Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or it may resemble a green ray shooting up from the sunset point. Green flashes are a group of phenomena stemming from different causes, and some are more common than others.Green flashes may be observed from any altitude (even from an aircraft). They usually are seen at an unobstructed horizon, such as over the ocean, but are possible over cloud tops and mountain tops as well.
A green flash also may be observed in association with the Moon and bright planets at the horizon, including Venus and Jupiter.
A larger-than-usual green flash in Santa Cruz, California
A mock-mirage green flash observed in San Francisco, California