A BLUE-GREEN FLASH: We’ve all heard of the green flash, the fleeting pulse of emerald light that sometimes appears just above the setting sun. But have you ever heard of the blue-green flash? Thom Peck photographed one from Poway, California, on July 25th:
Atmospheric optics expert Les Cowley analyzed this flash and found elements of blue mixed with the usual green–“in about equal measure,” he says. “This is a rare cloud-top flash.”
It turns out, blue flashes are a thing. They’re formed in the same way as green flashes: a mirage magnifies tiny differences in the atmospheric refraction of red, green and blue light. Blue flashes are generally harder to see than green flashes, because blue is strongly scattered by intervening air and small dust particles.
When Peck was watching the sun set on July 25th, however, conditions favored both colors. The clouds in Peck’s photo were trapped in a marine temperature inversion layer. “Strong temperature gradients produced a strong mirage with well separated colors,” explains Cowley. “And a very clean atmosphere allowed the blue to pass unscattered.”