at De Panne, Belgium . Florian
took this sequence on 31st March '03. This 'Etruscan
Vase' or 'Omega' sunset ends with a 1-2s long green
flash. The sunset was mainly an inferior mirage event but,
as usual, nature is richer - the sun is unusually distorted
and look at the two other green flashes 42 and 11s before
the sun disappeared! Larger images do better justice to this
fine sequence, check #1,
All images ©2003 Florian
Schaaf, shown with permission.
The classical green flash, a brilliant glint of green almost on the ocean
horizon just as the sun disappears, relies on a mirage
to magnify the usually small differences in refraction between red and
green light. The mirage occurs when there is warm air immediately over
the ocean and the air temperature gradient changes rapidly with height,
i.e. the temperature profile is curved. For the flash to be visible the
warm air layers must be below you.
First indications of something unusual are when a sliver of a 'second
sun' rises from the horizon as the 'real sun' descends. The lower sun
is an inferior mirage (inferior because the image is below the real sun).
As the sunset proceeds the lower sun eventually links with the upper sun
to form an Etruscan
vase or omega shape. The
two suns overlap more and more. At the moment when the suns are almost
gone the vertical magnification between the different paths of green and
red rays is strongest. The green flash then shines forth.
The mirage and green flash are sensitive to your height above sea level.
The strongest Etruscan vase effects are seen close to the sea surface
but you must be above the warm air layer to see the flash, too high and
little will be seen. The above sequence was from dunes 10m high but somewhere
a little lower could be optimum.
The horizon appearance can hint at whether a mirage and green flash might
be seen. Look at the horizon during the above sequence:
The peculiar tall and spiky 'waves' are not real. They are the fluttery
edge of the mirage and a sign that the sunset could be distorted with
a green flash at its end.