The intensity of the sky varies greatly over the day, and the primary cause of that intensity differs as well. When the sun is well above the horizon, direct scattering of sunlight (Rayleigh scattering) is the overwhelmingly dominant source of light. However, in twilight, the period of time between sunset and night and between night and sunrise, the situation is more complicated. Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible above the sun, usually for no more than a second or two, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena that stem from different causes, most of which occur when there is a temperature inversion (when the temperature increases with altitude rather than the normal decrease in temperature with altitude). Green flashes may be observed from any altitude (even from an aircraft). They are usually seen at an unobstructed horizon, such as over the ocean, but are also seen over cloud tops and mountain tops. Green flashes may also be observed at the horizon in association with the Moon and bright planets, including Venus and Jupiter.
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