Classical Green Flash

A sliver of sun brilliant green on the horizon, a classical green flash. Seen by Giuseppe Pappa at Giojosa Marea, Messina, Sicily on the Tyrrhenian sea.

All images ©Giuseppe Pappa, shown with permission.
About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed
Green flashes come in different flavours. Mock mirage flashes flicker on distorted layered suns still partially above the horizon but they are rarely visible to the unaided eye and it is dangerous to stare or use binoculars. Cloud top flashes similarly give rare and evanescent glints to cameras as the sun slides behind a cloud bank. Green is sometimes even seen by cameras as the sun sets behind a wind swept hillside.

But the much sought classical flash of literature needs no camera. At its best it blazes forth a brilliant and startling emerald light as the last slice of sun is poised on the horizon. A clear sky is maybe best but it is not crucial.

There are plenty of omens. The mirage conditions essential to form the flash show up early. As the sun sinks a second sun starts to rise from the horizon. They meet in an �omega� shape. The joined and distorted images then descend with a distinct lip visible at the sea surfaces. Finally there is the flash itself.

Believe not the all too many accounts citing differential refraction between red and green light producing an upper green edge to the sun that sets last. They miss the point, ordinary refraction effects are far too small to give a flash and it is not quite on the horizon anyway. A mirage is needed. Sun rays are refracted upwards as they slant across the temperature gradients linking a warm air layer close to the sea and cooler air above. Two images form, an upper �normal� one and a lower one that is inverted and rises. Where the images merge, first at the �omega� to give a red flash and then just before they both set (green flash) there is strong vertical magnification of the images. The extreme stretching makes the otherwise small colour separation visible.