High pollen levels in Eastern Japan create pretty rainbow suns

It probably doesn’t make the suffering any better, but it’s very pretty nonetheless.

On the morning of 8 March many early risers in the Kanto area of Japan were treated to an interesting and beautiful phenomenon called a “kafun kokan” or “pollen light ring” resulting in a rainbow-like halo around the rising sun.

While a delightful view for some, for those suffering from an allergic reaction to the pollen of the widely distributed Sugi (Japanese cedar) tree it’s a sign of terrible things to come.
As the name suggests, a pollen light ring is created when the light of the sun passes through an especially dense cloud of pollen. The tiny particles bend the sunlight in a way similar to how water droplets cause a rainbow to appear. However, in a rainbow the light passes through the water and refracts, whereas solid pollen is an obstacle that diffracts the sunlight.
As a result, rather than the large arc (or ring if you’re lucky) of a rainbow, the pollen light ring appears to encircle the sun in different bands of the visible spectrum.

Of course, there’s lots of junk floating around in the air, but pollen from the Sugi has unique characteristics that allow it to more easily produce pollen light rings.
First, the massive population of Sugi trees leads to very high concentrations of the pollen in the same area. For example, on the morning of the 8th, especially high levels of pollen were seen in Ibaraki Prefecture, directly to the east of prefectures like Chiba and Kanagawa and sitting between them and the rising sun.
The other factor is the shape of a Japanese cedar pollen particle. Rather than being completely spherical, they all have a little nub on one end, making them look like little umbrellas or Metroids.

Because of this, when hit by a single gust of wind, they will all orient themselves in the same direction which in turn causes even more intense diffraction to occur.

So just as every cloud has its silver lining, so too does every pollen cloud have its kafun kokan. However, this is probably small consolation to those who must suffer through the spring months with runny noses and itchy eyes.

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