Introduction to Green Flash
The green flash and green-ray are meteorological optical phenomena that sometimes occur transiently around the moment of sunset or sunrise. When the conditions are right, a distinct green spot is briefly visible above the Sun’s upper limb; the green appearance usually lasts for no more than two seconds. Rarely, the green flash can resemble a green ray shooting up from the sunset or sunrise point.
Green flashes occur because the atmosphere of Earth may refract, or force apart, the light coming from the Sun. Green flashes form as a family of related phenomena that arise from causes that are slightly different, leading to some types of green flashes being less frequent than others.
Seeing
Green flashes can be seen from any altitude. They are most often seen at an unobstructed horizon, such as over the ocean, but are also possible over cloud tops and mountain tops. They can occur at any latitude, although at the equator, the flash rarely lasts longer than a second.
The green flash also can be seen in association with the Moon and bright planets at the horizon, including Venus and Jupiter. With an unobstructed view of the horizon, green flashes are frequently seen by airline pilots, especially when flying westward as the sunset is slowed. If the atmosphere is layered, the green flash may appear as a series of flashes.
The first color shot of the green flash at sunset was taken by D.J.K. O’Connell in 1960 when he was watching at the Vatican Observatory.
Justification
Green flash happens because the atmosphere causes the light from the Sun to separate, or refract, into different frequencies. Mirages enhance green flashes by increasing refraction. A green flash is more likely to be seen in stable, clear air, when more of the light from the setting sun reaches the observer without being scattered. One might expect the light to flash blue because blue is refracted most of all and the blue component of the sun’s light is therefore the last to disappear below the horizon, but the blue is preferentially scattered out of the line of sight, and the remaining light ends up appearing green.
With slight magnification, a green rim at the top of the solar disk may be seen on most clear-day sunsets. However, for flash or ray effects, which require a stronger layering of the atmosphere and a mirage that serves to magnify the green from a fraction of a second to a couple of seconds, such magnification is easily obtained.
While simple atmospheric refraction or lensing explains the background gradient of red-amber twilight, the principal potential cause of the bright, verdant discontinuity from that gradient known as the Green Flash may be due to naturally-occurring coherent (laser) light.
This phenomenon was recently found by researchers at the Washington University School of Medicine, where infrared-laser light converts-up, or upconverting, to visible-green laser light, resulting in what the researchers there refer to as “a double hit” of photons on the retina, creating the perception of bright neon green from an originally invisible infrared laser.
Doubling the wavelength of green light yields roughly 1000-1100 nm infrared light, so the most likely hypothesis is that the Green Flash is coherent upconverted infrared (laser) light that “double-hits” the retina or camera lens, creating the effect of bright green (additionally, this explains why the Green Flash is also sometimes blue or purple; coherent infrared light can upconvert to any color of visible light if the conditions are met).
Kinds
The following is a list of some of the optical phenomena that are associated with the “green flash” Description:
| Type | Characteristics | Conditions | Best seen from |
|---|---|---|---|
| Inferior-mirage flash | Joule’s “last glimpse”; oval, flattened below; lasts 1 or 2 seconds | Surface warmer than the overlying air | Close to sea level |
| Mock-mirage flash | Indentations seem to “pinch off” a thin, pointy strip from the upper rim of the Sun; lasts 1 or 2 seconds | Atmospheric inversion layer below eye level; surface colder than air | 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 seconds | Observer below a strong atmospheric inversion | In a narrow height interval just below a duct (can occur at any height) |
| Green ray | Green beam of light either shooting up or seen immediately after sundown; usually a few degrees long, lasting several seconds | Hazy air and a bright green flash acting as a light source | Sea level |
The vast majority of the optical phenomena observed are inferior mirages or mock-mirages. Less than 1% of the other phenomena reported. Naming a few additional phenomena not found in the table above (and not well understood) are things like cloud-top flashes (as the sun sets into coastal fog, or cumulus clouds in the distance).
Flashes of blue
A “blue flash” can occasionally be seen when there is enough blue light present.
Green rim
When an astronomical object rises or sets against the horizon, the light from it travels through Earth’s atmosphere, which functions as a prism to separate light into its component colors. Color of the upper rim of an astronomical object can range from green to blue to violet depending upon the dilution of pollutants with their dispersion into an expanding volume of atmosphere.
The lower rim of an astronomical object is always red. A green rim is very thin and is generally impossible or can’t be clearly seen by naked eye. Usually, a green rim of an astronomical object fades away when the object is very low above the horizon because of atmospheric reddening, but sometimes the conditions are correct to see a green rim just above the horizon.
What was likely the longest observation of a green rim—which occasionally could have been a green flash—is described in the following quotation. Members of the Richard Evelyn Byrd party from the Antarctic Little America exploration camp saw it intermittently for thirty-five minutes in 1934:
There was a scramble for the surface and as eyes turned southward, they saw a small but bright green spot where the last ray of the upper rim of the sun hung on the skyline. It lasted an appreciable length of time, several seconds at least, and no sooner disappeared than it flashed forth again.
Altogether it remained on the horizon with short interruptions for thirty-five minutes.
When it vanished briefly it had appeared as though shut off by a tiny spurt-an inequality in the skyline caused by the barrier surface.
Even by moving the head up a few inches it would disappear and reappear again and after it had finally disappeared from view it could be recaptured by climbing up the first few steps of the antanea [sic] post.
1. What is the phenomenon? Green flash
The Green Flash is a rare but interesting atmospheric phenomenon that manifests just after sunset or before sunrise. It is a fleeting flash of green color that lasts only for a few seconds and is commonly seen over the horizon, particularly where the air is clean and pure. While many have been fascinated by it, the scientific explanation behind it has been a matter of argument for decades.
The Green flash is caused by atmospheric refraction. Different wavelengths of light will bend in different amounts, as sunlight passes into the atmosphere. The shorter wavelengths, such as blue and green, will be refracted more than longer wavelengths like red and orange.
At sunrise or sunset, because of the curvature of the atmosphere the sunlight splits, and if the conditions are just right, the wavelength of green light, for just a few seconds, becomes visible. The phenomenon is most often observed over the ocean or flat landscapes without an obstruction on the horizon.
But why is it so rare? The green flash only occurs if certain conditions come together. That the air be clear, the horizon visible, and the observer in a position without obstruction to duration of sundown/sunrise. It’s more likely to happen at coastlines or high altitudes, where the view of the horizon is unobstructed, and the conditions of the atmosphere align to create the effect.
2. Is There More Than One Kind of Green Flash?
There are actually many kinds of the Green Flash which possess their own unique features. The most commonly known Green Flash is the so-called “standard” Green Flash, which is a green spot or flash appearing just above sunset on the horizon line, but in different conditions, different forms exist.
One of the more famous varieties is the “Green Ray”, a brighter flash of green that may sometimes be seen just before sunset or just after sunrise. The other type is “Green Arch,” in which the green flash manifests as a fleeting arc of light on the horizon and may occur in the upper atmosphere. A less common model known as the “Green Flash Mirage,” happens under a layer of cooler air above warm air when a mirage effect split the light spectrum, boosting the scene’s green effect.
Scientists have recently recorded what’s called the “Blue Flash.” Not truly part of the Green Flash, the blue flash occurs significantly less often than the green, due to the same refraction processes. The blue flash is rarer and shorter in duration. These variations in the Green Flash make the phenomenon more complicated than one might imagine and indicate that factors about the light itself, the atmosphere and position of the observer come into play.
3. Why Is the Green Flash So Rare?
Due to the conditions that need to be right for it to happen, the Green Flash is a rare sight. On top of that, atmospheric quality is the major differentiator. If there are aerosols, dust, or pollution, that will scatter light and obscure the view of the horizon, making it impossible to see the phenomenon. And it does make for good conditions for Green Flash with clean air and some light pollution areas, remote beaches and mountaintops in favor.
Atmospheric temperature and pressure gradients are also an important aspect. This green flash is more nuanced — it’s more likely to occur when rays of light shine through a clear layer of cool air above the horizon and get the chance to refract properly. The location of the observer is also critically important because they need to have a clear and unobstructed view of the horizon where the brief flash will be visible.
If the Green Flash will fade slightly is a function of the time of year. Owing to the comment of the sun at some times of day for the months surrounding the equinox, the perfect atmospheric condition, which causes it to occur. It happens more easily in higher latitudes where the sun is lower to the horizon and light is more likely to be refracted.
4. Is it Possible to Predict the Timing of the Green Flash?
It is always a gamble to predict a Green Flash with certainty because so much depends on atmospheric conditions that are so finely balanced. But not smoking all but guarantees it won’t happen. Meet the right conditions: A clean sky, a steady air mass with no wind and a flat, free horizon are all essential for viewing the Green Flash.
The phenomenon is also a little more predictable at certain places, like coastal areas or high mountains where the horizon is clear and unobstructed. Some websites and apps offer predictions for when the Green Flash could be visible in specific areas, depending on weather conditions, particularly for areas that see the phenomenon more frequently.
The time of year and angle of the sun is one of the most important predictors. The phenomenon is more typically seen at equinoxes, when the sun’s rays strike the atmosphere at an angle favorable to refracted light. This effect is generally observed just before sunrise or just after sunset, when the sun is lower on the horizon.
5. What Gives the Green Flash Its Green Color?
Search: What makes the Green Flash green? Sunlight, or white light, is made up of different colors, each with a different wavelength. Since the sun is closer to the horizon, its light crosses a thicker section of the Earth’s atmosphere than when it’s directly overhead. This makes the shorter wavelengths of light — blue and green —scatter more than longer wavelengths, like red and yellow.
This scattering and bending of light make the green wavelength visible at sunset or sunrise, and particularly so for ephemeral moments when the sun sits just below the horizon. The blue wavelengths get scattered away, usually out of sight, so the colour that’s left is green. The occurrence is boosted by environmental elements such as temperature inversion or a high degree of atmospheric transparency, making the green spectrum more visible.
But the exact conditions that generate this momentary phenomenon are so sensitive. If atmospheric disturbance grows too great, the green flash will be missing entirely, or dim compared with the rest. It’s this rare balance of perfect conditions that makes the Green Flash so beautiful.
6. What Places Are Best to View the Green Flash?
Viewing of the Green Flash is more common in certain areas, notably those where the sky is mostly clear and there is no obstruction of the horizon. Coastal areas and mountaintops tend to be prime spots, since they offer long, flat horizons. Areas like the Mediterranean, Coastal California, or high-altitude zone close to the equator used to be frequent spots for witnessing the Green Flash.
In regions with greater particle concentrations — such as in conditions of heavy air pollution, haze or fog — this effect is normally not observed, as atmospheric particles introduce confounding effects. Normally, this light scattering spreads a haze far and wide over the sunset or sunrise making visibility of the green flash impossible. Hence, clean, pollute-free air is an important consideration for the development of a Green Flash.
Latitude is also a factor. The difference, though, is that the Green Flash is usually visible in places where the path of the sun in its cycle is at a steeper angle from the horizon, like higher latitudes, particularly during the equinox when the sun sets or rises with the right kind of angle for the effect to work.
7. Where Are Some Notable Places to Catch the Green Flash?
Some notable locations worldwide are known for ideal Green Flash viewing opportunities. Coastal areas, particularly high cliffs or wide views of the horizon, are frequently widely considered ideal locations. Popular destinations include, but are not limited to:
The Canary Islands: With their clear skies and dazzling sunsets, the Canary Islands is a prime area to witness the Green Flash.
California Coast: Clear skies and the region’s mountainous terrain make a great opportunity for this rare phenomenon, especially around Big Sur.
Hawaii: The Hawaiian Islands, famous for unobstructed ocean horizons and clear air, are known for frequent sightings of the Green Flash.
The Mediterranean: Places such as the French Riviera or coastal Greece are known for their colorful sunsets and clear air, and so hot spots for the Green Flash.
These places are characterized by natural beauty and good atmospheric conditions which make them the chosen locations to observe this ephemeral light phenomenon.
8. Can the Green Flash Be Seen From Space?
If we look upwards, however, we might see a green flash even in one of those polarization goggles, under some ideal conditions — those goggles are the only way to view this phenomenon, after all, because it is so rapidly extinguished by the air. On the International Space Station (ISS), astronauts have reported seeing a greenish tint during sunset or sunrise, but they are not experiencing anything like the phenomenon we see on Earth.
From space, the phenomenon is most likely due to the Earth’s atmosphere refracting sunlight in a way that is similar to how it happens on the surface. On the other hand, the effect in space is presumably subtle, and the specific conditions required for it to be visible from space are still not completely clear. The atmospheric layers at play in space differ from those on the Earth’s surface, and the precise angles of sunlight become a key point in this uncommon phenomenon.
While this view isn’t readily observable, astronauts have noted how breathtaking it is to see the horizon painted with colorful light, from the deep blue and green to the beautiful spectacle of colors. But the Green Flash on Earth is better understood and more thoroughly explored phenomenon.
9. Common Myths About the Green Flash
In fact, the green flash has inspired countless myths and legends over the ages. Another common piece of superstition is that if you see the Green Flash, good luck will come your way or some sort of magical event is about to unfold. The flash has been related to the concept of seeing the “true nature” of the world, and has provided a moment of insight, or illumination especially in certain cultures. Some saying that it brings a deeper understanding of life if you seek out for the Green Flash in a sunset.
Popular fiction would depict the Green Flash as a mystical, rare phenomenon that unlocks hidden powers, a mythic encounter between two individuals with the universe, a signpost to certain supernatural events. These and other myths may be amusing entertainments but, they only serve to gild the lily and the myth machine around this concept so that you can tell most people need it more than ever to expect magic.
The Green Flash is a subject of much speculation, but all the rumors aren’t true. Its beauty and the science behind it have captivated scientists and skywatchers alike.
10. How Do You Photograph the Green Flash?
The Green Flash is too brief and elusive to be photographed easily. Because the Green Flash occurs only for a few seconds, timing and the proper equipment are key. A good manual camera with a fast shutter speed will give the best chance of photographing this fleeting phenomenon.
The first and perhaps most vital consideration for photographing the Green Flash is an unobstructed horizon. A wide-angle setup will also capture a larger portion of the real horizon and raise the odds of photographing the precise second that the Green Flash occurs. Some photographers employ time-lapse, taking multiple shots over time in a gamble to catch the flash.
Photographing the Green Flash is tricky, but when skywatchers and shutterbugs do, it’s such a beautiful, ephemeral wonder of nature, it pays off.
