Capturing a Heavenly Halo at Greenland’s Summit Station
It’s easy to see a photo like this, pause for a moment, let out an “ooh” or an “ahh” or two, then continue on about your business. But that takes all the fun out of it!
Let’s stop for a moment, and really appreciate what’s going on in this picture. What is the science behind such atmospheric phenomena?
- Almost every optical phenomenon in the daylight sky, whether it’s a rainbow or a halo or a “sun dog”, occurs due to light bending through water that’s suspended in the atmosphere. That water could be liquid (like rainbows) or crystallized ice (like above), but it acts as a prism all the same.
- Start with the inner ring around the Sun. This is a solar halo. To be precise, it’s probably a 22˚ halo. This ring occurs when hexagonal ice crystals, randomly distributed and aligned in the atmosphere, refract light coming from the Sun back to the eye of the viewer. While every crystal of ice in the air is bending the light that hits it, only the hexagon-shaped crystals that happen to be sitting at exactly 22˚ away from the Sun AND oriented just right are able to bend light straight back at your eye. This is because of their particular six-sided geometry. Someone standing 50 feet to your right would see the same halo, but from a completely different set of randomly aligned ice crystals!
- What about those two orbs of concentrated light to the left and right of the Sun? Those are called "sun dogs". Aristotle used to refer to them as “mock suns”, chasing the real Sun through the sky. They usually are only seen when the Sun is low in the sky, and always at the same elevation as the Sun itself. What causes these? The same ice crystals as the 22˚ halo! You see, as the hexagonal crystals sink down to Earth, they begin to align. It’s the same thing that happens to a falling dart: the air resistance makes them stand up straight. Remember how the 22˚ halo was formed by whatever crystals just happened to be pointing our way? The sun dogs appear brighter because there’s simply more “ice prisms” pointing in the right direction to send light into your eyes. You can even see a red-to-blue prism effect at work, since different wavelengths of light get bent at slightly different angles.
- And that blob at the top of the halo? That’s an upper tangent arc. It’s origin lies in the same alignment of ice crystals that form the sun dogs, but instead of falling vertically, they are arranged horizontally. If you look closely ou can even see the far edges arcing down like wings, suspending the orb of light in mid-air. The arm of light reaching upwards like a solar pillar is the result of simple reflected sunlight off of the icy faces of suspended crystals.
- The outer ring is perhaps the most rare. I’m pretty sure that’s a 46˚ halo. The same hexagonal ice crystals are at play, but instead of bending light through their six-sided faces, they are bending it through their base. That particular geometry of refraction gives bends light at a wider angle, meaning the halo is larger. It’s fainter, because there are fewer ice crystals randomly oriented that way, and can only be seen when the Sun is low and the air is clouded with frozen water.
There’s definitely more in there that I’m not mentioning, but wow … right? Sights such as these would be carry plenty of beauty without further explanation, but a little knowledge certainly enriches nature’s grandeur, no?
(via Alan Boyle, photo by Ed Stockard)
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