Manotick resident Bob Caines at least had a sense of humour about getting his car on the road Monday after using some real elbow grease to get the ice from his windows first. Julie Oliver/Postmedia
Julie Oliver / Postmedia
Freezing rain comes from the air, but Canada’s senior climatologist says it’s the shape of Ottawa’s ground-level landscape that makes ice storms happen so often here.
Warm air and cold air meet and create freezing rain because this is a valley. This geography brought us heavy ice for a week at the end of 1998, and we had a smaller replay of the same pattern on Monday.
This week’s storm actually began days before it reached us. At Environment Canada, David Phillips traces it to the other side of the continent.
“If we look at the origin of the storm, it probably is California. We saw a lot of snow and rain there and as it crossed the United States it picked up moisture from the Gulf of Mexico,” he said late Monday. “That always is a big issue for freezing rain and it’s similar to what we saw back in 1998 — cargoes of moisture coming up from the Gulf of Mexico” and joining another storm system.
Monday brought ordinary rain in southern and central Ontario but freezing rain from Kingston east.
Ottawa had about three hours of light snow, a couple of hours of ice pellets, then at least 12 hours of light but steady freezing rain, with a few short breaks. East winds with gusts near 60 kilometres an hour added pressure to wires and branches coated with ice: Tree branches fell, and in Gatineau the storm tore a piece of wood loose from the abandoned E.B. Eddy factory, forcing traffic to detour.
But why does the freezing rain so often fall here? Why is Ottawa 14th out of 100 cities in terms of amount of freezing rain per year?
“I think the valley is a big determination. The fact that you’re low,” Phillips said. “And I know it doesn’t look like a huge valley, but it’s there. Just that low topography is enough for that low, very heavy dense air just to settle in there and hug the ground.
“The winds can’t kick it out. It just stays there.”
Add in a flow of warm southern air with rain. “In the Ottawa area that moisture came northward and typically that warm air is lighter and it overrides the cold air.”
So, rain is coming from warm air on top and passing through cold air below.
If there’s a thick layer of cold air near the ground, the rain coming through it will turn to snow or ice pellets. Either way we can shovel them out of the way.
But if it’s a thinner layer of cold air, the raindrops stay liquid while becoming supercooled — colder than freezing temperature, but forced to stay liquid because friction with the air makes the drop wobble around too much to solidify, like rapids in a river.
Everything at ground level — rooftops, trees, roads and power lines — is uniformly cold. The conditions are set. That super-cold water drop hits one of these surfaces, spreads out in a thin layer and freezes almost instantly.
“It forms a very smooth, thin veneer of slick ice. And it’s the hardest ice that nature can produce. If you hit it with a hammer maybe you’ll break it but it’s tough, and it just goes layer after layer.”
None of it drips off; instead it builds up to a great thickness — in the range of five to 10 millimetres Monday in Ottawa, as opposed to 67 mm in the three waves of freezing rain 20 years ago.
Ottawa has on average 16 days with freezing rain each year, and 64 hours of freezing rainfall.
“So it’s very much part of your climate and I think it’s that valley location that makes it so.”
The capital region has been used in the past for federal government research on how freezing rain affects aircraft, because we have a reliable supply of it.
There was a little good news Monday: By late afternoon Ottawa police had recorded 34 collisions since midnight — which isn’t a particularly high number compared with the rough average of about 50 on a normal day. Bad snow days can push the total up around 150, police said. Traffic on Monday was well below average.
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