Green flashes are real (not illusory) phenomena seen at sunrise and sunset, when some part of the Sun suddenly changes color (at sunset, from red or orange to green or blue). The word ``flash'' refers to the sudden appearance and brief duration of this green color, which usually lasts only a second or two at moderate latitudes. There are several quite different phenomena commonly lumped together under the name of ``the green flash'', and this intermingling of disparate phenomena has fostered confusion. So I prefer to say ``green flashes'' and avoid the definite article.
You have probably heard something about green flashes, but may not have seen one. If so, you'll be happy to find that a number of pictures of green flashes are available on the Web.
A number of Web pages offer incomplete explanations of green flashes. All of those listed here have some virtue as well as some defects. I am developing my own page to counter the weaknesses of the ones I've found. I also have some simulations (i.e., gif animations) of the commoner forms, as evidence that things are not so simple as the textbooks would have you believe.
| kind | characteristics | conditions | best seen from ... |
|---|---|---|---|
| inferior-mirage flash | Joule's ``last glimpse''; oval, flattened below; lasts about 2 seconds | surface warmer than the overlying air | 3 to 5 meters above the sea |
| mock-mirage flash | indentations seem to ``pinch off' a thin, pointy strip from the upper limb of the Sun; duration 2 to 5 or even 15 sec. | atmospheric inversion layer below eye level | the higher the eye, the more likely; flash is most obvious when the eye is just above the inversion |
| sub-duct flash | large upper part of an hourglass-shaped Sun turns green for up to 15 sec. | strong atmospheric inversion | in a narrow height interval just below a duct (can occur at any height) |
| green ray | green beam of light that seems to shoot up from a green flash, or is seen immediately after sundown. Usually only a few degrees long; lasts no longer than a couple of seconds | hazy air and a bright green flash of one of the kinds named above that acts as a light source | ?? |
Remarks: Most (about 2/3 or 3/4) of all observations involve inferior-mirage flashes. The rest are for the most part mock-mirage flashes. The two other kinds are rare and constitute only about 1 per cent of all reports. Not all kinds are named here -- there are also some that are not understood, such as the cloud-top flash (which usually is seen as the Sun sinks into the coastal fog, but sometimes also at distant cumulus clouds). And Alistair Fraser's flashes, which are seen in hilly country, and are a variant of the mock-mirage flash in places where inversions are pushed up over hills.
This side bar was transferred from: http://mintaka.sdsu.edu/GF/papers/Zenit/glance.html
By now, you may be eager to run out and see some green flashes for yourself. Here's some advice on how to do that. If you don't know what you're doing, you can look for years in vain. If you do know what to do, you can see green flashes in most sunsets.
There's a wealth of information out there -- much of it, unfortunately, wrong. Here are some common misconceptions about green flashes, each contrasted with the facts of the matter.
There's no substitute for a good book (or magazine article). Here's a recommended reading list on green flashes. It is not complete, but will provide additional words (to be taken with a grain of salt) and pictures. A complete bibliography is also available. (Because the bibliography is a huge file that will load slowly, links to it are set in boldface as a warning.)
Want more? Here's an "omnium gatherum" of links to Web pages where green flashes are mentioned. You may be surprised at the number of commercial uses of green flashes -- as trade names, as tourist lures, etc.
The Above was Transferred From: http://mintaka.sdsu.edu/GF/