Sun"vex, adjective. (a modern atmospheric effects portmanteau of sun and convex.)

This adjective means: Arched in the sky, the open end of which points away from the sun.

The only widely-confirmed arcs to display this attribute are the Parry arc and the Circumzenith arc. Why are the overwhelming majority of atmospheric effects suncave? Consider this: light from the sun and moon emanate from their sources as a perceptual circle. Reflecting that light to your eye with anything other than the source at its center requires that a number of complicated factors be just right, and the skies are ordinarily too chaotic for that.