The Green Flash

For years, Castle Parkers have watched sunsets from various locations along the shores of Castle Park in hopes that a special night will come when they will see ‘The Green Flash’. Many have claimed to see the elusive ‘green flash’, and others wonder if it really exists. Resident expert, John Clarke, offered the following explanation and short videos. View as an animated image or as a movie. If you chose the later, you will need a viewer that plays .mov files. You can download a free .mov file player at QuickTime.

Explanation

This is a series of images of the setting sun, taken with different exposures as the sun dims while setting. The “green flash” is the greenish color that appears when the last bit of the setting sun dips below the horizon. It is an optical effect of the passage of sunlight through the Earth’s atmosphere along the path toward the horizon, and only is seen when conditions are right along that long path through the atmosphere near the horizon. It is best seen when there is a clear view to the horizon, for example watching the sun set over Lake Michigan.

The optics of the formation of the image of the green flash is fairly complicated, but it has to do with the refraction of the light through the atmosphere, and the scattering and absorption of blue and red colors of the sun by particles in the atmosphere. It is also like a mirage – for example when you see an image of water on a hot highway far in the distance, this is actually an image of the sky, with the light refracted by hot air above the highway pavement. More detailed explanations can be found online by googling “green flash”.