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What you need to know about COVID-19: August 12, 2020
It’s the first weekend of spring, so should I stop talking about snow? Of course not.
There’s more snow on the way late Monday and another dusting by the end of the week. You’ll find more about the spring snow in my weather video at www.weatherbyday.ca
Right now, I would like to talk about a photo that was taken Wednesday morning in Bridgewater, N.S. Ann Joudrey-Nauss was very impressed by the way the snow rolled down her windshield. She wasn’t the only one who saw this fairly unusual phenomenon.
At about the same time, Charlie Wainwright walked out to his car and found what looked like a cotton roll on his windshield. Charlie lives in Halifax and says the same thing happened on another car just up the street.
On Wednesday morning, Charlie and Ann had snow rollers on their vehicles.
Most snow rollers are spotted in wide-open spaces and are much larger, but the science is the same and the basic ingredients still apply.
• The ground – or in this case, the windshield – must be covered by a layer of ice.
• The layer of ice must be covered by wet, loose snow with a temperature near the freezing point.
• The wind must be strong enough to move the snow rollers, but not strong enough to blow them too fast; ideal wind speed is about 45 km/h.
• Alternatively, gravity can move the snow rollers.
These balls, or rolls, are formed naturally as chunks of snow are blown along the ground by the wind. Once they get moving, they pick up more snow along the way; it’s a lot like rolling snowballs to make a snowman. However, unlike snowballs made by people, snow rollers are usually cylindrical and are often hollow since the inner layers, which are the first layers to form, are weak and thin compared to the outer layers and can easily be blown away. The hollow centre makes them look like a “Swiss roll.” Remember those? Snow rollers have been seen to grow as large as one metre in diameter.
Because gravity can help these free rollers along, they are more common in hilly areas … or on windshields.