What Causes the Green Flash at Sunset and Sunrise?

Jonathan Belles
Published: February 7, 2018

It's a visual too brilliant to ignore: as the sun pierces the horizon, either at sunrise or sunset, a small green light appears as the yellows and oranges blaze brightly.

These green flashes have mystified onlookers and even scientists and are a prized catch for photographers, but their cause is well-known.

One lucky photographer caught the green flash in San Diego earlier this week. 

How does this flash occur?

Our atmosphere acts like a prism that splits light into a visible spectrum and our atmospheric prism separates different strengths of colors from the sun.

When the sun is overhead, it is seen as an off-white or yellowish color because our atmosphere scatters the blue light into what we usually see as a blue sky. What's left over in the color spectrum is hued yellow.

But at sunset, the sun's rays are much more oblique to the eyes, so the light coming from the sun has much more atmosphere to go through and more colors are scattered away.

The process of refraction, or the bending of light, through a prism.

Usually, most colors are "refracted," or bent away from our eyes, with the exception of the warmest colors – red and orange – as the sun greets the horizon. This is why the sun usually appears warm as it hits the surface.

Typically, greens and blues are bent away from our eyes as the sun sets. Just for a quick moment, a small bit of green light survives its trek through the atmosphere to our eyes.

Other prisms are bike reflectors and sometimes glass, which can turn daylight into a small rainbow on the ground.

How to See the Green Flash

First of all, never stare at the sun for a prolonged period of time. Let your camera do the hard work.

The best time to set up your camera is right before sunset because this is when the atmospheric optics are best for this effect. You could also catch the green flash as the sun rises above the horizon, but this takes some guesswork since you'll need to approximate where and when the sun will come up.

The green flashes last only a moment or two. You should be in an area with a distant, flat and cloudless horizon to see through as much of the atmosphere as possible. Stable, sinking air commonly found in high-pressure areas can boost your chances of seeing a green flash.

Sometimes, if you're extraordinarily lucky, you might even see a blue flash like the one captured below.

A remarkable series of seven images taken in extremely rapid succession show both a green flash and a rarer blue flash.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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