Key Takeaways on the Green Flash:

• The green flash is a rare, brief optical event of green, yellow, and sometimes blue that colors the horizon, occurring at sunrise or sunset most often in the western U.S.

• Refraction and dispersion cause the green flash, and there are four different types of green flashes that can occur.

• The green flash is tied to pirate folklore.

From resolving matters of the heart to promises of good weather, the enigmatic green flash holds a stake in meteorological folklore.

Occurring at either sunset or sunrise under only the most perfect weather conditions, the elusive phenomenon has reached mythical status for those hoping to one day glimpse a green speck along a horizon.

Although rare, Bennett Maruca, associate professor of physics and anatomy at the University of Delaware, says that calling the illusion a “flash” is a bit of a misnomer.

“It’s really not a flash,” says Maruca, adding that the phenomenon is a brief pop of green, yellow, and sometimes blue coloring along an otherwise unadulterated horizon line.

What Is the Green Flash?

A green flash can occur at either sunrise or sunset but is more easily observable during the latter. On a cloudless evening, Earth’s atmospheric sunlight distortion is most pronounced when the sun is just about at or slightly below the horizon.

“A green flash is a very brief phenomenon, lasting a few seconds or less,” says Maruca.

Four types of green flashes are recorded in the Farmer’s Almanac, which the American periodical describes as inferior mirage, mock mirage, subduct flashes, and green ray.

Each type ranges from an oval and flat flash, to a thin or pointy strip that also only last between 1-2 seconds and 15 seconds. The green ray is the rarest type, as it occurs as a beam of light that shoots up after the sun sets.

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What Causes the Green Flash

The green flash is caused by refraction and dispersion. Refraction is the bending of light, while dispersion scatters it.

“Earth’s atmosphere is not uniform – not only is it curved, but its density and temperature also vary with altitude and time. This makes it very effective at refracting light,” says Maruca.

Similar to how a camera bends or changes direction as the sun sets, the differing thickness of Earth’s atmosphere bends the light, allowing the image of the green flash to be observed. Light bends in a denser medium, like thick air on a warm evening. In essence, the atmosphere acts as a lens.

“Our atmosphere bends the light around, so you get an extra five minutes or so of the sun being up,” says Edward Guinan, professor of astronomy and astrophysics at Villanova University.

Dispersion is a related phenomenon in which different wavelengths of light are refracted at various angles.

“A prism produces a ‘rainbow’ by dispersion – the different colors of light are refracted, or bent at different angles, which separates them into a spectrum,” explains Maruca. “Earth’s atmosphere can act just like a prism – refracting and dispersing light from an astronomical source, such as the Sun.”

An Optical Event

As Earth’s atmosphere spreads light during a sunset or rise, it also acts as a “little prism” to disperse differing wavelengths of light. The atmosphere scatters shorter wavelength light like blues and violets more than it does red, so when the sun sets, these colors are scattered before they reach the human eye, creating red-hued sunsets.

The moment the “flash” occurs is when the sun is already below the horizon, and the Earth’s atmosphere refracts sunlight, bending shortwave light more than longwave light, before the light becomes dispersed. The longest and shortest wavelengths, red and violet, respectively, become obscured, leaving the mid-length colors of green hues.

“When you’re looking at the horizon, you’re looking through the full lower layer of the atmosphere,” says Maruca.

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Where to See the Flash

Guinan adds that the conditions for viewing the green flash must be just right. The horizon must be clear, the surface flat, and there must be few clouds with little or no haze.

“As the last sliver of the sun sets, if things are right, it will turn kind of a greenish, weird color of green that’s not really sharp,” says Guinan. “It’s like a mirage in a way. One of the simplest types of these mirages is often seen while driving on a warm or hot sunny day. The road ahead looks shiny or wet. This refraction occurs because the sun has heated the road surface, which heats the layer of air near it.”

Because the sun always sets to the west, those living along the western U.S. have the best chance of seeing the green flash. The horizon must be clear without distortion. Guinan reminds green flash seekers to avoid looking directly into the sun and to use binoculars or a camera viewpoint instead.

Article Sources

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Madison’s reporting focuses on marine and environmental issues, climate change, and novel scientific discoveries related to health and technology. Raised on an island in southeast Alaska, Madison is now based in western Montana. Her writing has been featured in Time, Snopes, Business Insider, Mountain Journal, EcoWatch, and Alaska Magazine, among others. When not writing, Madison teaches yoga, raises chickens, and fosters adoptable dogs and cats.