15 January 2018

Snow Rollers: Nature’s Winter Treat

If you live in one of world’s colder, snowier regions you may have seen them – but even then the chances are remote.  A rare and seemingly mysterious treat of nature, the snow roller is a natural phenomenon created without human intervention.  They are known by other names too: snow donuts, snow cylinders or even snow bales. Whatever you prefer to call them, it seems that nature, at least, is trying to make the most of the snow and have a little fun.
Whether or not nature can be anthropomorphized is a matter for debate. However, one’s first reaction is, inevitably that it must have been a person or persons unknown who took the time to gather a handful of snow and then slowly and carefully roll it until it became too large to push anymore. Yet a brief inspection of the snow around reveals no footprints.  So how were they created? There is science behind this magical and mysterious apparition.

Image Credit Aida Lundquist
Image Credit WSDOT
Snow rollers can only form in a very specific set of infrequent conditions.  First of all there must be a good, thick layer of snow on the ground.  Then, there must be sunshine – snow rollers will not form while snow is falling. Failing sunshine, the temperature has to be rising: the snow on the top must be just about to melt so it becomes just a little bit sticky.

Image Credit wcn47
Image Credit wcn47
Image Credit wcn247
That is not all.  Next you need wind: not a gale but one strong enough to effectively peel off the top, sticky layer of snow.  As the wind blows, the snow will roll and gather some of the powdery snow beneath it. A natural slope or hill also helps to get the snow roller started.  The wind will push it along until the roller becomes too heavy for it to be driven any further. 

Image Credit wcn47
Image Credit Pospisil
One final element completes the puzzle.  In order to reach their optimum size there must be a sufficiently long surface of smooth, unbroken snow.  Even a few tufts of grass sticking up can stop a snow roller in its tracks before it properly gets going.

Image Credit wsdot
Image Credit Sergey Yeliseev
Image Credit Mark Cameron
So why are they often hollow?  The structure on the inside is the weakest and so can be blown away (this can happen to an entire nascent roller if the wind is too strong).  Although the whole structure looks strong it is in fact quite fragile so even when a roller forms the interior can be blown away if the wind becomes just a little stronger.

Image Credit wsdot
Image Credit wsdot

Snow rollers can often be stopped in their tracks - and then their shapes become even stranger.

Image Credit Wikimedia
Image Credit Ninian Reid
Even so, sometimes snow rollers can form in the most unlikely places.  If you are lucky, you may even get some on your roof.

Image Credit Wikimedia
Image Credit Mark Cameron
Although they are often seen singly or in pairs, snow rollers can sometimes appear as a host - that seems like a suitable collective noun for them.

Image Credit yogabint
Image Credit cuproff
Put all the factors together and there you have the reason why snow rollers are not ubiquitous.  Very precise conditions are needed for this phenomenon to occur. Yet as the word ubiquitous amply describes our species and its penchant for camera phones. So although we may not get to see a snow roller where we live, we can at least enjoy them at a distance.

Image Credit John
Image Credit Miranda Granche
First Image Credit WSDOT

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