Why Green Flash Sunset Sightings Are Surging Across Coastal Skies This Winter
Dispersion alone explains why colors separate, but it takes a mirage phenomenon to make that sliver large enough to see with the naked eye. In a uniform atmosphere, the green rim measures only a fraction of an arcminute thick, far too slender for casual human observation. The flash needs atmospheric magnification.
That magnification requires a temperature inversion layer, where a pocket of warmer air sits directly atop cooler, denser surface air. The density boundary acts like a refractive lens, bending the rays of the setting sun upward or downward along an irregular curve. This creates an optical duct that stretches, magnifies, or stacks slices of the solar limb.
Depending on the exact thermal profile of the coastal boundary layer, observers see different mirage variations:
| Mirage Profile | Thermal Setup | Visual Signature | Typical Duration |
|---|---|---|---|
| Inferior Mirage | Ocean water is noticeably warmer than the surface air layer directly above. | An inverted second solar image merges upward, pinching into an hourglass or oval. | 1, 2 seconds |
| Mock Mirage | Sharp inversion layer hangs 50, 200 meters above sea level, well above the observer. | Striped horizontal notches appear; a detached green sliver floats briefly above the rim. | 1, 3 seconds |
| Sub-duct Flash | Strong, persistent atmospheric duct traps light along flat surface layers. | An extended green ray pulses upward as an optical streak rather than a flat rim. | 0.5, 1.5 seconds |
| Green Ray (True Beam) | Extremely stable haze-free duct with uniform horizontal density lines. | A narrow vertical beam shoots several degrees straight into the twilight sky. | Fraction of a second |
The classic inferior mirage accounts for roughly 85% of coastal California sightings. When the sea retains residual warmth compared to crisp evening breezes, the lower sun flattens into an omega shape before the top corner snaps into an unmistakable emerald pinpoint.
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