explanation

The green flash is an atmospheric event. when conditions are right, at the last moment of the setting sun, its upper edge or limb blazes with an emerald green color for a few seconds before disappearing below the horizon. To understand the green flash requires some background knowledge. Even the most cursory everyday observation reveals that the sun fades and appears to turn reddish-orange as sunset approaches. This reddening is caused by Rayleigh scattering of light by molecules in the atmosphere. These molecules are very small compared to the wavelengths of visible light, which has the consequence that the scattering is proportional to the inverse fourth power of the wavelength. Thus scattering of violet light (400 nm) is 7 times more effective than that of red light (650 nm). The result is that the sun looks red because so much blue light is removed from the line of sight. Astronomers use the term "airmass" (sumbolA) to describe the Earth's atmospheric thickness in the line of sight. An object overhead (altitude h = 90 degrees) is seen through one airmass, at altutude 30 degrees through two air masses, and so forth.

A more detailed examination of the setting sun shows that it also appears somewhat flattened and may show horizontal structure and banding. The flattening or ellipticity is due to atmospheric refraction, which raises the sun's lower limb by about 35 minutes of arc while the upper limb is raised by only about 29', when the lower limb is tangent to the horizon. The banding effects are due to layers of differing temperature and density in the atmosphere. The setting sun's disk is made up of light of all colors. Green and blue light are refracted by air slightly more than red light, so the disk actually consists of a flattened red disk, with a yellow disk slightly above it, a green disk above that, and blue and violet disks at the top. This phenomenon is called atmospheric dispersion and is easily visible to anyone who looks through a telescope with high magnification at a bright star or planet low in the sky.

In the case of the setting sun, the upper limb is green (or rarely blue). If the horizon is provided by the sea or by very distant flat land, and the air is very clear for a hundred kilometers or more towards the sunset, then it is possible to see the green flash. As the upper rim of the setting disk approaches the horizon, it begins to spread into a thin bar of light, then runs through the spectrum from orange to yellow, then pale green, and finally reaches a deep emerald color for two or three seconds. Under favorable conditions, a brief blue blob of light may be seen after this, but exceptional clarity of the air is needed.