Sun Dogs

Sun dogs
© Martin Ruegner/Getty
You aren't seeing triple. Sun dogs, brightly coloured blobs of light sometimes visible on either side of the sun (pictured above), are really mirror images of our star.

They appear when it is cold enough for tiny ice crystals to be present in the atmosphere. The illusion is the result of sunlight bending or refracting through these floating crystals, which act as prisms. Consequently, sun dogs are rainbow-hued: the inner edges, closer to the sun, are tinged with shades of red, while the outer edges are blue. They belong to a group of optical phenomena called halos, which all involve sunlight interacting with ice crystals.

Sun dogs are officially called parhelia. Their more common name derives from their appearing to follow the sun, like a dog follows its master. The best time to try to spot them is when the sun is close to the horizon, around sunrise or sunset.

Frost flowers

Frost flowers
© Henri Koskinen/Alamy
A truly rare sight, these beautiful but short-lived ice sculptures form when the sap inside plants freezes. This can happen when the air temperature is freezing, but the ground is not yet frozen. The sap inside long-stemmed plants expands in the cold, creating cracks in the stem. Now water can ooze out and freeze, forming intricate and fragile ribbons or flower-like structures (pictured above).

Snow rollers

Snow rollers
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As big as an oil barrel or as small and fragile as a stick of candyfloss, snow rollers (pictured above) form on windy hillsides when strong gusts blow moist snow along the ground. "You get a chunk of snow that peels, bends and curls over itself," says Jeffrey Deems at the National Snow and Ice Data Center in Boulder, Colorado. The result is a hollow, cylindrical roll of the stuff with a thin, weak inner layer. Depending on its size and shape, it might be called a snow roller, a snow bale or a snow doughnut.

Snow rollers are rare, with sightings every few years only when conditions are just right. First, there needs to be a thin surface layer of wet snow. Underneath that, there has to be a layer of powdery snow or ice so the surface layer can lift up and roll as the wind blows without sticking to it. The wind must have a Goldilocks element to it, too - it must be strong enough to lift and roll the snow, but not so strong as to blow the roller away.

Hoar frost

Hoar frost
© David Jones/PA Archive/PA Images
The name derives from the Old English word har, meaning white or grey with age, because hoar frost deposits its long, spiky crystals onto objects like trees and fences, transforming their appearance. Like regular frost, this variety (pictured above) forms when water vapour turns to ice on a surface that is below freezing. If there is a high-enough deposition rate, which depends on moisture and temperature gradients from the atmosphere to this surface, says Deems, the crystals can grow into the feathery kind typical of hoar frost.

Because the interlocking patterns of crystals grow in size and complexity, some nickname the phenomenon "frost on steroids".

When it forms on snow, the result can be a giant field of feathery crystals called surface hoar.


© Rolf Nussbaumer Photography/Alamy
Graupel (pictured above) is perhaps the ultimate whimsical weather phenomenon, a case of unorthodox snow that forms inside a cloud. More precisely, it involves supercooled water droplets condensing on top of falling snowflakes. The droplets freeze, and as they do so they change the shape of the snowflakes, turning them into small squishy balls or pellets. "When you get a snowflake forming in the cloud, the liquid water inside the cloud freezes on to the snowflake as rime, and then it hits the ground as graupel," Deems says.


© plainpicture/ganguin
The thick layer of styrofoam-like ice sometimes seen on top of parked cars, trees and telephone poles is called rime (pictured above). It tends to form straight on to surfaces in cold, foggy environments with lots of water vapour in the air.

For this to succeed, the tiny droplets of cloud or fog must be supercooled - in other words, cooled to below freezing point but able to stay liquid, often thanks to impurities in the water. When these droplets then come into contact with any cold object, like a parked car, they immediately freeze on to it.

© USDARime on snow crystals seen through an electron microscope (above and below). If a crystal is entirely covered by rime, it becomes graupel
Lasting fog can lead to huge build-ups of rime. When it forms from supercooled water vapour turning directly to ice, "it looks like spiky, sparkly crystals", says Deems, but if the process involves tiny droplets freezing, the rime has a chunkier consistency because the resulting ice crystals are randomly oriented.