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The Green Flash |
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Definition and Description: The green flash is an atmospheric refractive phenomenon where the top edge of the sun will momentarily turn green. It is seen rarely by the naked eye, primarily because it requires specific conditions to occur, but also because it requires the observer to know what to look for. Despite the name, there is no "flash;" the event only lasts from a fraction of a second to at the longest, a few seconds. |
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The Basic Cause: Refraction bends the light of the sun. The atmosphere acts like a weak prism, separating the light into diferent colors. Blue light is bent more strongly than red. But this refraction is very weak even at the horizon, which is why the sun isn't seen as being multi-colored in the daytime. The effect is magnified by the atmosphere, and at the horizon there is a lot of atmosphere between the sun and the viewer. Layering in the atmosphere causes an effect similar to a horizontal cylindrical lens: the separation of the color bands is exaggerated in the vertical direction. |
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Conditions: The green flash is best observed when you have a clear view of the horizon, with no objects or pollution in the line of sight. Usually you need to be able to see a distance of several miles out, almost to the point where the curvature of the Earth defines the limit. Because of this, the green flash is most often reported by the ocean.This is due to the additional amount of atmosphere one is looking through at the horizon when the sun is setting. Also, the line of sight is nearly parallel to the horizon. |
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The Setting Sun: When the sun is a few degrees above the horizon, it appears distorted and flattened in the vertical direction. The closer it gets to the horizon, the more flattened it gets. Soon, the edge of the sun will become notched on either side. Thes notches appear to ride up the sides of the sun from bottom to top; however, in reality the notches are caused bythe atmospheric layering and aren't moving--it's the sun that's setting through them. When the notches reach the top of the sun, they meet and "pinch" off the edge of the disk. This is the part that suddenly turns green: a green flash. |
