A green flash is a brief burst of green light that appears at the very top of the sun just as it slips below the horizon at sunset (or just before it rises at sunrise). It typically lasts only one to two seconds, which is why many people have never seen one despite spending plenty of time watching sunsets. The phenomenon is entirely real and well understood, caused by the Earth’s atmosphere bending and separating sunlight into its component colors.
Why the Sun Turns Green
The green flash comes down to two properties of Earth’s atmosphere: refraction and dispersion. Refraction bends sunlight as it passes through the atmosphere, the same way a straw looks bent in a glass of water. When the sun is near the horizon, its light travels through a much thicker layer of atmosphere than when it’s overhead, so refraction is at its strongest. The total bending at the horizon is about 0.53 degrees, enough to make the sun appear slightly higher in the sky than it actually is.
Dispersion is what makes the flash green specifically. Different colors of light bend by slightly different amounts. Blue and violet bend the most, red bends the least, and green sits in the middle. This means the atmosphere creates a tiny stack of colored images of the sun, each slightly offset. The red image sits lowest, the green image a fraction higher, and the blue image highest of all. The total spread between red and blue is only about 0.006 degrees (roughly 20 arc seconds), far too small for your eye to resolve when the full disk of the sun is visible.
But in the final moment before sunset, when only a sliver of the sun remains, that tiny color separation becomes visible. The last bit of sun you see isn’t the red image. It’s the green one, sitting just above. For a second or two, that green sliver is all that’s left, and it can appear as a vivid emerald spot on the horizon.
Why Green Instead of Blue
If blue light bends more than green, you might expect a blue flash instead. The reason you don’t see one comes down to scattering. The same process that makes the sky blue, called Rayleigh scattering, strips blue and violet wavelengths out of sunlight as it travels long distances through the atmosphere. Short wavelengths scatter far more effectively than longer ones, so by the time sunlight has traveled all the way to the horizon, most of the blue and violet light has been scattered away in every direction. Green is the shortest wavelength that survives the trip in enough quantity to be visible.
In exceptionally clear air, however, enough blue light can survive for a blue flash to appear. These sightings are rare and usually reported from high-altitude locations or over very clean ocean air.
Different Types of Green Flashes
Not all green flashes look the same. The most common type is the inferior mirage flash, sometimes called the “last glimpse” flash. This happens when the surface below the sun (usually the ocean) is warmer than the air above it, creating a mirage layer. As the sun sets, a mirage image rises to meet the real sun, and the last tiny sliver visible at the top flashes green. You need to be only a few meters above the warm surface to see this type, which is why beachgoers report it most often.
The mock mirage flash requires the opposite conditions: a temperature inversion, where a layer of warm air sits above cooler air. This type tends to be more dramatic and is the kind most frequently captured in photographs. Instead of a single dot of green, the sun can appear to pinch into multiple thin strips, with green appearing at the top edge of each one.
The sub-duct flash is rarer and more spectacular. It occurs within strong temperature inversions, often over cold ocean currents, and can last noticeably longer than the other types. These flashes are larger and take on a characteristic triangular shape. They appear higher above the horizon than mock mirage flashes, which helps distinguish them. In some sub-duct events, green light can spread into faint rays extending above the sun’s position after it has set, a phenomenon sometimes called a “green ray.”
How to See One
The green flash is often described as rare, but it’s really more accurate to say it’s rarely noticed. With the right conditions and a clear horizon, green flashes happen fairly regularly. The key is knowing what to look for and where to look.
You need a distant, unobstructed horizon. The ocean is ideal because it provides a clean, flat line with no buildings or terrain to block the final moments of sunset. Elevated vantage points help too, since being higher up gives you a longer line of sight to the horizon and slightly extends the moment when the last sliver of sun is visible.
Clear air matters more than almost anything else. Dust, smog, haze, and high humidity all scatter and absorb light before it reaches you, reducing the chance of seeing green. Humid, muggy coastal cities are poor choices despite their ocean views, because moisture in the air amplifies scattering. Dry coastal areas, mountaintops, and remote ocean locations offer much better odds. That said, researchers have noted that a surprising amount of reddening haze can be present at the horizon and still allow a green flash to come through.
Watch the sun as it approaches the horizon, but focus your attention on the very last second. The flash happens in the final one to two seconds before the sun disappears completely. If you blink or look away at the wrong moment, you’ll miss it. At sunrise, the flash appears in reverse: a green spot on the horizon just before the sun’s upper edge emerges.
Eye Safety While Watching
A common question is whether it’s safe to stare at the setting sun while waiting for a green flash. The sun near the horizon is dimmer than at midday because its light passes through so much more atmosphere, but it can still be bright enough to cause retinal damage. The National Eye Institute warns that looking directly at the sun can damage cells at the back of the eye and cause permanent vision loss. Regular sunglasses, even very dark ones, do not provide adequate protection from direct solar viewing.
The safest approach is to avoid staring at the sun while it’s still visibly bright. Keep it in your peripheral vision or look away until only the very last sliver remains at the horizon. At that final moment, the sun is at its dimmest and the flash itself is extremely brief. Many experienced observers watch the horizon just above where the sun is setting rather than the sun itself, letting the flash appear in their field of view without prolonged direct exposure.
Photographing a Green Flash
Capturing a green flash on camera is challenging because it’s so brief and small. A long focal length is essential. Experienced green flash photographers use lenses of 500mm or longer, with some shooting at 1000mm to make the sun’s disk large enough in the frame for the green sliver to show up clearly. At these focal lengths, a sturdy tripod is non-negotiable. Even small vibrations will blur the image, so checking your tripod head for any looseness before shooting is worth the effort.
Exposure settings need to be fast to freeze the moment. In clear conditions, shutter speeds around 1/1000 of a second at a narrow aperture work well. In hazier air, opening up to a wider aperture and slowing the shutter to around 1/500 of a second compensates for the dimmer light. The trickiest part is color balance. The sun at the horizon throws off enormous amounts of red and orange light, which can overwhelm the green in photographs. Some photographers use tungsten-balanced film or white balance adjustments on digital cameras to shift the color rendering and bring out the green more naturally.
Shooting a burst of frames during the last few seconds of sunset gives you the best chance of catching the flash. With digital cameras, continuous shooting mode and a fast memory card let you fire off dozens of frames in those critical seconds, improving the odds that at least one frame captures the green at its peak intensity.