What is a Green Flash?

By Manzanillo Sun Reporter from the January 2013 Edition

Manzanillo is well known internationally for the green flash phenomenon during sunsets, as well as the warm waters of the ocean, deep sea fishing, port and tourism.

Well, what are green flashes?

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.

Green flashes are enhanced by mirage*, which increase the density gradient in the atmosphere and therefore, increase refraction. A green flash is more likely to be seen in clear air, when more of the light from the setting sun reaches the observer without being scattered. One might expect to see a blue flash, but the blue is preferentially scattered out of the line of sight, and remaining light ends up looking green. With slight magnification a green rim on the top of the
solar disk may be seen on most clear-day sunsets, although the flash or ray effects require a stronger layering of the atmosphere and a mirage, which serves to magnify the green from a fraction of a second to a couple of seconds. Very occasionally, the amount of blue light is sufficient to be visible as a “blue flash”.

*A mirage of an astronomical object is a naturally occurring optical phenomenon, in which light rays are bent to produce distorted or multiple images of an astronomical object. Many times there are “sparkles” in the water at night, these are caused by phosphorus elements in the waters surrounding Manzanillo. People living on the Bay of Manzanillo can see these often on moonlit nights.

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