The green flash is a rare sunset phenomenon that occurs when the atmosphere is especially transparent. The limb of the setting Sun is distorted into horizontal ridges by layers of different-temperature air. A fragmentary "green fringe" floats on top, the result of dispersion placing the blue and green images of an astronomical object slightly higher than the yellow and red. The Sun is flattened to an oval shape by the greater atmospheric refraction at the horizon, where the thick air also absorbs and reddens the light more. The two commonest forms of green flash are associated with the inferior mirage and the mock mirage. This photograph shows a mock mirage flash.