'Round Rainbow' Fills Bloomingdale Sky

A Bloomingdale resident caught a rare glimpse of a fully circular rainbow on Matthews Terrace Tuesday.

Brian Brenkert said a neighbor called him to tell him of the unusual phenomena and he grabbed a smartphone snapshots to share the right.

By definition, all rainbows are round. When we see them, we tend not to see the half cut off by the horizon. That leaves us viewing the traditional half-circle rainbow to which most are accustomed.

But not today. Brenkert said his neighbor explained the sight as a "22 degree halo formed by ice crystals."

A halo is a ring of light surrounding the sun or moon, explains the University of Illinois at Urbana-Champaign's WW2010 weather website.

Most halos appear as bright white rings but in some instances, the dispersion of light as it passes through ice crystals found in upper level cirrus clouds can cause a halo to have color.

Halos form when light from the sun or moon is refracted by ice crystals associated with thin, high-level clouds (like cirrostratus clouds). A 22 degree halo is a ring of light 22 degrees from the sun (or moon) and is the most common type of halo observed and is formed by hexagonal ice crystals with diameters less than 20.5 micrometers.

In this instance, the light witnessed went through two refractions, passed through an ice crystal and was refracted into the shape of a circle.

"I thought it was really cool, no chance of finding any pot of gold with this rainbow," Brenkert told Patch. "There is no end!"