Natural Phenomenon - Green Flash

How Does Green Flash Happen?



Green flashes are real (not illusory) phenomena seen at sunrise and sunset, when some part of the Sun suddenly changes color (at sunset, from red or orange to green or blue). The word “flash” refers to the sudden appearance and brief duration of this green color, which usually lasts only a second or two at moderate latitudes. 



A green flash, which occurs more commonly at sunset — but can also occur at sunrise — is a phenomenon in which part of the sun can be observed suddenly and briefly changing color. It usually lasts only a second or two — which is why it is referred a flash — as the sun changes from red or orange at sunset, for example. 

The green flash is viewable because refraction bends the light of the sun. The atmosphere acts as a weak prism, which separates light into various colors. When the sun's disk is fully visible above the horizon, the different colors of light rays overlap to an extent where each individual color can't be seen by the naked eyes. 
 
But what actually causes the green flash? It is the result of the combination of two optical phenomena: a mirage and the dispersion of sunlight.
 
 The first ingredient is a mirage. Like the famed desert mirage, the mirages that cause green flashes are the result of sunlight bending on its way to your eye and creating an image of an object somewhere other than its actual location.



Dispersion is the second ingredient. When the light bends (or refracts) on its way to your eye, the air acts as a prism to split up the light into its constituent colors. So, first the red light from the sun sets, followed by yellow, green, blue and violet.  

How Green Flash Happen?

When the sun starts to dip below the horizon the colors of the spectrum disappear one at a time, starting with those with the longest wavelengths to those with the shortest. At sunrise, the process is reversed, and a green flash may occur as the top of the sun peeks above the horizon.


It is a primarily a green flash because more green light gets through and therefore is more clearly seen. Sometimes, when the air is especially clear, enough of the blue or violet light rays make it through the atmosphere, causing a blue flash to be visible. However, green is the most common hue reported and captured in photos.
 

To the untrained eye, all green flashes may look the same. In fact there are a total of four types:

- the inferior-mirage flash

- the mock-mirage flash

- the sub-duct flash and

- the green ray.

 

What conditions are required to see the green flash?

 

The green flash is best observed when you have a clear view of the horizon uncluttered by foreground objects and pollution free. This usually means you need to see a distance of several miles "out", almost to the point where the curvature of the Earth defines the limit. This is primarily why stories of seeing the green flash frequently occur at the ocean. This is due to the additional amount of atmosphere one is looking through at the horizon when the Sun is setting. In addition - and equally important - is the fact that the line of sight is nearly parallel to the horizon.   

 

Why is it called a "green" flash and not a "blue" flash? 

 

Because contamination in the atmosphere scatters blue light removing it from the line of sight. More green light gets through and therefore is more clearly seen. In extraordinary conditions, a "blue" flash might be seen. 

 

Actually, all celestial objects experience the same effect near the horizon; it is possible to see "green flashes" from the setting Moon, Venus, or bright stars like Sirius. 

 

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.