Have you ever seen the “green flash”? If not, don’t be discouraged. I’ve witnessed this rare meteorological phenomenon on only three occasions, the last of these just a few days ago. As with so much evanescence in nature, attempt to hunt an experience down and you must be prepared for disappointment. The green flash is no exception.
I was reminded of my encounters with it, both here and on top of Mowing Word in Pembroke, through reading Kai Bird’s monumental biography of the great atomic physicist Robert Oppenheimer recently. The author describes Oppenheimer inviting a companion one brilliant sunset to take a walk at Cruz Bay in the US Virgin Islands:
“Sure enough, just as the sun sank below the horizon, [they] saw a flash of green light. Robert quietly explained the physics behind what [they’d] seen … layers in the Earth’s atmosphere functioned like a prism, creating just for a second a flash of green.”
You’ll not see one every time you go looking. You have to know where to be. Two of my three encounters have been from here, the bare slopes of white rock and umber heather above South Stack cafe on Anglesey. With the sun almost gone, its upper radius briefly changed colour to a brilliant green. Then came the dynamic element! An intense light darted across the sky.
Technically an optical illusion caused by refraction of sunlight, the atmosphere separates the sunlight into its composite colours. The shorter wavelengths – blue, violet – refract more strongly than longer ones of yellow, orange and red, which are absorbed by the atmosphere. Green light, with its mid-range wavelength, is left most visible when the sun has slipped just below the horizon.
Thus the green flash. I first saw it after a climb on these same sea-cliffs in the late 1960s with a visiting German climber, Reinhard Karl, who died in an ice avalanche on Cho Oyu in 1982. Looking out to sea over his shoulder, a linger of sunlight suddenly condensed into a tracer bullet of vivid green light. It rushed straight at me, glanced off my shoulder into infinity. Beautiful, disturbing somehow. I needed the father of the atomic bomb’s words to explain it to me.
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