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  Green Rim

  Not the way green flashes form. The rays of the setting sun are refracted by the atmosphere to curve slightly downwards. The sun appears flattened and raised in the sky. Green light is refracted more and the 'green sun' is very slightly higher than the red one. But the effect is small and cannot be seen with the unaided eye. Green flashes need something more!
Some textbooks and most web pages say that the green flash is produced by refraction under ordinary atmospheric conditions. That is not true, if it was then we would see flashes much more often.

A standard stable atmosphere gets cooler with increasing height and the air density falls smoothly and monotonically. The lower and denser portion acts as a giant lens bending rays from the setting sun towards the earth. As a result, the rays appear to be coming from a point higher in the sky and the sun appears to be raised up. Green light is refracted more strongly than red and so different coloured images of the sun become very slightly vertically separated. As the sun sinks it develops a green** upper edge and a red lower one. But the effect is small usually only visible in binoculars - do not ever look without full eye protection, even for an instant.

To make a green flash these tiny refraction effects need somehow to be considerably magnified. We need a mirage ...

Why a green edge, why not blue? Why are green flashes green?    Blue light is refracted more strongly than green or red and the sun's upper edge might be expected to be rimmed with blue outside a band of green. This rarely happens because so little blue light remains in the rays of the setting sun. Blue light is strongly scattered by air molecules, small dust and aerosol particles out of the direct rays. There is a further effect, traces of ozone absorb the orange light which makes the contrast between the red sun and green flash even more dramatic.

For all that, sometimes we do see blue flashes!