What causes the green flash at sunset?

If you've ever watched the Sun dip below a flat horizon—over the ocean, a great lake, or an open prairie—you might have glimpsed a sudden, vivid green spark just as the last sliver of light disappears. That's the green flash, a fleeting optical phenomenon that has inspired sailors' legends and photographers' persistence for centuries. It usually lasts one or two seconds, and catching it feels like a small reward for watching a sunset all the way through. But what actually causes this rare burst of green, and how can you increase your odds of seeing it?

What Is the Green Flash?

The green flash is a brief, brilliant spot of green light that appears at the upper edge of the Sun just as it sets below the horizon. In rare cases, observers report a blue or violet flash, but green is by far the most common and visible. The phenomenon also occurs at sunrise, though it's much harder to catch because you'd need to know exactly where the Sun will appear and stare at an empty sky moments before it rises. For most people, it's a sunset reward—a tiny, dazzling punctuation mark at the end of the day.

Contrary to popular belief, the green flash isn't a single type of event. Scientists categorize several variants, including the "green rim" (a thin green edge visible for a few seconds), the "mock mirage flash" (common over the ocean), and the "sub-duct flash" (seen when the Sun sets behind a distant cloud or mountain). But for the casual sky-watcher, any green burst counts as a success. If you're planning a trip to a prime viewing spot, it helps to check your precise sunset time on sunset.now and arrive early to scout the horizon.

What Causes the Green Flash?

The green flash is caused by atmospheric refraction—the bending of sunlight as it passes through Earth's atmosphere. The atmosphere acts like a weak prism, splitting sunlight into its constituent colors. Because green light bends more than red light, it follows a slightly higher path through the sky. As the Sun sinks below the horizon, the red and orange rays are the first to disappear behind the curve of the Earth, leaving the green light visible for an instant.

This effect is strongest when the Sun is very close to the horizon, because the light passes through the thickest layer of atmosphere. That's why the green flash only appears during the final seconds of sunset. The bending also gets magnified by temperature inversions (when warm air sits above cooler air near the surface), which act like a lens. For a deeper dive into how sunlight behaves at different times of day, see our guide on why sunsets are red.

Why Is It Usually Green and Not Blue?

You might wonder: if violet light bends even more than green, why don't we see a violet flash? The answer lies in scattering. Blue and violet light are scattered by the atmosphere much more efficiently than green or red—the same Rayleigh scattering that makes the daytime sky blue and sunsets red. By the time the Sun's light travels through the thick, dusty air near the horizon, most of the blue and violet wavelengths have been scattered away. Green is the shortest wavelength that survives the journey to your eye.

This also explains why green flashes are more common in places with exceptionally clear, dry air, such as deserts or tropical oceans. High humidity or haze scatters green light too, washing out the flash entirely. If you're hoping to photograph one, pay attention to how clouds affect sunsets—a low, uniform band of cloud can actually block the flash, while high cirrus clouds might leave the horizon clear enough for a sighting.

How to See a Green Flash

Seeing the green flash requires three things: a very clear, flat horizon (the ocean is classic), stable air with minimal turbulence, and precise timing. Stand at a high vantage point if possible—a beach overlook or a dune—to get the most unobstructed view. Watch the exact top edge of the Sun at the instant it disappears. Never stare at the bright Sun before it gets low; wait until it is comfortably dim to look directly. Sunglasses can help reduce glare, but they also cut the green, so try naked-eye viewing as the final second approaches.

Timing is everything. The flash is so brief that if you blink, you'll miss it. Use sunset.now to check your exact sunset time for your location and be ready with your eyes fixed on the horizon about 30 seconds before the predicted moment. For the best odds, pick a day with low humidity and no clouds near the horizon. Some dedicated flash-hunters travel to the best cities in the USA to watch the sunset where coastal views offer a reliable stage. And don't be discouraged if you don't see it on the first try—many observers report that their first green flash came after several patient attempts.

Common Myths About the Green Flash

One widespread myth is that the green flash is an illusion or a trick of the eye. In reality, it's a real physical phenomenon that has been photographed and studied for over a century. Another misconception is that you need special equipment—but the green flash is visible to the naked eye under the right conditions. Binoculars or a telephoto lens can help you see the green rim more clearly, but they also narrow your field of view, making the timing more critical.

Some people believe the green flash only occurs over the ocean. While water provides the flattest, most stable horizon, the flash can also be seen over large lakes, flat plains, and even from mountaintops looking across a valley. The key is a distant horizon that is perfectly level. If you're curious about how the Sun's path changes with location, read about how latitude affects sunset times—being closer to the equator can give you a more vertical sun drop, which may reduce the green flash window.

Can You Photograph the Green Flash?

Absolutely, but it requires preparation. Use a camera with a zoom lens (at least 200mm) and a tripod. Set your focus to manual and prefocus on the horizon during the late afternoon, then switch to a fast shutter speed (1/250 or faster) as the Sun sets. The flash is so brief that you'll need to shoot in burst mode starting about 10 seconds before the predicted sunset time. For more on capturing the perfect moment, check our tips on sunset photography camera settings.

Many photographers find that the green flash is easier to capture in video, then extract the frame. Modern smartphone cameras with high frame rates can also work if you have a telephoto attachment. But don't get so caught up in your settings that you miss seeing it with your own eyes—sometimes the best memory is the one you don't view through a screen. For links to tools that help you track sunset times and plan shoots, visit our apps and tools page.

Frequently asked questions

Is the green flash real, or just a myth?

Yes, the green flash is a real atmospheric optical phenomenon. It has been photographed and scientifically studied for decades. It occurs because the atmosphere refracts sunlight and separates the colors, with green being the last visible wavelength as the Sun sets.

How long does the green flash last?

Most green flashes last only one or two seconds—sometimes less. The green rim variant can linger for several seconds, but the classic flash is extremely brief. That's why precise timing and careful observation are essential.

Can you see the green flash at sunrise?

Yes, the green flash can also occur at sunrise, but it is much harder to see because you need to know exactly where the Sun will appear and stare at that spot before it rises. Most observers find the sunset flash far more accessible.

What's the best place to see the green flash?

The best places have a very flat, unobstructed horizon and clean, stable air. Oceans and large lakes are ideal. Deserts and high-elevation plains also work well. Check the weather for low humidity before heading out.

What causes the green flash at sunset? — sunset.now guide