Rare Cloud-Top Blue Flash ~ Captured by Steve Mattan at Sunset Beach, Cape May, New Jersey. The sun is well above the horizon, its upper limb sinking behind cloud. The shrinking sliver first develops green and then deep blues before being extinguished. Cloud-top flashes are rare, occur much further from the horizon than ordinary flashes and are not wholly understood. This is a fine example.     All images ©Steve Mattan, shown with permission
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Cloud top flashes are mirages. Like all mirages, they result from refraction across the strong density gradients between air layers of different temperature. Parts of the mirage are vertically magnified, separating colours which are seen as a flash.

Cloud-top flashes are a form of mock-mirage produced when cooler denser air is below warmer.

Nearly all mirages are close to the horizon. Cloud top flashes sometimes occur higher up and blue rather than green flashes can be the result. Atmospheric scattering of shorter wavelength violets, blues and greens is much weaker away from the horizon thus giving more transmitted blues. The light of normal flashes is more denuded of blues yielding – at most – a green or yellow-green.

My thanks to Andrew Young for his very helpful comments.