The green flash is a rare, momentary optical phenomenon. This fleeting spectacle occurs precisely when the sun disappears below the horizon at sunset or reappears above it at sunrise. The event is celebrated for its sudden, brilliant burst of color, which typically lasts for no more than a second or two. It offers a stunning demonstration of the Earth’s atmosphere acting as a giant lens refracting light.
What Exactly Is the Green Flash?
The green flash presents as a brief, intense spot of emerald or cyan light appearing on the very top edge, or upper rim, of the solar disk. This burst of color occurs only when the sun is almost entirely obscured by the horizon, making it an incredibly transient event. It is a verifiable optical effect that has been captured by scientists and amateur observers alike.
While the name suggests only green, the color can range from lime green to a bluish-green or even violet, depending on atmospheric clarity. The flash is typically a small, concentrated point of light that can appear to detach itself from the setting or rising sun. Its brevity makes it desirable, as it serves as the final farewell of the sun’s light. The explanation for why this particular color is the last to be seen is rooted in the physics of atmospheric optics.
The Role of Atmospheric Optics
The sun’s light is composed of all the colors of the visible spectrum, which are separated by the Earth’s atmosphere, much like a prism separates white light. This separation is due to atmospheric refraction, the bending of light as it passes through air of varying densities. When the sun is near the horizon, its light travels through the densest and longest path of atmosphere, which maximizes the bending effect.
The atmosphere bends shorter wavelengths of light (blue and violet) more than longer wavelengths (red and orange). This process creates a stack of colored solar images, with the blue/violet image sitting highest and the red image lowest. As the sun sets, the red, orange, and yellow images disappear first, leaving the blue, green, and violet images remaining at the very top.
Although blue and violet light are refracted the most and should theoretically be seen last, they are also scattered most effectively by air molecules (Rayleigh scattering). This scattering removes most of the blue and violet light from our line of sight, leaving the green light as the most visible color at the sun’s upper rim. The green image is briefly visible before the entire disk finally sinks below the horizon. The effect is often enhanced by a temperature gradient, where a layer of warm air sits above cooler air near the horizon, known as a temperature inversion.
Different Forms the Flash Can Take
The appearance of the green flash is dependent on specific atmospheric conditions, leading to several recognized variations, most of which are associated with mirages.
Inferior Mirage Flash
This is the most commonly reported type, occurring when the air near the surface is warmer than the air above it, often over the ocean. It appears as the last glimpse of the sun’s upper rim just before it disappears.
Mock Mirage Flash
This form requires the observer to be above a temperature inversion layer where warmer air rests over cooler air. This flash appears to pinch off a thin, pointed strip from the sun’s upper limb, momentarily detaching from the main solar disk.
Subduct Flash
A rarer form, the Subduct Flash can be the most spectacular, sometimes lasting up to fifteen seconds. It is characterized by the upper portion of an hourglass-shaped sun turning green.
Green Ray
The rarest manifestation is the Green Ray, which is not just a spot but a column of green light that seems to shoot upward immediately after the flash. This ray is thought to be a scattering effect, where the light from a bright green flash is diffused by hazy air.
How to Maximize Your Viewing Chances
Seeing a green flash requires clear atmospheric conditions and an unobstructed view of the horizon. The ideal setting is a flat, distant horizon, such as over the ocean or a large lake, though flashes have also been spotted over mountain ranges. The air must be exceptionally clear, with minimal haze, humidity, or pollution, as these factors scatter the green light and reduce its intensity.
Flashes are often associated with a temperature inversion layer, so looking for stable, clear weather conditions will increase the odds. Never look directly at the sun while it is high in the sky, even at sunset, to avoid eye damage. Only when the sun is nearly completely set, with only a tiny sliver remaining, is it safe to look for the fleeting burst of green. Observing from an elevated position, such as a hilltop or tall building, can also be beneficial, as it lowers the apparent horizon and provides a clearer view above any surface haze.