photo #4302237

Kay Cheng (Cloud Appreciation Society Member 43,028) saw a tree with a halo in Kiba Park, Tokyo, Japan. The effect shows the overlap of two distinct optical phenomena caused by ice crystals that often appear in the sky at the same time. One of them, which appears very faint here, is a 22-degree halo. This is a common light effect caused by sunlight shining through hexagonal columns of ice – shaped rather like minuscule pencils – that are likely tumbling chaotically as they fall through the sky. Named after the angle you get when you point one hand at the Sun and the other at the ring, which is always 22 degrees, this light effect can appear as a complete ring of light around the Sun or Moon. Only a faint part of the halo shows here, up to the top right of the tree, likely because the hexagonal columns of ice are falling in a more organised way, perhaps lined up with their long sides horizontal. Far brighter is the other optical effect known as an upper tangent arc. This effect appears in the sky as if it is touching the top of the 22-degree halo. An upper tangent arc appears as the light shines through hexagonal columns of ice that are aligned with their long sides being horizontal. Lining up like this can happen due to the effect of air resistance on column-shaped crystals as they fall through the sky. A lot of work goes into making a halo for a tree.