The reason we get freezing rain might surprise you
Thursday, Nov 16, 2017 06:00 am
Freezing rain is a weather phenomenon many people could do without. Every time it falls, motorists and pedestrians find themselves slipping and sliding, fighting to stay upright and avoid collisions. We have our colder middle atmosphere in Canada to thank, according to Alysa Pederson, senior meteorologist with Environment Canada.
“Here in Canada, all of our precipitation actually starts as snow because it’s really cold in the middle levels of the atmosphere,” she said. “When (the snow) falls through, it will fall through a layer of temperatures above zero so that will melt the snow and then it will become rain in that layer. If the surface temperatures are below zero, then the rain will hit the surface and on contact it will freeze.”
Ice pellets are similar to freezing rain, according to Pederson, in that they form in the same way but the layer of below zero temperatures is thicker allowing the snow to reform into pellets before it reaches the ground.
Airdrie received freezing rain Nov. 10, but remarkably, Airdrie Fire Department Deputy Fire Chief Garth Rabel said crews did not receive any calls to attend serious vehicle collisions.
The incidence of freezing rain can also be impacted by the location above sea level, according to Pederson.
“Because Calgary sits at 3,500 feet above sea level, Calgary only had maybe 1,000 to 1,500 feet thick of air that was below zero so it wasn’t able to refreeze that rain. It didn’t freeze until it hit the surface,” she said. “Brooks, further east, they’re actually only at 2,400 feet above sea level so that cold layer was a lot deeper than it was over Calgary just because of how much lower to sea level the ground is. They saw pellets.”
In other parts of the world, closer to the equator, Pederson said a different process creates the rain droplets.
“Up here, we’re always going to have lower temperatures at the levels that precipitation begins to form,” she said. “It’s a cold process. (Precipitation) always forms as ice crystals.”
The temperature of the ground also influences how the precipitation we receive acts.
“If the ground temperatures are well below zero – like -6C – (precipitation) will freeze much more quickly, whereas if your ground temperatures are -0.5C – barely below zero – the more and the harder the rain falls, the more that it’s going to release heat when the rain (reaches the ground),” Pederson said. “The rain, when it freezes, releases some heat. So, if you’re closer to zero…it will not be as treacherous on the roads because it will be heating up as it hits the ground.”
Unfortunately, the temperature in and around Airdrie Nov. 10 was closer to -6C, meaning the rain was freezing upon contact with the ground, according to Pederson, causing all that slipping and sliding motorists and pedestrians hate.