Parry Arcs over England & Belgium September 21, '08. David Johnson's arc was seen at Newcastle and Jorgen Ravoet's on the Belgian coast at Blankenberge. Images ©David Johnson & Jorgen Ravoet, shown with permission.
The rare Parry arc is the colourful upper half of the 'eye' shape, the lower half is the much more common tangent arc.
Parry arcs result from a seemingly highly improbable crystal orientation. To make them, column crystals must have their long axes horizontal AND two prism side faces also horizontal. We see the arcs because although the orientation is rare, the arcs themselves are formed very efficiently from low concentrations of crystals.
Rays passing through two side faces inclined 60° to each other form the arcs. They can do this in four ways. Two arcs are above the sun, concave and convex to it. Two arcs are below the sun. The visibility and shape of each arc depends on the solar altitude. The arc seen here is the upper suncave Parry arc, the upper sunvex arc forms when the sun is low.