brought with it rainy weather by the end of the day.