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‘Ultra-low density’ snow responsible for big drifts

A local weather forecaster says “ultra-low density” snow was the most significant factor in Sunday’s big storm that deposited as much as 60 centimeters in some parts of West Kootenay.

Jesse Ellis, a weather specialist at the Southeast Fire Centre in Castlegar, explains that snow density can be expressed as a ratio of what falls on the ground versus what is produced as a liquid once the snow melts.

One centimetre of melted snow is typically equivalent to 10 to 12 cm of fallen snow, for a ratio of 10:1 or 12:1.

Ellis says heavy, wet snow may be closer to 7:1 (one centimetre of melted water produces only seven centimetres on the ground) and low density snow is often around 20:1.

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However, on Sunday “many areas reported densities that were much lower: closer to 30:1 or 40:1! If this event had come with ‘normal’ snow density, we would have seen closer to 15 cm or 20 cm total.”

Ellis said several factors contribute to low snow density including how cool it is at the surface, mid-elevation, and at the elevation the snow is forming.

Wind is also a factor at both the ground level and in the layers of the atmosphere between the ground and where the snow is being formed.

“From my experience, cool temperatures and light winds are the best combo for producing low density snow but it is a complex field of snow science,” he says.

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