Atmospheric
Optics
Green.. ..& Red Flashes
Sunset images by Marcella Giulia Pace at Marina di Ragusa, Sicily.
A classical inferior mirage type green flash is caught poised just above the ‘horizon’. Not the real horizon but instead the ragged edge of a mirage. The mirage that produces the flash. Turbulence flakes the mirage to filmy fragments.
All images ©Marcella Giulia Pace, shown with permission
A thin inversion layer gives the second main type of mirage - a mock mirage - and another small flash.
Red Flash
Earlier in the sunset Marcella caught the red flash.
The sun's upper image with a lower red edge slides downwards. Its miraged mirror image climbs.
Where the two meet there is strong vertical magnification and colour separation. the same effect that gives the green flash. The normally thin red edge spreads like a sticky liquid between the two suns. The red flash.
The flash, there for but an instant, is the final sliver of sun doubled and mirrored by the mirage.
One sliver descends, the other ascends. They merge and then are gone. Sea warmed air beneath colder stuff produces the mirage.
Why green? Near to the horizon the sun nearly always has an upper green rim. Normal atmospheric refraction raises the sun�s image. Shorter wavelengths refract most giving blue and green sun disks highest. The resulting coloured rim is only visible in large scale images � never look for it visually no matter how dimmed the sun.
The mirage reveals the colours. Where the two slivers meet there is strong colour separation showing us � highly magnified � the sun�s upper green edge. Beneath the sun there is a red one, see down the page.
Why not blue? Near the horizon rays have a long path through the air. Rayleigh scattering depletes the blues and violets. Blue flashes do occur but they are rare.
