Green flash - Green Rim

Green Flash - Exploring the Phenomenon of the Green Rim

Have you ever witnessed a breathtaking green flash during a sunset? This mesmerizing optical phenomenon has captivated skywatchers for centuries. While textbooks and websites often attribute the green flash to ordinary atmospheric refraction, the truth is far more intriguing. In this article, we will delve into the science behind the green flash and explore the phenomenon of the green rim.

The Formation of Green Flashes

Contrary to popular belief, green flashes do not occur under ordinary atmospheric conditions. To understand how they form, we need to examine the behavior of sunlight as it passes through the atmosphere during sunset. As the sun descends towards the horizon, its rays undergo refraction, causing them to curve slightly downwards. This refraction is responsible for the sun appearing flattened and elevated in the sky.

During this refraction process, green light is refracted more strongly than red light. Consequently, the "green sun" appears marginally higher than its red counterpart. However, these effects are extremely subtle and cannot be perceived with the naked eye alone. To witness a green flash, something more extraordinary is required.

The Role of Mirage

To amplify the minute refraction effects and give rise to a green flash, a mirage becomes essential. A mirage occurs when there are variations in air density due to temperature gradients. In a stable atmosphere, temperature decreases with altitude, resulting in a smooth decline in air density. However, localized temperature fluctuations can create regions of denser air.

In such instances, the denser air acts as a giant lens, bending the rays of the setting sun towards the Earth. This lensing effect causes the rays to appear as if they are emanating from a point higher in the sky, thereby elevating the position of the sun. Simultaneously, due to differential refraction, different-colored images of the sun become slightly vertically separated. As the sun sinks below the horizon, it acquires a green upper edge and a red lower edge, resulting in the elusive green flash.

The Mystery of the Green Color

One might wonder why green flashes are green and not blue, given that blue light is refracted more strongly than green or red light. The answer lies in the behavior of sunlight during sunset. Blue light is significantly scattered by air molecules, small dust particles, and aerosols, diverting it away from the direct rays of the setting sun. Consequently, only a minimal amount of blue light remains in these rays, making it unlikely to contribute to the coloration of the green flash.

Additionally, traces of ozone in the atmosphere absorb orange light, intensifying the contrast between the red sun and the green flash. While blue flashes are rare, they can occasionally be observed under certain atmospheric conditions.

The Ephemeral Nature of Green Flashes

Green flashes are fleeting phenomena that demand keen observation and favorable atmospheric conditions to witness. Due to their subtle nature, they are often difficult to perceive without optical aids such as binoculars. However, it is crucial to exercise caution and never attempt to view a green flash without proper eye protection.

In conclusion, the allure of the green flash lies in its enigmatic formation and ethereal beauty. This atmospheric phenomenon, born from the interplay of refraction and mirages, captivates those fortunate enough to witness it. So, keep your eyes on the horizon during sunset, for you may just catch a glimpse of nature's breathtaking green rim.

Not the way green flashes form. The rays of the setting sun are refracted by the atmosphere to curve slightly downwards. The sun appears flattened and raised in the sky. Green light is refracted more and the 'green sun' is very slightly higher than the red one. But the effect is small and cannot be seen with the unaided eye. Green flashes need something more!

Some textbooks and most web pages say that the green flash is produced by refraction under ordinary atmospheric conditions. That is not true, if it was then we would see flashes much more often.

A standard stable atmosphere gets cooler with increasing height and the air density falls smoothly and monotonically. The lower and denser portion acts as a giant lens bending rays from the setting sun towards the earth. As a result, the rays appear to be coming from a point higher in the sky and the sun appears to be raised up. Green light is refracted more strongly than red and so different coloured images of the sun become very slightly vertically separated. As the sun sinks it develops a green.. upper edge and a red lower one. But the effect is small usually only visible in binoculars - do not ever look without full eye protection, even for an instant.

To make a green flash these tiny refraction effects need somehow to be considerably magnified. We need a mirage ...


Why a green edge, why not blue? Why are green flashes green? Blue light is refracted more strongly than green or red and the sun's upper edge might be expected to be rimmed with blue outside a band of green. This rarely happens because so little blue light remains in the rays of the setting sun. Blue light is strongly scattered by air molecules, small dust and aerosol particles out of the direct rays. There is a further effect, traces of ozone absorb the orange light which makes the contrast between the red sun and green flash even more dramatic.

For all that, sometimes we do see blue flashes!

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Reference Atmospheric Optics

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