The Green Flash

Does this sound familiar? You have just had a full day of diving; you're sitting with some new friends on the hotel terrace, having a drink and watching the sun sink into the Caribbean, when one of them says, 'Wonder if we'll see the green flash tonight?' You don't know what he's talking about, but you play along and like everyone else you stare at the sun until it's gone. There was no flash, and you think you've been had. Well, you haven't. There is a green flash, but you have to be very lucky to see it.

The green flash was apparently first documented by Captain Back of the H. M. S. Terror while in the arctic during its expedition of 1836-1837. Referring to a January 17 event he wrote:

               'In the morning however, at a quarter before ten o'clock while standing 
                on an ice hummock about seventeen feet high, and looking toward the east, 
                I had observed the upper limb of the sun, as it filled a triangular cleft 
                on the ridge of the headland, of the most brilliant emerald colour, a
                phenomenon which I had not witnessed before in these regions.' (Murray 1838).

At first glimpse of a rising sun or final glimpse of a setting sun is when you might see a momentary burst of dazzling green light. It is not, as some have suggested, simply a product of retina fatigue whereby the removal of the brilliant orange of a sunset induces its complementary color to be seen. The green light is physically there, and has been photographed (Meinel 1983).

To understand this phenomenon one must begin with the fact that air is a refractive medium --- it bends light. Although the bending produced by a given layer of air is very weak (index of refraction at sea level is ~ 1.0003), it becomes progressively stronger as light approaches the earth's surface and the air density increases. At sunset the cumulative effect is to 'elevate' the sun's image by about half a degree above its true position. Moreover, like most optically transparent media, air is also dispersive --- it bends light of different frequencies by different amounts. This is easily demonstrated with a telescope at sunset by viewing the sun alternately through blue and red filters and seeing the sun's blue disk about 20 seconds of arc higher than the red disk (see the figure below). The naked eye's resolution limit is only about 120 seconds of arc, which explains why we don't normally see a blue fringe at the top of the sun and a red fringe at the bottom.

Suppose now that you are viewing the setting sun through an 'ideal' atmosphere: clear air, sharp horizon, air density monotonically decreasing with increasing altitude. The top of the sun's 'red disk' is tangent to the horizon and is setting normal to it. In 1.4 seconds the sun's 'blue disk' will sweep towards the horizon and vanish. During that time you will see light that has progressively more colours subtracted from it, first red, then orange, then yellow, etc., until in the last split second before the sun totally disappears there should only be blue. So why don't we see a blue flash? Some people do. Lord Kelvin, in 1899, referred to exactly that when he described the sun rising over Mont Blanc, Switzerland. But there are good reasons why the flash should usually be green, and not blue.

The light sensitivity of the human eye is a function of wavelength. Under normal daylight ('photopic') conditions the peak stimulus is light in the yellow-green portion of the spectrum, as shown below.

At sunrise and sunset, when the oblique rays from the sun travel a longer distance through the atmosphere, the light reaching the earth has had a considerable portion of its blue content removed by Rayleigh scattering (that's why the sun looks red). The scarcity of blue light causes the peak stimulus to shift slightly towards the yellow, making a blue flash less likely and a greenish-yellow one more likely. But it is well known (Wilson and Brocklebank 1961) that when the eye sees a small patch of light adjacent to a large one, the small patch will be perceived as being slightly shifted in colour in the direction of the colour that is complementary to that of the large patch. With a large expanse of red sky and a wisp of greenish-yellow at the sun's rim, the wisp of greenish-yellow therefore appears to be biased towards the blue-green, and the flash appears to be emerald green. A blue flash can occur only if the atmosphere is unusually clear, and scattering does not deplete the sunlight of too much of its blue component.

Why is the green flash so elusive? A necessary condition for seeing it seems to be a well-layered atmosphere, where the index of refraction increases without interruption towards the surface of the earth. In fact, the gradient of the index of refraction should be greater than usual, to accentuate the dispersion of colours. This is not at all a common atmospheric condition. Usually the atmosphere has some turbulence, with anomalies in temperature and humidity from layer to layer. Such inhomogeneities cause abnormal refraction, which can produce a confused separation of colors. At the extreme, the required ordering of colours (blue above red) could be reversed, which would make it impossible to see a green flash.

Click here for a Green Flash photo!

This article has been translated into Romanian by Alexander Ovsov.

REFERENCES:

Glenn E. Shaw, "Observations and Theoretical Reconstruction of the Green Flash", Pure and Appl. Geophysics, 102, 223-235, 1973.