People are less likely to see the green flash if:
1) they spend time looking at the sun before the last remnant remains.
2) the horizon is disturbed with larger waves.
3) they expect a moment of brighter light. (it's quick, not intense)
4) the air is humid or hazy or clouds are present.
The spectrum is produced at the limb of the earth in the same manner that white light is refracted in a simple cardboard slit spectroscope. There is no special inversion or reflection of the spectrum and it can be seen on land with the requisite flat horizon such as salt flats or a flat plain. If you consider the visible light as red-orange-yellow-green-blue-indigo-violet; the middle segment (yellow-green-blue) can be briefly distinguished as the "green flash". Of course, we all easily see the early diffused red & orange, so nobody exclaims this "flash" and thoretically one could see the indigo-violet flash, but this requires an even greater sensitivity. The green flash phenomenon is clear and simple optical physics and no mystery. Take care and joy, Aythya crew