|UK Halos, Wales ~ The UK saw widespread halo displays on 28th January. Graham A Stephen (site) took these images while walking in the Carneddau Mountains of Wales. The top picture is from the slopes of Carnedd Gwenllian looking towards Carnedd Llewelyn and Yr Elen All images ©Graham A Stephen, shown with permission|
A 120° parhelion blazes far from the sun and crossed by the parhelic circle.
|Horizontal hexagonal plate crystals, horizontal columns and some kind of randomly oriented hexagonal prisms, perhaps in clusters, produced the display.
An intense and spectral circumzenithal arc from plate crystals is overhead. Tangent to it is a fainter supralateral arc, its widely spread colours showing to the right of the top image. Sun rays entering a side face of column crystals ant leaving through a hexagonal end face generate this arc, often mistaken for a rarer 46° halo. Similar column crystals produced the gull wing shaped upper tangent arc atop the 22° halo.
A bright sundog (plate crystals again) merges into the white parhelic circle. Crystals of all types contribute to the latter.
Another false sun, a 120° parhelion, is found some distance around the parhelic circle. This parhelion is always white and often overlooked when it appears against cloud. Rays internally reflected three times inside plate crystals make the 120° parhelion. It is brightest when the plates are thick or when they are of triangular aspect - alternate hexagonal sides long and short. Crystals are traditionally drawn with regular hexagonal cross sections but in reality they can be quite irregular. Their interfacial angles are always constant though.
At left a zenith centred HaloSim ray tracing shows the halos.