January 26, 2008
Green flashes, elusive phenomena observed at sunset or sunrise, result from atmospheric refraction near the horizon. The shorter the wavelength the larger the refraction, thus at sunset the refractive delay of the setting Sun is longer for shorter wavelengths. Therefore, the "green" Sun sets a split-second later than does the "yellow" Sun. To observe this phenomenon, a clear atmosphere is essential.
On occasion, the yet rarer blue and violet flashes can be detected from high altitudes, where the air is even clearer than at the surface and there is less atmospheric reddening. The blue and violet flashes shown here have been shot from the Roque de los Muchachos observatory (Canary Islands of Spain) at an altitude of 2,150 m above sea level (7,054 ft), where on this day the Sun set in an "ocean of clouds" rather than on the horizon. Note that the background picture shows the western horizon ten minutes before sunset. Click on image for larger view. Photo taken on September 30, 2007.