The green flash is a brief burst of green light seen at the upper rim of the Sun in the last one or two seconds before it sets, or the first one or two seconds after it rises. It is not an illusion, an afterimage or a trick of a tired eye — it is atmospheric refraction, and it has been photographed thousands of times. Earth's atmosphere bends light, and it bends short wavelengths slightly more than long ones, so the atmosphere splits the Sun into a stack of overlapping coloured images with the blue-green one sitting fractionally higher than the red one. As the Sun sinks, the red, orange and yellow images vanish below the horizon a moment before the green one does, leaving a green rim as the last thing visible. Violet and blue are scattered out of the beam by the long path through the lower atmosphere, so green is the colour that survives. The separation involved is tiny — on the order of ten arcseconds — which is why a green flash almost always needs a mirage to magnify it into something the naked eye can catch.
The Atmosphere as a Weak Prism
Air is denser near the ground than aloft, and light entering that density gradient at a shallow angle curves. Near the horizon the effect is large: the atmosphere lifts the Sun's apparent position by roughly half a degree, which is about the Sun's own angular diameter. The practical consequence is startling and worth stating plainly — when you watch the lower edge of the Sun touch the sea, the Sun is already geometrically below the horizon. You are looking at an image bent over the curve of the Earth.
Because refraction is wavelength-dependent, that lifting is not identical for every colour. Green light is bent a little more than red, so the Sun's green image is raised a little more than its red image. The two are displaced vertically by only about ten arcseconds — roughly one two-hundredth of the Sun's diameter — and the discs overlap almost perfectly, so through most of the sunset all the colours cancel out into ordinary white and yellow. Only at the very last moment, when nearly the whole disc has dropped below the horizon, does the tiny offset matter: the red image sets first, then orange, then yellow, and for a second the only part of the Sun still above the horizon is the top of the green image.
The missing colours are explained by scattering rather than refraction. Blue and violet are bent most of all and should in principle appear last, but light reaching the eye from a setting Sun has travelled through some forty times more atmosphere than light from overhead, and Rayleigh scattering strips the short wavelengths out of it — the same process that reddens the setting Sun in the first place. Green is the shortest wavelength that reliably survives the journey. On exceptionally clean, dry air over cold water, observers do occasionally report and photograph a blue flash, and much more rarely a violet one.
Why the Horizon Does Half the Work
Ten arcseconds is at or beyond the resolution limit of the unaided human eye. On its own, pure dispersion would produce a green rim visible only through a telescope. What turns it into an event is a mirage, which acts as a vertical magnifier.
When the sea is warmer than the air above it — the usual situation over open ocean, and strongly so in the tropics — a shallow layer of warm air sits against the surface, and light passing through it curves the wrong way. This produces an inferior mirage: an inverted image of the Sun's lower edge rising to meet the true image, which is why a setting Sun often takes on a flattened, hourglass or Etruscan-vase profile just before it disappears. That distortion stretches the last sliver vertically by a factor of ten or more, and it is this stretched sliver, not the raw dispersion, that the eye registers as a distinct green patch.
This is why the flash is so closely associated with the sea. A sea horizon is sharp, distant and level, and the water surface reliably supplies the necessary temperature contrast. It is not strictly required — flashes are seen over deserts, over lakes, from aircraft, and from mountains looking down onto a cloud deck — but any of those substitutes has to provide both a clean horizon and a thermal gradient. Related mirage physics, worked at much greater scale, produces the fata morgana, where a thermal inversion stacks and stretches distant objects into cliffs and towers.
Five Kinds of Flash
Atmospheric optics researchers distinguish several distinct varieties, which look different and occur under different conditions:
- Inferior-mirage flash: the common one. A warm surface under cooler air produces the hourglass Sun, and the flash appears as the narrowing waist pinches off. Typically one to two seconds. This is the flash most sea-level observers see.
- Mock-mirage flash: produced by a thermal inversion rather than a warm surface. The Sun appears sliced into horizontal strips, and a thin strip detaches from the top of the disc and turns green as it fades. Best seen from elevation — a clifftop, a ship's bridge, an aircraft — looking down through the inversion layer.
- Sub-duct flash: the rare and dramatic case, requiring the observer to be inside an atmospheric duct. The Sun appears to set as an hourglass and the flash can persist for many seconds, occasionally longer.
- Green ray: not a rim at all but a beam of green light shooting upward from the point of sunset, needing haze to scatter the beam into visibility. Distinct enough that it is worth counting separately.
- Green rim: the underlying effect itself, continuously present at the top edge of the low Sun and visible any time with binoculars or a telescope — but only through optical aid, and never safely while the Sun is high.
Where the Odds Are Best
Three things matter, in order: a genuinely sharp horizon, clean stable air, and patience.
West-facing tropical and subtropical coasts are the classic ground, and Hawaii, Florida's Gulf coast, the Caribbean, coastal California and the Canary Islands all have local traditions around it. High-altitude observatory sites do well because the air above them is thin and unusually steady; the desert observatories of northern Chile, sitting above the coastal inversion in the exceptionally dry air of the Atacama Desert, produce some of the best-documented flashes anywhere. Cold water under warm air, as off the Pacific coast of South America or over the Great Lakes in spring, is also favourable.
Polar regions are a special case and by some distance the most productive. When the Sun's path is nearly parallel to the horizon rather than crossing it steeply, the whole sunset can take hours instead of minutes, and the conditions that produce a flash persist correspondingly longer. The most celebrated observation on record comes from members of Admiral Richard Byrd's Antarctic expedition at Little America in 1929, who reported watching a green flash persist, on and off, for around thirty-five minutes as the Sun skimmed along the horizon at the end of the polar night.
The practical method: pick a west-facing sea horizon with no cloud bank sitting on it, arrive early, and do not stare at the Sun while it is still bright — wait until it is dim enough to look at comfortably, which is usually when the lower edge has already touched the water. Watch the very top edge of the disc, not the whole Sun, through the final seconds. A camera with a long lens shooting continuously will catch flashes the eye misses entirely.
Verne, Rohmer and the Reputation Problem
The flash owes much of its fame to Jules Verne, whose 1882 novel Le Rayon Vert built a plot around a young woman determined to see one, and attached to it a folk claim — that anyone who witnesses the green ray can read their own heart and the hearts of others. The legend appears to be largely Verne's own invention, but it took hold, and the book set off a wave of attempts by readers who had never previously heard of the phenomenon. Éric Rohmer borrowed the title and the idea for his 1986 film, which ends on a genuine flash.
That literary history left an odd legacy: for decades the green flash carried a whiff of the mythical, and it was widely dismissed as an afterimage produced by staring too long at a bright orange Sun. Two facts settle the question. Photographs and video record it, and film has no retina to fatigue. And it appears at sunrise as readily as at sunset — before any observer has looked at the Sun at all. It is a straightforward consequence of the same refraction that also makes the low Sun look flattened, and of the same scattering that gives the sky its colour and, at night, allows the faint moonlight of a moonbow to reach the eye at all.