|Smile in the Sky
Sandy Robertson captured this halo display in Hertford, England.
The 'smile in the sky' or 'upside down rainbow' or 'circumzenithal arc' or 'upper symmetric 46° plate arc' is the finest of all the ice halos.
Look for it directly overhead when the sun is lower than 32°. The first sighting is always a surprise, an ethereal rainbow fled from its watery origins and wrapped improbably about the zenith.
But liquid water has no part in its making. It is made by hexagonal plate ice crystals drifting with their large faces horizontal in high and always cold cirrus cloud.
Other crystals were also there in Sandy's display. A faint coloured halo curves downwards from the circumzenithal arc. This is a supralateral arc produced by hexagonal ice prisms drifting with their long axes horizontal.
All images ©Sandy Robertson, shown with permission
|Refraction through 90° prisms produces widely spaced and almost pure spectral colours|