The green flash was explained by James Prescott Joule in a letter to the Manchester Literary and Philosophical Society in 1869. It is a greenish flash seen at the upper limb of the sun at sunset (and also at sunrise, but it is harder to observe then). There are several kinds of green flashes, but the "true" ones are mirage phenomena. There are also the green ray, a vertical column of green light after sunset, and greenish crepuscular rays, which are different things. The photograph shown is from the wonderful and excellent website The Green Flash by A. T. Young, and was taken by him. The reader interested in the green flash is highly recommended to consult this website for everything there is to know about the subject, and many more pictures.
This subject has a very large literature. A good introduction is given in M. Minnaert, The Nature of Light and Colour in the Open Air (New York: Dover, 1954), pp. 53-63. It is closely related to the phenomena of the mirage, involving nearly horizontal rays of light, atmospheric temperature gradients, and the spectral composition of twilight.
Composed by J. B. Calvert
Created 26 January 2003