Blue Flash from Canary Islands
December 28, 2008
Green, blue, and violet flashes can be observed regularly from high altitudes, such as from the Canary Island of La Palma (Spain) off the African coast. And if the atmosphere is clean enough, this otherwise rarely observed phenomenon can even be detected when the Sun is setting behind a wall of clouds on the horizon. Interestingly, it’s not only the Sun’s disk that’s capable of producing a green flash, but also the upper edges of a cloud bank (as shown above), well above the setting Sun, can on occasion show green and blue peaks. Evidently, the atmospheric dispersion is working on the cloud, as it usually does on the Sun, to produce a blue or green upper rim and a red lower one. The colors on the upper edge of the clouds appear only where atmospheric motion or a temperature inversion have caused a little broadening of the colored upper rim; in other places the rim is compressed, and so it isn't visible. Note that the lower panel is a zoom into the upper-panel image. Since the Sun isn't visible, it's hard to know if the lower flash was actually hugging the Sun or lifted above it (a mock flash). Atmospheric inversions generally produce mock flashes. Photo taken on January 7, 2008.