Seen by Jim Grant in Southern California December 30, '09. "We had kind of broken skies tonight. So this GF really surprised me". Images©Jim Grant, shown with permission.
This is a classical green/blue flash, the type where there is a brilliantly coloured flash for a second or so just as the last sliver of sun sinks below the horizon.
Other flash types are above the horizon on the top of a still visible part of a distorted sun. These latter flashes are more often photographed but are less brlliant and long-lived. They are hard to see with the unaided eye.
The events presaging a classical flash become evident several minutes earlier. The sun’s disk sinks slowly towards the ocean but then, before it reaches it, a second sun starts to rise up from the waves. A mirage has started. The two suns soon meet to form an ‘Omega’ shape. Then they overlap more and more.
Finally, as the last trace of the two suns reaches the horizon, conditions are right for the flash. A particular zone of the mirage has considerable vertical magnification and this is now close to the horizon. The very slight green or blue rim of the sun resulting from the ordinary different refraction of green and red rays by the atmosphere is considerably magnified. It becomes easily visible – the ‘flash’.
Without the preamble of the omega mirage a flash is unlikely to be seen. Look for the early signs when flash hunting.
Technically, the classical flash is an 'inferior' mirage – in the early stages there is an inverted image of the sun beneath the ‘real’ one. The effect is similar to the mirage seen in a hot road surface or in the desert and the air conditions are the same. The mirage and flash requires a layer of warm air (usually provided from contact with a sun-warmed ocean) beneath cooler air.
Blue flashes are a rare compared with green ones. Why are not all flashes blue? Sunset rays are reddened because shorter wavelength rays are preferentially scattered away by the atmosphere. A particularly clear atmosphere is needed for sufficient blue to be preserved in the direct rays. However, these images demonstrate that cloudless skies are not a requisite.