Looking directly upwards in Saaremaa, Estonia, Paula Henk spotted a rare pair of optical phenomena. The white ring is known as a parhelic circle, while the brightly coloured arc is a circumscribed halo.

Both of these optical effects can appear as sunlight shines through ice crystals in the atmosphere. Paula’s arcs were caused by the crystals in high Cirrus clouds. From the style of light effects, we know that the crystals were in the form of hexagonal plates with their large faces horizontally aligned.

Paula was lucky to spot a complete parhelic circle like this. The white ring results from sunlight reflecting off the side faces of the tiny hexagonal plates of ice. The effect is always at the same elevation as the Sun and appears centred around the zenith. The brightly coloured arc is a part of a circumscribed halo. When the Sun is high in the sky, as it was for Paula, this looks very similar to the perfectly circular ring around the Sun known as a 22-degree halo. It has brighter colours, however, and isn’t a perfect circle for all but the highest Sun elevation.

Looks like someone’s trying out their brand-new geometry set for the sky.