photo #5128403

One advantage of looking up regularly is you’re more likely to spot the subtle arcs and halos that can appear as sunlight shines through the ice crystals of high clouds. Kees Neve (Cloud Appreciation Society Member 57,271) did so by his office in The Hague, Netherlands and saw a display of many optical effects formed by crystals in the high-layer cloud Cirrostratus. Towards the top was the distinctive and brightly coloured ‘cloud smile’ known as a circumzenithal arc. At the bottom was a much fainter 22-degree halo, which is the ring of light encircling the Sun, just out of frame. Overlapping the top of the 22-degree halo was a slightly brighter up-curved arc, known as an upper tangent arc. And just above this, forming an eye-like outline with it, was the rarest of optical effects present that day.This last one was a Parry arc, named after the English explorer Sir William Edward Parry who recorded the rare and elusive light effect in 1820 while his ship was trapped in ice on an expedition in Arctic Canada in search of the Northwest Passage. The arc’s shape changes according to the elevation of the Sun, varying from a sharp ‘V’ when the Sun is on the horizon to a shallow curve like this one when it is higher. The rarity of the Parry arc comes from the fact that it only appears when sunlight shines through a specific shape and orientation of ice crystals. They need to be hexagonal column-shaped crystals, like minuscule pencils of ice, that are aligned horizontally with one of their long sides facing directly down. Ice crystals in clouds aren’t shaped and aligned like this often, so it’s best to be glancing up when they are.