What causes the strange green flash at sunset and sunrise on Earth?

Jun 25, 2023
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I am a sailor and skywatcher and have seen the flash about a dozen times. About 20 years ago I was sailing near the Golden Gate Bridge when the conditions were ripe for a green flash. Sure enough, a huge flash appeared. These last less than a second so if you blink, you miss it. The amazing part was that someone in Berkeley caught the flash on film as I was sailing by. In doing research the next day, the image showed up in a search and there I was in the picture. The flash is very evident and framed by the bridge. What an amazing moment and the fact that I found the pic is even more amazing.
 
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Jun 25, 2023
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One more flash story: I was sailing off Mexico in a race from San Diego to Cabo with 150 boats. It was the first night of our 750 mile sail. The boats were all within VHF radio range and the conditions looked perfect for the flash. I got on the radio to announce that it might happen on this evening and to be on the lookout. An old timer comes back and says to the fleet, I have been looking for the green flash for 40 years, what the hell are you talking about? I told him and the rest of the fleet to be patient and see what happens. There are 500 people on the boats around us looking at the sunset. Sure enough, there is a spectacular green flash! The old timer comes back, Craig, you are a genius, bless you.
 
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Jun 26, 2023
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I've never seen the green flash. I've talked with guys that have seen the green flash....twice in the same day. SR-71 pilots whose mission plan had them crossing the terminator multiple times.
 
Best place is any water horizon. If there are no clouds on the horizon it is virtually certain to show up. Atmosphere must be stable, no inversions, good lapse rate. I have seen it many times (a dozen?). Usually clouded out where I have lived, but has never failed to show up if horizon is unobstructed. The problem is clear air to the horizon is rare in the humid eastern US, except during wintertime Canadian high frigid air masses. But you need to be at the beach to see it.
I have seen it in the Adirondaks, over distant mountain horizon. Also, it is extremely short, maybe half a second and very tiny.
 
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