A major problem for people who have never seen a green flash is not knowing what to look for. Flashes are not always green; they are not a “flash” in the sense of a sudden burst of brightness (except at sunrise). They do not (usually) light up the sky, but are often small and inconspicuous. Fortunately, there are some photographs here that show the commoner forms of green flashes.
However, green flashes usually are brief: a duration of one or two seconds is typical at moderate latitudes. The simulations will give you some notion of the time scale of these events. Of course, if you can get to very high latitudes, you can prolong them greatly.
Once you know what to look for, you'll find that green flashes are surprisingly predictable , as experienced observers have found.
And speaking of experienced observers, let me recommend an introduction to green flashes by Walter Nissen, who vividly describes what he has seen, and very reasonably complains about the term “green flash”.
If you want to see green flashes, you must be in the right place. The major difficulty that people have in trying to see GFs is to insist on standing someplace where green flashes are almost never visible.
The primary consideration is that green flashes are related to mirages . By a long-established theorem of atmospheric optics, mirages of astronomical objects can only occur below the astronomical horizon (assuming the layers in the atmosphere are “horizontal” — a condition that is not always met in the real world). So your apparent horizon must usually be formed by some obstacle belowe eye level.
Of course, the ocean meets this requirement perfectly; so a sea horizon is guaranteed to be favorable. A water horizon helps in another way: the water's large heat capacity almost always produces a temperature difference between the water and the air above it; and these temperature differences make the mirages that make green flashes big enough to be seen easily.
But, to use a common scientific phrase, while the sea horizon is sufficient , it isn't necessary . You can obtain a low enough horizon by standing on top of a mountain, or (in flat regions) a tall building. (I hear that flashes are regularly seen from the Empire State Building in New York; but any building tall enough to surmount everything within view will do.)
Airplanes and balloons also provide a high vantage point, and as you'd expect, there are lots of reports of green flashes seen from these as well. It doesn't matter much what forms the visible horizon, though a sharp, straight line works better than a cloudtop or an ill-defined surface such as brush or a forest. Still, many people have seen cloudtop flashes.
Most people who have never seen a green flash have no idea what to look for; or, worse, a completely mistaken idea of what to look for. To give you some idea of what to expect, I have prepared some oversimplified simulations of a few common types of sunset. These are just little animated cartoons of typical sunsets, showing what different conditions produce.
Obviously, everybody in the polluted Midwest, or Europe, is usually out of luck. Try to avoid big cities, particularly those in damp, muggy climates, where the high relative humidity increases the scattering cross-section of whatever aerosol is dirtying the air. But even in the Midwest, there are a few hours of clear air after the passage of an Arctic cold front; so green flashes have been seen over the Great Lakes.
Even so, it's surprising how much reddening haze can be in the lower atmosphere, and still allow green flashes to be seen. I've seen a flash when I'd have sworn the haze was too heavy to allow it.
Using optical aid, such as binoculars, or a camera viewfinder behind a long telephoto lens, you should be able to see some green-flash phenomena in most sunsets. Here in San Diego, I probably see one or more flashes in 5 out of 6 sunsets over the ocean. Without magnification, that would be more like 1 in 6. But I know what to look for; most of the flashes detectable by a trained eye are barely visible and certainly not conspicuous, and those would pass unnoticed by a novice. So, magnification makes a big difference.
But, be careful. The Sun is safe to look at, even with binoculars, when it's right down on the astronomical horizon. But it's a lot brighter just a few minutes earlier at sunset, or a few minutes later at sunrise. The brightness changes by a factor of two every minute near sunrise and sunset, so an error of just a minute or two can make the difference between eye safety and eye injury.
Be sure your horizon is really lower than where you stand; and don't look at the Sun when it's more than its own diameter above that low horizon. If your thumb, extended horizontally at arm's length, can cover up the Sun while touching a sea horizon, the Sun is low enough to look at safely.
A sensible rule is: If the Sun is too bright to look at comfortably, don't look at it.
There are also visual effects at sunset if you've been staring at the red or orange Sun. These reduce your red sensitivity, and often make the nominally yellow stage of a flash appear green to the eye. To avoid these effects, don't look at the Sun until it's nearly down (have a helper tell you when to look, or watch only a strongly attenuated image.) But some people prefer the effect of the enhanced green seen with a bleached retina ; so pick the state of adaptation you prefer.
There seems to be a story going around that wearing sunglasses makes seeing green flashes more difficult. To the extent that sunglasses diminish the brightness of the retinal image, they might reduce the degree of retinal bleaching at sunset, and make marginal flashes more likely to appear yellow than green. As sunsets are essentially unpolarized, Polaroid sunglasses are irrelevant.
The viewfinder of a camera attenuates the image somewhat; and, with a telephoto lens, you have useful magnification. So this brings us to photography.
Remember that the Sun is just about 1/100 of a radian across, so its image on the film is 1/100 as big across as the focal length of the lens used. With a standard 50mm lens on a 35mm camera, you get a solar image only half a millimeter in diameter. (Miniature cameras and throw-away cameras usually give even smaller images).
A good rule of thumb is that you don't see all the detail in a photograph that you can see with your eyes if the focal length of your lens is less than about 150mm.
You begin to get an image that shows more detail at 300mm focal length. I started out with a 500mm lens, then decided I needed larger images, and added a doubler, to give 1000mm focal length. That's super-telephoto by most people's standards. It works pretty well.
I used to photograph sunsets fairly often — certainly several a month, on the average. I tried to photograph a green flash in a large fraction of those sunsets — perhaps 4 out of 5. After getting a lot of yellow flashes by tripping the shutter too soon, I learned to wait almost a second after I first saw green, if I wanted to get it on film. Then I got something green in about 1 sunset in 3; but of course, most of those are little inconspicuous flashes, or the exposure is off, and the picture isn't fit to show in public. So it takes a lot of attempts to get a decent GF photograph; I have a few dozen.
If you are seriously interested in green-flash photography, I have some more technical advice here.
Different circumstances favor different types of flash. In general, you want to place your eye near the level of some atmospheric thermal structure that favors the production of mirages. Where you go depends on the weather, your latitude, and the time of year.
Inferior mirages, which produce the classic “last glimpse” flash that is reported most often, require a surface warmer than the air, and an eye position just a few meters above that warm surface. For practical purposes, that surface has to be water: a sea or lake horizon. The water will be warmer than the air most often in winter and least often in summer. A day after the passage of a cold front is very favorable. The water is more often warmer than the air in low latitudes than in high ones; but these flashes are seen over the Gulf Stream in the British Islands, which are fairly far from the Equator. In San Diego, these flashes are commonest around Christmas and New Year's, but I've photographed them as late as March.
Inferior-mirage flashes are best seen from heights only a few meters above sea level; they're often seen by people standing low on a beach, or on the deck of a small ship. One consequence of being so close to sea level is that the thin region from which the flash is visible rises very slowly as the Sun sets. That means you can often see the flash twice, or at least prolong its visibility, by starting from a sitting position, and then jumping up as soon as the flash appears. My simulations indicate that the inferior-mirage flash should appear larger, the closer you are to sea level; so sitting near the water's edge, where you can just see over the waves, should be most favorable.
On the other hand, mock-mirage flashes, which seem to be photographed most often, require the opposite type of thermal structure: a thermal inversion. These generally occur most often in late spring and early summer (except for Mediterranean climates, like Southern California, where they occur during the “Santa Ana” season of late fall). To see these, your eye must be above an inversion — preferably, just above it. Obviously, the higher you are, the more likely there is an inversion below you; but the higher you are above the inversion, the smaller its flash will be. Inversions are more common at high latitudes than at low ones; but we see a lot of them here in San Diego, at 34° latitude.
Other types of flash occur in special circumstances: at cloud tops, over distant mountains, beneath very strong thermal inversions at very high latitudes. Where to look depends on your circumstances. Some trial and error may be required to learn the best conditions at different times of year.
G. P. Können of the KNMI (Royal Dutch Meteorological Institute) has asked me to add one more bit of advice: at sunset, keep looking for several seconds after the Sun has disappeared. You might see a “Green Ray ” display, or the Sun might reappear and give a green flash, owing to waves on the water or in the air. This is more likely at high latitudes, where the pace of a sunset is more stately than near the equator.
So far as I know, a real “Green Ray” has never been well photographed, so this advice is particularly important for photographers. Note that the green ray is usually (though not always) several times larger than the Sun itself, so even an ordinary camera should be able to capture this phenomenon.
You might be interested to compare my advice with the results of a statistical study of some 600 sunsets performed near Rome. Not all of the interpretation is sound; but the observational data are, I think, in general agreement with my own experience. The author offers advice on how to predict the appearance of green flashes.
Unfortunately, it's written in rather broken English; but if you can make your way through the linguistic difficulties, you'll find some useful information. In particular, there are guidelines for choosing the proper exposure for photography. (If you read Italian, you'll be happier reading the Italian version of that page.)
Although these “rules” will help maximize your chances of seeing a green flash, they are not without exceptions. Many people have reported green flashes over distant mountains that were certainly higher than eye level; I have been surprised at occasionally seeing flashes even in fairly murky conditions; and of course there are flashes that are big and spectacular even to the unaided eye — a couple have even been photographed with ordinary cameras. The extreme variability of green-flash phenomena allows them to break almost any rule occasionally. But, granting all this, I still insist that the recommendations above will greatly increase your chances of seeing a flash. Though exceptions do occur, they are uncommon.
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