To see green flashes a sea horizon is
best but the view from a tall building or a hillside can suffice.
Flashes are also visible from aircraft. A low horizon is needed because
pure I-Mir and
M-Mir flashes take
place below the astronomical
horizon. The apparent sea horizon dips lower by an amount
depending on your height above sea level. Flashes made by wavy
inversion layers do not need such low horizons.
Stand a few metres above sea level to see I-Mir flashes. M-Mir flashes
require that you are above the inversion layer but the ideal height
is a compromise because the higher you are above the layer, the shorter
is any flash. In any event, inversion layers are often only a few
feet above the surface.
Don't be deterred by thin haze. It will dim the sun, thick haze will
hide the sun completely, but a good
flash might still be visible.
Do not stare at the sun or look at it through
any optical device - your eyesight could be permanently damaged.
An I-Mir flash, the type most seen by eye rather than a camera, occurs
just before the sun disappears altogether. Looking at the sun earlier
will, as a minimum, bleach the red sensitive receptors of your retinas
and impair your ability to see the flash faithfully. Don't
Photography needs a long lens and a very firm tripod. A 300mm
lens on a 35mm camera is probably a minimum but 1000mm is better.
Exposure will change as the sun sinks. Setting the camera to take
automatic spot readings might help but it is better to develop your
own technique by practising on 'ordinary' sunsets.
Take a whole series of images. Understanding how a flash was generated
is more certain if there are several well spaced images including
ones of the earlier shape of the sun. However, do not run out of batteries
or memory just before the flash occurs!
Archive the images and do any further processing only on copies.
Records. The main thing is to soak in the spectacle of a green
flash with your eyes and mind and not be distracted by taking records
or fiddling with camera settings.
However, if you would like to contribute to the scientific study of
green flashes and the structure of the lower atmosphere then you could
make a record immediately after the event.
Date and time will be stamped in the EXIF data on digital images (provided
that the camera clock is set accurately) but a separate record is
a useful check. Note your exact position and make an estimate of height
of the camera above sea level - a GPS measurement of latitude and
longitude is useful (GPS heights are not accurate enough). Which way
was the wind blowing? How fast? Gusty? If near the sea, how high were
the waves? What did the sun look like before the flash?