||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