Chasing the Green Flash
Hunting down one of the most elusive atmospheric phenomena
Picture the scene. You’re on a tropical beach, cocktail in hand, at sunset as the Sun begins to sink. The huge, fat, solar disc, distorted by the haze of our atmosphere, drops towards the horizon. But something strange happens — just as the final sliver of Sun disappears behind the edge of the world, a strange green light flashes out for a millisecond or two. Before you can even register its presence, it’s gone.
What you're seeing is a green flash, a rare sight occurring at sunrise and sunset as very edge of the Sun passes over the horizon. As the first or last few rays of light to reach you arc through the atmosphere, a couple of things happen.
The first is that the rays with the longest wavelengths — red light — are bent downward by refraction from the air in the atmosphere. Essentially,the reddest bits of the Sun set slightly before or rise slightly after the rest of it, for the same reason that a spoon appears to bend in a glass of water.
The second is that the rays with the shortest wavelengths — blue and violet light — are more likely to hit particles of dust in the atmosphere and bounce off. The reason the sky looks blue, rather than black, away from the Sun is because we see these bounced rays.
If you take out the red light (which has been bent below the horizon), and the blue light (which has been bounced out), you’re left with one colour — green.
So why don’t we see them all the time? The answer is simple. Most of the time we’re not looking. Even if you do look, the effect can be stymied by many factors — clouds covering the Sun, land or trees obscuring the true horizon, hazy air or even an untimely blink.
If you do want to see one, or even photograph it, you’ll need preparation, the right equipment and a little bit of luck.
The first factor is location. You’ll want to stand, facing the sunrise or sunset, at a place where the horizon is formed by something lower in the sky than your eye is. A field is pretty good. A mountain or a forest is bad. The sea is perfect. Standing in an elevated position yourself — on top of a cliff, mountain, or even in an aeroplane or balloon — can help achieve this.
The second factor is clear air. If the sky is dusty, damp, smoggy or hazy then it’s likely that much of the green light will be scattered out with the blue and violet, leaving a yellow flash instead that’s impossible to tell apart from the Sun. Big cities are often bad places to see a green flash — get out into the countryside if you can.
You’ll also probably want some form of magnification. Bring a zoom lens or a pair of binoculars to help you. Green flashes occur by definition far away on the horizon, so they’ll appear pretty puny to you.
Of course, it goes without saying that you shouldn’t look at that Sun with the naked eye through any kind of magnification until it’s right on the horizon and very dim. Even a few minutes beforehand (or afterhand if you’re trying to capture a dawn flash), the brightness can cause serious eye damage.
To photograph a green flash, you’ll want to do a few things.
Firstly, a tripod is useful so that you don’t have to worry about blurriness. Set your camera to as quick a shutter speed as possible without underexposing the photo. Manual-focus on the horizon if you can, to stop the autofocus going weird in the slightly unusual light conditions.
Then, a second or two before the first or last edge of the Sun’s disc (in the morning and evening respectively) is about to cross the horizon, hold down the shutter button and take as many pictures as you can. With any luck, one of them will have captured the green flash.
Finally, if you’re extra-lucky, you might get to see a blue flash. These are extraordinarily rare and require extremely clear weather. Normally, as discussed above, the blue light is scattered out but on exceptionally clear evenings and mornings, you can sometimes see a rim of blue after the green fades.If you ever get to glimpse one, consider yourself very lucky indeed.
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