Green flash at sunset. Flash of light (upper centre) tinged with green and blue rays, seen above the setting Sun (centre). This phenomenon is caused by a temperature inversion above the clouds seen across the Sun. The exact effect and the length of time it is present varies depending on the exact conditions in the atmosphere at the time of viewing. The green flash will be present for longer if mirage conditions are present. A mock-mirage flash results in a thin strip appearing above the upper limb of the Sun, and lasts for 1 or 2 seconds.