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! REFERENCES: Glenn E. Shaw, “Observations and Theoretical Reconstruction of the Green Flash”, Pure and Appl. Geophysics, 102, 223-235, 1973. Aden and Marjorie Meinel, “Sunsets, twilights, and evenihg skies” (Cambridge University Press, New York, 1983), Chapter 3. Sir George Back
