December 2021

Halos on the Ski Slopes

When Claudio Cattaneo (Member 13,236) took to the ski slopes of Crans-Montana, Switzerland early on a cold December morning, he was greeted by a dramatic display of halo phenomena. Caused here by ice fog, known as diamond dust, glittering in the morning air, these arcs, spots and rings of light appear when sunlight is refracted and reflected as it shines through tiny ice crystals floating in the air.

The ice crystals are in the form of minuscule hexagonal plates and columns. They have the usual six-sided symmetry of the intricate, filigreed crystal shapes that inspire December holiday decorations, but they have far simpler shapes. Their sides are flat and even, and so they behave like glass prisms through which the sunlight can sparkle. And just like glass prisms, the ice causes the sunlight to bend, or refract, as it passes through. The collected effect of light glinting through these countless crystals give rise to the halo phenomena. Each ring, spot or arc has a name, and some of the ones Claudio spotted are rare.

With such an abundance of optical effects in Claudio’s photograph, we decided to seek advice on identifying them. Les Cowley (Member 014) runs the authoritative Atmospheric Optics website and is a world expert on light effects in the sky just like these. “This is a spectacular and rare display,” he told us. “I wish I’d been there to see it.” Les sent us annotations detailing all optical effects present on the slopes that day. Some effects were familiar to us, like the ring of light around the Sun known as a 22-degree halo, the spots of light known as sundogs touching the halo on either or both sides of the Sun, and the vertical beams stretching above and below it known as sun pillars. But there were other, far rarer light effects present, with exotic names like ‘Parry supralateral arc,’ named after Arctic explorer William Edward Parry who, in 1820, saw and carefully recorded a display of rare halo phenomena while icebound off Melville Island in the Canadian Arctic. “The Parry halos, including the helic arc, are the rare ones here,” explained Les Cowley. Polar conditions are usually needed for the ice crystals that produce such dramatic optical displays.

The optical effects spotted by Claudio Cattaneo (Member 13,236) all have names, and they were identified for us by Les Cowley (Member 014), a world expert in atmospheric optical effects.

But the crystals that caused Claudio’s halos were likely thanks to the snow machines that had been working the pistes overnight. As well as preparing the slopes for the skiers, snow blowers at ski resorts can, when conditions are just right, lead to very small and very pure ice crystals that float as diamond dust in the air after sunrise. These are likely what led to the display on this December morning in Switzerland.

Les’s tip for halo spotters is that dramatic displays towards the Sun like these are almost always accompanied by others. “Whenever there is something spectacular in the sky,” he advises, “remember to look in the other direction. There are often more marvels to be found.”

A rare display of halo phenomena spotted by Claudio Cattaneo (Member 13,236) on the ski slopes of Crans-Montana, Switzerland.

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