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30 theWeather / theWeatherClub.org.uk

Weather Front Science lesson

Arctic rolls

ILLUSTRATION BY SPENCER WILSON

Snow rolls—supernatural playthings or natural phenomena?

Fairies, aliens, playful spirits, even a strange animal building itself a peculiar and uncomfortably cold nestsnow rolls have drawn a variety of explanations through the years, most of them invoking the supernatural. And it is easy to see why. Snow rolls look like giant Swiss rollsalbeit without the jamrandomly scattered across snow-covered fields. As well as their unusual appearance, there are several deeply peculiar aspects to these objects that have helped inspire these otherworldly explanations. Firstly there has seldom, if ever, been a report of a single snow rollthere are generally loads of them scattered far and wide across whichever

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field they appear in. So whatever demented being decided to go frolicking in the snow clearly had assistance. Secondly there are never any footprints left behindin fact the creators leave absolutely no trace of themselvesso whatever is making these things can clearly fly. Thirdly the rolls are pretty much impossible to make with human handsjust how do you get freshly fallen snow to act like a rolled-up piece of carpet or your favourite childhood cake? Give it a try it won’t work. In fact, it turns out that the secret to creating snow rolls lies not with the supernatural but instead with the snow and the wind and

the ground and the air temperature and the geography of the landscape. In fact it turns out that an awful lot of environmental conditions have to be just so for this phenomenon to occur at all, all of which makes snow rolls extremely rare, but nonetheless entirely explicable without reference to monsters. For snow rolls to form, the ground must be covered by a layer of ice to which snow will not stick. The ice must be covered by wet, loose snow, the temperature of which must be close to the melting point of the ice. The wind must be strong enough to start the snow rolls rolling, and strong enough to continue blowing them along, but not

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31 theWeather / theWeatherClub.org.uk

CUMULUS CLOUDS BY JOHN DEED

Cloud spotting for beginners

These natural sculptures will always surprise and mystify, simply because they’re so rare that most people have never even heard of them

THE CUMULUS CLOUD

strong enough to blow them too fast, in case they collapse or blow apart. It also appears to help if the wind is blowing down a slope so that gravity can help things alongsnow rolls are definitely more common in hilly areas than on the flat. Met Office meteorologist Frank Barrow explains their formation: “They start off with a nice thick layer of snow, with the top snow just on the point of melting, either because of general temperature or sunshine on the surface. The top snow layer becomes a bit sticky and you then need a fairly strong wind. The sticky layer can be peeled off the colder and more powdery snow underneath by the wind, forming a roll.” Varying in size from the width of a golf ball to as much as 2m across, these natural sculptures will always surprise and mystify observers, simply because they’re so rare that most people have never even heard of them, let alone seen one. Stumbling across a familiar field full of arctic rolls that were not there the day before would be a bit of a shock even if you had some idea what they were. Just imagine what kind of thoughts would spring to mind if you didn’t.

Close your eyes and think of a cloud. What image comes to mind? I bet it’s a Cumulus cloud. This feels like the generic cloud, the one that stands for cloudkind in general. It is the one drawn by every five-year-old child, beside the Sun, above the house and the family. It is the ‘fair-weather cloud’, the most light-hearted of all the cloud forms, and it feels like the best one with which to kick off a column on the pleasures of cloudspotting. The reason Cumulus is a fair-weather cloud is because it often forms as a direct consequence of the sunshine warming the ground. When it does so, the warmth can cause the air just above it to expand slightly and float upwards in invisible currents, or thermals. Whenever air rises, it expands. When air expands, it cools. And this cooling can cause the moisture carried upwards in the thermals to change form. If it cools enough, the water vapourthe invisible gaseous form of water, in which it is bouncing around as individual moleculescan condense into droplets of liquid. These countless, minuscule droplets mean the water is no longer invisible. They scatter the sunlight and, as they are borne upon rising

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Gavin Pretor-Pinney is founder of The Cloud Appreciation Society, author of The Cloudspotter’s Guide, The Cloud Collector’s Handbook and The Wavewatcher’s Companion thermals, form the distinctive, brilliant-white mounds of Cumulus clouds. Cumulus are distinguished from other cloud types by being low, with their bases typically at altitudes of between 1,000 and 5,000 ft, as well as having flatish bases, heaped, cauliflower tops and (assuming they are not in the process of evaporating away) crisp, distinct edges. They are a common cloud that forms right around the worldexcept, that is, in Polar regions, where the Sun never rises high enough in the sky to produce thermals of any consequence. Cumulus are also the most comfortable clouds to sit on. At least, they look like they would be. They’re like gentle tufts of cotton wool on which dreamers can kick back and shoot the breeze. This fact was not missed by the Italian painters of the Baroque period who rarely depicted a saint or an angel without a comfy Cumulus for them to perch upon. Whilst there are many rare and fancy cloud types vying for our attention, cloudspotters should never forget to appreciate this basic, most generic of clouds. Cumulus may be common, but how many others can claim to be the sofas of the saints?

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Arctic rolls  
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