The sky around sunrise and sunset produces a handful of optical effects that feel almost like tricks of the eye. A flash of green at the moment the sun vanishes, beams of light fanning across the sky, and mountains glowing pink long after the sun has set are all real, repeatable phenomena. Each one comes from the way light bends, scatters, and gets blocked as it travels through a thick slice of atmosphere near the horizon. Knowing how they form makes them far easier to spot and to photograph.
Here is what causes the green flash, crepuscular rays, and alpenglow, and how to give yourself the best chance of seeing each.
The Green Flash
The green flash is a brief burst of green light that can appear at the very top edge of the sun as it disappears below the horizon, or as it first rises. It happens because the atmosphere bends different colors of light by slightly different amounts, acting like a weak prism. Red light bends least and green and blue bend more, so for a second the green-tinted upper rim of the sun stays visible after the rest has dropped from sight.
- You need a clean, distant horizon. An ocean horizon is the classic spot because the view is flat and unobstructed for miles.
- Clear, stable air helps. Haze and clouds near the horizon block the effect entirely.
- Watch the last second. The flash lasts barely a moment, so keep your eyes on the final sliver of the sun.
- Protect your eyes. Only look right as the sun is nearly gone, not while it is still bright.
Crepuscular Rays
Crepuscular rays are the beams of light and shadow that seem to radiate from the sun, often when it sits behind broken clouds or a mountain ridge. The beams are columns of sunlit air separated by the shadows of clouds. They appear to spread out from a single point purely because of perspective, the same way straight railway tracks seem to converge in the distance. The rays are actually close to parallel.
When the beams appear on the opposite side of the sky from the sun and seem to converge toward the far horizon, they are called anticrepuscular rays, caused by the same effect seen from the other end.
- Look for partly cloudy skies. You need gaps in the clouds for the light to stream through.
- A little haze in the air makes the beams visible by giving the light something to scatter off.
- Turn around. Check the sky opposite the sun for the fainter anticrepuscular version.
Alpenglow
Alpenglow is the soft pink or reddish glow that lights up mountains, clouds, or the eastern sky after the sun has dropped below the horizon, or before it rises. Because the sun is below the horizon for the observer, the light reaching the high peaks has traveled a long path through the atmosphere, which scatters away the blue and leaves the warm reddish tones to paint the high ground.
True alpenglow appears when the direct sun is already gone and the remaining light is reflected off particles in the air onto the mountains. It is most vivid on snow and pale rock, which take on color readily.
- Watch the high ground after sunset, not the horizon. The glow lands on peaks while the valleys are already in shadow.
- Snow and light-colored rock glow strongest, so high ranges show it best.
- It also works at dawn, when peaks flush pink before the sun reaches lower elevations.
Why These Effects Cluster at Sunrise and Sunset
All three share a root cause. When the sun is low, its light passes through a much longer path of atmosphere than it does at midday. That long path does three things: it bends the light enough to separate colors, it scatters away the short blue wavelengths and leaves warm tones, and it lets clouds and terrain cast long, dramatic shadows across the sky. The same low angle that makes the light golden is what sets up every one of these optical events.
How to Photograph Them
These effects are faint or fast, so a few habits improve your odds of capturing them well.
- For the green flash, use a long lens and burst mode aimed at the final sliver of sun, then check frames afterward since the effect is too quick to time by hand.
- For crepuscular rays, a slightly darker exposure deepens the contrast between the bright beams and the shadow gaps. Include a foreground for scale.
- For alpenglow, shoot the lit peaks while keeping the shaded foreground from going pure black, and use a warm white balance to keep the pink tones honest.
- Shoot raw in every case so you can recover subtle color and contrast later.
Bringing It Together
The green flash, crepuscular rays, and alpenglow are not rare miracles, they are predictable results of low-angle light moving through a deep layer of air. A flat clear horizon sets up the flash, broken clouds and a touch of haze reveal the rays, and high peaks catch the afterglow once the sun is gone. Learn the conditions each one needs, watch the right part of the sky at the right moment, and these quiet displays will start showing up far more often than you expected.