Today's Feature - Mirages and Green Flashes

Last updated on September 16, 2023

Today's Feature - Mirages and Green Flashes

Mirages and green flashes are fascinating atmospheric phenomena that occur during sunset. When the sun is low on the horizon, its rays pass through air layers of different temperature and density, causing refraction and creating mirages. These mirages can produce multiple images of the sun, with some appearing erect and descending, while others are inverted and ascending. The interaction between the cold Californian offshore ocean current and warm onshore air can lead to stacked temperature inversions, resulting in complex and fantastical shapes in the mirage.

But mirages don't stop there; they can also give rise to green flashes. As the sun approaches the horizon, its upper limb takes on a green color. This occurs because green rays are refracted more strongly by the atmosphere compared to red rays, causing the "green sun" to appear slightly higher. Normally, this green rim is too narrow to be visible. However, mirages have the ability to greatly magnify the solar image in the vertical direction, leading to the phenomenon known as a green flash.

The green flash is a brief burst of green light that can be observed just as the sun dips below the horizon. It lasts only a few seconds and is often described as a fleeting moment of enchantment. This optical phenomenon occurs when the atmosphere acts as a prism, separating the colors of light and allowing the green light to momentarily dominate.

Here are some additional details about mirages and green flashes:

• Mirages occur due to the bending of light rays as they pass through air layers with varying temperatures and densities.
• Temperature inversions play a crucial role in mirage formation, creating an environment where light is refracted in unusual ways.
• Multiple layers of temperature inversions can lead to complex mirage shapes and multiple images of the sun.
• The interaction between offshore ocean currents and onshore air can enhance the formation of temperature inversions, increasing the likelihood of mirages.
• Green flashes are most commonly observed during sunset or sunrise when the sun is close to the horizon.
• The green color of the flash is caused by the refraction of green light, which is more strongly bent by the atmosphere compared to red light.
• Mirages can magnify the solar image vertically, allowing the narrow green rim to become visible and creating the illusion of a green flash.
• Green flashes are relatively rare and require specific atmospheric conditions to occur.
• Observing a green flash requires a clear view of the horizon and patience, as the phenomenon is brief and easily missed.

In conclusion, mirages and green flashes are captivating optical phenomena that occur during sunset. They are the result of light rays bending as they pass through air layers with different temperatures and densities. Mirages can create multiple images of the sun, while green flashes are fleeting bursts of green light that can be observed just as the sun sets below the horizon. These phenomena add an element of wonder and enchantment to our natural surroundings, reminding us of the intricate interactions between light and the atmosphere. So next time you find yourself witnessing a beautiful sunset, keep an eye out for mirages and the elusive green flash.

Miraged Sunset & Green Flashes - Imaged from San Francisco my Mila Zinkova (site).

Sunset mirages result when low sun rays are refracted between air layers of different temperature and density. In this sunset, the cold Californian offshore ocean current and warm air from onshore have produced multiple stacked temperature inversions. A single inversion layer can produce a mirage having three images of the sun. The upper one is erect and descending, one of the lower ones is also erect but the other is inverted and ascending! Multiple layers give us the complex and fantastical shapes here.

Mirages do more - they can produce green flashes. Close to the horizon the sun's upper limb is always coloured green because green rays are refracted by the atmosphere more strongly than red making the 'green sun' appear slightly higher. This green rim is too narrow to be visible but mirages can greatly magnify the solar image in the vertical direction. The result - a green flash.

Image ©Mila Zinkova, shown with permission.

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Reference Atmospheric Optics

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