Vancouver Rainbow Ring. Thin cirrus clouds made of millions of hexagonal ice crystals 3 to 6 miles high, each a tiny prism refracting the sun's light. The upper troposphere is below freezing so ice-crystal displays are year-round. I've seen weak sundogs in July. Truly good displays in the US are most common in the fall, winter, and spring, when the jet stream descends southward, drawing Arctic air masses with their treasure-trove of jewel-like ice prisms.