Monday, April 21, 2008


The Green Flash Phenomenon

Nearly all my life, I've heard of the Green Flash. Just as the sun sets over a broad expanse of water, so the story goes, the sky will flash over green. I've never seen it. Here is what's reported to be a photo of it from California near La Jolla two years ago. The ocean heats up, on the surface, and creates an inversion layer which acts as a lens and screens out all the other colors (somehow) just as the last of the sun is obscured by the horizon. I hope I live long enough to see it once.


"The ocean heats up, on the surface," - um, no.
Just as I have never seen it, I have never understood the explanation for it. I believe that's what the explainer said. I cannot vouch for the accuracy. You're of course right about the Ocean; lately it has cooled, not heated at all.
Roger, You do know that the sun doesn't actually touch the ocean when it sets, even though it looks like it does.
Good one, Andy. I seem to remember the temperature of the Gulf of Mexico water in Summer getting colder the lower you dove. Perhaps that was my inability to temperature regulate.
Actually, the ocean (or any large body of water) is (in the summer) warmer on the surface than it is deeper. (We neglect local phenomena like volcanic vents.)

In fact, there is normally a sharp temperature drop a few feet below the surface of the water. For more information, google "thermocline".

The green flash effect is caused by a chromatic aberration in the lens effect of the atmosphere. ("Chromatic aberration" is another good google term.)
If it's just the last bit of the sun going green, I've seen that. I have been told, however, that it involves the whole sky. Like Rick in Casablance, I guess I was misinformed.
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