What’s a green flash and how can I see one?

View at EarthSky Community Photos. | Julia Vaughan caught this green flash at Ocean Beach Fishing Pier, San Diego, California, on July 14, 2023. She commented: “A lot of people think the flash is just a myth, but it isn’t. It’s very real.” So true! Thanks, Julia!

What is a green flash?

People say that when you go to the beach to look at the sunset, you can try to see a green flash. But what is it? The green flash is an optical phenomenon that you can see shortly after sunset or before sunrise. It happens when the sun is almost entirely below the horizon, with the barest edge of the sun – the upper edge – still visible. For a second or two, that upper rim of the sun will appear green in color, sometimes blue. It’s a brief flash of the color green, and quite exciting to see, especially if you’ve been looking for one!

Green flashes do play a role in some legends. It’s said that once you’ve seen a green flash, you’ll never again go wrong in matters of the heart.

Mock mirage and green flash over the Pacific, seen by Jim Grant in San Diego. Published with permission.

How can you see one?

You just need two things to see a green flash:

1. A clear day with no haze or clouds on the horizon.

2. A distant horizon, and a distinct edge to the horizon. You can see the green flash from a mountaintop or high building. But it’s most often seen over the ocean, by people on beaches or in boats.

Important tip: Don’t look at the sun until it is nearly entirely below the horizon. If you do, you will dazzle (or damage) your eyes and ruin your green flash chances for that day.

Since you need to know exactly where to look along the horizon, and since most of us aren’t up before dawn, green flashes are most often seen after sunset. But diligent observers can see them before dawn, too. And, although they’re most often seen over the ocean, you can see green flashes over land, too, if your horizon is far enough away.

View at EarthSky Community Photos. | Jan Null took this photo on July 21, 2023, from Pigeon Point, California. Jan wrote: “Photographing Pigeon Point Lighthouse and capturing the green (and occasionally blue) flash are two of my favorite subjects on the San Mateo County Coast of California. There was slight inversion on one of the few fog-free days this summer and I drove there hoping to possibly catch one or the other. Fortunately, I was able to get both with probably the most distinct blue color I have seen, with a very strong blue spike on the histogram.” Thank you, Jan!

What makes a green flash?

According to Atmospheric Optics:

Inferior mirages are produced by warm air at the ocean or earth’s surface and an air temperature gradient changing rapidly with height. Rays from a low sun are refracted back upwards as they pass between the cool and warm layers. Refraction always tends to deflect rays towards the denser layer. An observer above the layer sees two solar images or parts of them (1) an erect image from rays that pass relatively undeflected above the warm layer and (2) a lower inverted image from rays mirrorred upwards by the warm layer. Each sun image is as ‘real’ as the other. The effect is not dissimilar to the mirage seen above a hot road surface.

As the sunset proceeds the upper and lower images approach, touch and eventually overlap to form an ‘omega’ shaped sun.

A green flash occurs because at a later stage the deflection by the warm layer/cooler air boundary becomes very sensitive to the angle of incidence of the sun’s rays. Small deviations are vertically magnified including the difference in deflection between red and green rays. This amplification provides the separation between green and red that refraction through a normal atmosphere cannot accomplish.

What is the green ray?

The flash can be like a flame that shoots above the horizon. In that case, it’s called a green ray. I’ve seen lots of green flashes, but never a green ray, although I was once walking on a beach in Mexico and turned away just as my companion saw one.

I did not find any photos of flamelike green rays (if you know of one, let me know), but the photo below suggests the beginnings of a ray.

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Mock mirage (explained at Atmospheric Optics) and green flash seen from San Francisco. Image via Brocken Inaglory/ Wikimedia Commons.

Green flash photos from the EarthSky community

View at EarthSky Community Photos. | Jim Grant caught this green flash at the Ocean Beach Pier in San Diego, California, on July 19, 2023. Jim wrote: “This sailboat was drifting close to the Ocean Beach Pier, I knew the sunset was going to be stunning and I started tracking the boat hoping to get it centered in the sun. The green rim and green flash above were a bonus.” Thank you, Jim!
View at EarthSky Community Photos. | Jim Grant in San Diego, California, captured this green flash on July 12, 2023. Jim wrote: “I took this from an elevated deck directly across the street from the Ocean Beach Pier. I was 40 feet above sea level on a pretty clear day with a slight inversion layer in place.” Thank you, Jim!
View larger at EarthSky Community Photos. | Bill Miller caught this green flash in Sint Maarten on April 27, 2020. He wrote: “We see green flashes frequently but it is always a challenge to get a good picture of one … timing is everything.” Thanks, Bill!
Green flash atop sun pyramid, in 2014, via Colin Legg. Published with permission.
Jim Grant photographed this green flash on April 27, 2012, off the coast of San Diego. Published with permission.

Bottom line: How to see the elusive green flash.

July 19, 2023

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