Monday, September 19, 2011

Green Flash

September 19, 2011
3:39 P.M.

"It was the best green flash I've ever seen" - Mila Zinkova, Santa Cruz, CA, January 29, 2006

I've written two entries on interesting phenomena that I've seen this summer. I have seen Kelvin-Helmholtz waves and a massive superior mirage, and you can see my posts on those. This year, I saw another interesting atmospheric phenomenon - the "green flash." It is subtle, and if you aren't specifically looking for it, it can be hard to see, but it is very interesting when you do see it.

If you see a green flash, there is no need to duck for cover; Lord Voldemort isn't trying to kill you. A green flash is a very rare sliver of green light most usually seen right after sunset or before sunrise.

Let's use a setting sun as an example, as green flashes are most commonly seen at sunset. Green flashes occur because light is refracted as enters the atmosphere. Light moves more slowly in the denser air nearer the ground than the thinner air aloft. Out of all the colors in the visible spectrum, infrared (the color) has the longest wavelength, and ultraviolet (once again, the color, not the separate type of  wave) has the shortest. Shorter wavelengths follow closer to the curvature of the Earth when the sun is right at the horizon. Therefore, as the sun sets, the longer wavelengths (first red, then orange, then yellow) disappear because they don't follow the curvature as closely as the shorter wavelengths (green, blue, violet) and therefore travel faster since they are higher up, where the air is thinner. Green, meanwhile, stays in the lower atmosphere and travels slower, so green is the last color we see, with it being more yellowish at first and more blueish at the very end before it dies out.

Why don't we get blue or violet flashes then? These types of light are generally scattered throughout the atmosphere as the sun sets. Otherwise, we'd see violet flashes. On very rare occasions, however, you can see a blue flash, especially if the air near the surface is very dense and the air aloft is much lighter (like an inversion).

There are four types of green flashes. These are the inferior-mirage, the mock-mirage, the sub-duct, and the green ray. The inferior and mock-mirage flashes are by far the most common, making up 99% of all green flashes, with the inferior being the commoner one. I saw an inferior green flash, and the picture at the top is an inferior green flash as well.

Inferior-mirage green flash looking west from Madagascar - Vic and Jen Winter, 2001

Inferior green flashes are formed when the air at the surface is warmer than the air above. They are best seen at sea level.

Mock-mirage green flash - Mila Zinkova, San Francisco, CA, September 17, 2006

Mock-mirage green flashes are formed when the air at the surface is colder than the air above. They are best seen above sea level.

Sub-duct green flash - Andrew T. Young

First of all, a duct is a horizontal layer within the atmosphere that traps various types of waves below it. I've seen these before; in fact, the picture below shows a duct (from the superior mirage i saw earlier this summer). That top of the "fog bank" is the duct, and although it isn't perfectly lined up, you can see how the "boat" flattens out just before that.

Sub-duct flashes are very rare and are incredible. They last for around 15 seconds as opposed to the second or two you see inferior-mirage green flash for. I'd suggest checking out this link. Incredible pictures of the whole process, including a violet flash at the very end! Highly, highly recommended. If I was a teacher, I'd require everybody to look at this page, so go ahead a take a peak.

 Green ray (I think) - Guillaume Dargaud, Dome C, Antarctica, March 2004 (58C below 0!)

It is hard to find pictures of green rays because they are so rare and "green flash" and "green ray" are often used interchangeably, even though the "green ray" is a specific time of green flash. This is the best picture I could find.

And finally, a video of a standard inferior-mirage green flash.

Three hours later, I'm done and knowledgeable about green flashes! I can't do super long posts when school starts, but I'll still be up on the blog. I move in on the 22nd and start classes on the 28th. UW baby!

Thank you SO MUCH for reading my blog!!! I really appreciate it.

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