Green and Blue Flashes
May 15, 2013
Photographer: Oliver Stiehler
Summary Authors: Oliver Stiehler; Jim Foster
When taking photos of a sunset earlier this spring, I happened to catch these scintillating green and blue flashes -- mock mirages. The sky was cloudless with very little haze, and I was at a location approximately 900 ft (275 m) above sea level. With patience, a clean sky and a clear view of the horizon, green and even blue flashes can on occasion be observed at sunset and sunrise. They’re easier to detect at sunset since you don’t have to guess the exact spot where the solar disk will break the horizon.
The flashes result from atmospheric refraction when the Sun is low in the sky. But the refraction must be sufficiently strong, through atmospheric temperature gradients, that a mirage occurs. When this happens there's a marked vertical magnification of the images for parts of the mirage. Then, and only then, will color separation produced by differential refraction be visible to the eye or camera. Of course, always use extreme caution when looking toward the Sun. Photo taken on March 2, 2013.Photo details: APO Televid 82 with DSLR adapter; a focal length of about 800 mm; aperture of f/9.8; 1/8000 second exposure; ISO 100, photos cropped to 35 percent of the original size.