The Green Flash!

greenflashContributed by Paul Dempsey/B. McWilliams

ONE OF NATURE’S MOST UNUSUAL AND BEAUTIFUL DISPLAYS, the “Green Flash” leaves a lasting impression on everyone fortunate enough to have seen it. Jules Verne was struck by its exquisite beauty and in his novel, Le Rayon Vert, expressed his sentiments in verse:

“Only those who have seen it,
Can know the peace without compare,
That comes to those touched by its greenness,
The mystique of le rayon vert.”

The rare “Green Flash” occurs when the last sliver of the sun momentarily turns a vivid brilliant green, just as it disappears below the horizon. The phenomenon is caused by a combination of refraction of the sun’s rays by the atmosphere, the filtering of sunlight through the air and an unusual thermal structure in the upper air, all occurring simultaneously.

As rays of light pass through the atmosphere, they are “bent” or refracted a little, causing the image of the sun that you see near the horizon to be slightly higher than it ought to be. The amount of refraction depends on the wavelength of the light, therefore the “blue image” of the sun is displaced slightly higher in the sky than its “red image.” In the absence of any other complication, the setting sun ought to have a blue rim along the top.

In turn, the blue light is filtered very effectively by molecules of air and dust, leaving the colour green as the shortest wavelength to survive the long trip through the atmosphere when the sun is near the horizon. Usually, this green upper rim cannot be seen with the naked eye and only becomes visible in rare circumstances when the thermal structure of the atmosphere makes the air act like a giant magnifying glass. At the moment when the green rim passes through this magnifying region, it is momentarily enlarged, producing a hauntingly beautiful and spectacular green flash.


1. Watch the sun set over the ocean; this provides a straight uninterrupted horizon.

2. Choose a cloudless evening. The changes of temperatures with height in the lower layers of the atmosphere are likely to be greater than usual, increasing the variations in the extent to which the different colours of the sun’s rays are refracted.

3. Conduct your vigil around the time of the summer solstice. At this time of year, the sun makes a relatively shallow angle with the horizon when it sets. If it occurs, a green flash will then last longer.

4. Don’t look directly at the sun, as this may injure your eyes. Observe from an oblique or circumspect angle.

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