The green flash is a brief optical phenomenon sometimes visible just as the Sun disappears below a clear horizon, where its upper rim appears to turn emerald green for one or two seconds. This rare sighting occurs during a green flash sunset when atmospheric conditions allow the Sun's green light to be refracted toward the observer while red and blue light scatter away. Though often dismissed as a myth, it is a well-documented effect that requires specific timing, geography, and clear skies.

What Causes the Green Flash?

The green flash happens because Earth's atmosphere acts like a weak prism. As the Sun sets, its light passes through increasingly thicker air near the horizon, which bends shorter wavelengths (blue and violet) more than longer ones (red and orange). However, blue light scatters strongly in the atmosphere, leaving green as the most visible refracted colour.

For the flash to appear, several conditions must align. The horizon must be sharp and unobstructed — typically over the ocean or flat land. The air must be stable and clean, with minimal haze or turbulence. Even then, the flash lasts only a fraction of a second, so timing and location matter greatly. Those wanting to observe it can check accurate sunset times to plan their viewing window.

Types of Green Flash

Not all green flashes look the same. Meteorologists and photographers recognise several varieties, each linked to atmospheric conditions:

  • Inferior-mirage flash: The most common type, seen over water. The Sun's lower edge appears distorted by a mirage, and the green rim appears just above it.
  • Mock mirage flash: The upper part of the Sun appears rippled or "cut" by layers of warm and cool air, producing a green segment that detaches from the disk.
  • Subduct flash: A rare, fleeting green spot visible as the Sun sinks behind a distant cloud or ridge.
  • Green ray: A mythical-seeming beam of green light shooting upward from the Sun, observed in very stable atmospheric conditions.

The inferior-mirage flash is the easiest to catch, especially from coastal viewpoints like Honolulu's Pacific horizon, where flat ocean expanses and clean air increase the odds.

Where and When to See One

A green flash sunset is most reliably observed from locations with a clear, distant horizon and low atmospheric turbulence. The ocean is ideal because the horizon line is sharp and uniform. Flat farmland, deserts, or high mountains can also work, provided the view is unobstructed for many kilometres.

Timing is equally specific. The flash occurs only in the last second or two before the Sun completely vanishes. It is never visible when the Sun is high in the sky. The best seasons are spring and autumn, when temperature inversions near the coast create stable air layers. Even then, no more than one in ten clear sunsets produce a visible flash to the naked eye.

To improve your chances, watch for a moment when the Sun's disk turns from yellow to orange to red. If the very top of the disk still appears greenish as it nears the horizon, the flash may occur. This is also a good time to compare sunset colours between the Sun's upper and lower limbs, as the contrast hints at how much refraction is occurring.

The Role of Atmospheric Refraction

Atmospheric refraction bends sunlight more strongly when the Sun is near the horizon. At sea level, the Sun's apparent position is roughly half a degree higher than its true position — that is about one solar diameter. This bending separates colours slightly, with green shifted upward relative to red and blue.

The green flash exploits this colour separation. Normally, the Sun's white light masks the effect. But at the instant of sunset, the red and blue components disappear: red sinks below the horizon first, blue scatters away, and green briefly dominates the uppermost rim. The flash ends when the Sun fully sets and the green rim vanishes too.

This same phenomenon, though far less dramatic, can be observed at sunrise in reverse. At dawn, the first sliver of the Sun sometimes appears green before turning yellow. Understanding the Sun's path across the sky — how its angle and speed change with latitude and season — helps predict when these events are most likely.

Common Misconceptions

Many people assume the green flash is a myth or a camera artefact. It is neither. The effect has been scientifically explained since the 19th century and can be captured with standard photography. Another error is expecting the entire sky to turn green; the flash is a small spot no larger than a fingernail held at arm's length.

Some also believe the flash only occurs over tropical oceans. While it is easier to see at low latitudes due to cleaner air, it has been observed from Scotland, Iceland, and even the Swiss Alps. The key remains a flat, unobstructed horizon and stable atmospheric layers.

Photography Tips

If you wish to photograph the green flash, use a telephoto lens of at least 300mm to magnify the Sun. Set your camera to manual focus and expose for the Sun's disk, not the surrounding sky. Shoot at the fastest shutter speed possible — 1/1000 second or faster — to freeze the fleeting moment. A tripod is essential, and a remote shutter helps avoid vibration.

For best results, start recording video a full minute before sunset, then review the footage frame by frame. The green flash often appears in just one or two frames, so patience and preparation pay off.

Wrap-up

The green flash is a rare but real reward for those who watch the horizon at the right moment. With accurate sunset predictions from a reliable source and a clear view to the west, any patient observer can increase their odds of seeing this brief emerald blink. Whether you call it a trick of light or a gift of physics, it remains one of nature's most fleeting wonders.