Rick uses a 1100mm F/7 lens/scope extended to 2200mm to capture unusual sunsets from Blind Pass between Sanibel and Captiva Islands, Florida.
Mostly his distorted Florida** sunsets are of the inferior mirage type [warm air beneath cooler] leading to a classical green flash - "..the bottom edge of the sun on the horizon closes in and becomes narrow, in many cases it narrows so much it leaves a segment of the sun floating briefly a little above the apparent horizon. If the air is clear enough this segment will turn green and produce a Green Flash."
This one was quite different - "There was a thin line of sun right on the horizon that lasted for about 14 seconds after the line became flat."
As the sun descended green flashes were visible on the top. The remaining disk became more and more flattened until eventually only a bright line remained that then broke into fragments before disappearing.
The sequence has been by called a 'tin-hat' mirage by an early researcher after those worn the American WWI Expeditionary Force.
A temperature inversion, abnormally cooler air beneath warmer, is necessary to produce the mirage. Sun rays are refracted downwards towards the ocean by the temperature gradient between the two air layers. The rays produce an inverted slice of the sun beneath the ‘normal’ image. Rays passing deeper into the inversion layer produce a third erect image closest to the horizon.
If the temperature inversion gradients are strong enough the ray bent toward the ocean can travel for long distances around the curvature of the Earth. This ‘ducting’ often produces highly flattened sunset mirages. It is possible that the Florida mirage was produced by a very thin duct close to the ocean surface. In that case the line is an extremely flattened image of the whole solar disk.
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In contrast to Californian sunset mirages where a cool offshore ocean current combined with warm air mostly gives mock-mirages. |
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