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Catching the elusive 'green flash'

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Successful observers of the green flash should have an unobstructed view of the horizon and the air must be clear, which is the reason green flashes are most commonly seen over water. (Photo: Brocken Inglory)

The Green Hornet and the Green Lantern are fictional superheroes who are continuously eluding those who try to catch them. The "green flash" is not a superhero, but can be just as elusive. For some, catching a glimpse of it can be a lifelong quest.

The green flash is a phenomenon that occurs at sunset and sunrise when conditions are favorable, and results when two optical phenomena combine: a mirage and the dispersion of sunlight. As the sun dips below the horizon the light is being dispersed through the earth's atmosphere like a prism. As the light passes through the familiar Roy G. Biv of the spectrum, sometimes a flash of green can be seen for a few seconds.

A mirage is also necessary for the phenomenon to be seen, because the flash is dependent on a variation in astronomical refraction near the horizon. There are actually four types of green flashes, each dependent on a different type of mirage:

  • The inferior mirage, or I-Mir, which is the type usually seen by the naked eye just as the last of the sun dips below the horizon (the same type of mirage that is seen over asphalt on a hot day);
  • The mock mirage, or M-Mir, which is caused by an atmospheric temperature inversion and looks similar to the I-Mir;
  • The sub-duct flash, where a mirage causes the setting sun to form an hourglass shape and turns the upper part green for up to 15 seconds; and
  • The very rare green ray, which is a beam of green light that shoots up from the green flash or is seen immediately after the sun sets. There is no known photograph of the green ray.  

Although the green flash usually lasts between one and three seconds, it was observed on and off for a full 35 minutes on October 16, 1929 by Admiral Byrd's expedition at the Little America base on Antarctica.

The expedition's meteorologist, W. C. Haines, gave this account:

 "The sun was skirting the southern horizon, its disk disappearing at intervals only to reappear again a few moments later.... The irregularities in the snow surface permitted the upper limb of the sun to appear in one or more starlike points of light from adjacent notches. These points or flares of light would sometimes have a greenish color on their appearance or disappearance. The length of time during which the green flare was visible varied from a fraction of a second to several seconds.... When the sun sank too low to be seen from the ground, it was still visible from elevated points such as the anemometer post or radio towers. The above effect was seen at intervals during a period lasting over half an hour. Conditions were more favorable for its occurrence when first observed. Later the green appeared for shorter and less frequent intervals, and the orange and red flares increased in frequency."

The name "green flash" may have come from Jules Verne's novel "Le Rayon-Vert" (The Green Ray), published in 1882, which popularized the phenomenon. In his novel, he described the color as "a green which no artist could ever obtain on his palette, a green of which neither the varied tints of vegetation nor the shades of the most limpid sea could ever produce the like! If there is a green in Paradise, it cannot be but of this shade, which most surely is the true green of Hope."

Successful observers of this phenomenon should have an unobstructed view of the horizon and the air must be clear, which is the reason green flashes are most commonly seen over water. They have, however, appeared just over the tops of clouds located over a water horizon. The green flash can also be seen on a distant horizon when looking down from a high location; it is said to be regularly visible from the top of the Empire State Building, as well as from airplanes and mountaintops.

Andrew T. Young, an adjunct faculty member of the Department of Astronomy at San Diego State University and an expert on the green flash, has compiled a comprehensive website of the phenomenon that includes many photos, including some of the even rarer blue flash.

Those who live in an area with a view of the sunset over water have a better chance of observing this phenomenon; for those who live in an area where the western horizon is obstructed or the air isn't clear enough, the prospect of observing the flash becomes a quest on those rare occasions when they might be in an optimal locale at the right place at the right time.

For those not willing to take any chances, one of the best places on Earth to see the green flash is at Cerro Paranal, Chile, home of the European Southern Observatory's Very Large Telescope. Here, ideal conditions occur on a regular basis for observing the green flash. In fact, conditions here are so ideal that an extremely rare green flash of the moon was captured on photograph in 2011.