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| Observing & photographing green flashes |  |
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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 take risks. 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? |
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