Omega sun

From Glossary of Meteorology

omega sun

A two-image inferior mirage of the setting or rising sun in which the erect image of the solar disc, which forms the top of the omega, touches the inverted segment of the lower image, which forms the base of the omega.

When the sun is even lower, all that is seen are two images of the upper segment of the sun floating back to back above the optical horizon and having an outline similar to that of an American football. The stage at which only two images of the upper rim are seen is one way in which the green flash is formed.