Two conditions are essential for freezing rain. First, there has to be a low-lying layer of cold air with its temperature below the freezing point. And there has to be a large mass of warmer, humid air above it.
When those two conditions are met, ice crystals form in the upper clouds. Falling down through the warm air mass, the snowflakes melt and turn into raindrops.
As they continue downward, the raindrops hit the cold air mass and become supercooled. That means they stay liquid even below the freezing point. Then when they strike a cold surface, they freeze on contact.
Freezing rain is hardly a rare occurrence in a Québec winter. In the Montréal area, between November and March, there are, on average, a dozen episodes of freezing rain. But some have been historic.
It’s hard to measure freezing rainfall, by its very nature. That’s because as the masses of air move, freezing rain can change into melting snow or ice pellets. Also, the amount of freezing rain that falls varies from place to place, depending on the relief of the landscape and time of day. Ice meters can measure the amount of ice on the ground, but not that on buildings or electrical towers. Casts and direct measurements on power lines are more accurate. That’s why the amounts of ice measured in the 1998 ice storm vary a lot from one municipality to another.
The 1998 ice storm was exceptional because of two unusual situations that occurred hundreds of kilometres away from southwestern Québec. First, El Niño caused a large mass of warm air to form over the Gulf of Mexico. Due to prevailing winds, this warm air mass moved to Québec and then another one formed. Second, the usual pattern of west-to-east prevailing winds stopped for a few days as a result of a major high-pressure system that stayed put over Newfoundland and Labrador.
The scope of the 1998 ice storm can therefore be explained by three successive freezing rainfalls over an area of 40,000 km2 within a very brief span of time. It was an unprecedented event that many have called a natural disaster, or act of God or nature.
According to Environment Canada, there were three episodes of freezing rain, and the average accumulation was between 50 mm and 70 mm. The regions of the Outaouais, Montérégie, Beauce and Montréal were the hardest hit.
The maximum freezing rainfall was recorded in Montérégie. A total of over 100 mm of freezing rain fell in the area between the towns of Saint-Hyacinthe, Granby and Saint-Jean-sur-Richelieu, dubbed the “triangle of darkness.”
The first observable effect of the ice storm was the difficulty getting around. Walkers slipped and fell, while cars were stuck in the ice or could only advance slowly, due to poor road conditions. Public transit moved at a crawl. People preferred to stay home, going out only when they had to.
As freezing rain continued to fall, plants and trees were coated with ice, which many amateurs photographers found spectacularly beautiful. Some branches bent under the weight. Others broke, littering streets, yards, parks and woods. Many fell onto cars, and pedestrians were at risk of being hit.
Starting January 6, the ice storm struck the power transmission system in and around Montréal hard.
The collapse of the transmission system led to a breakdown in power supply. Although hydropower stations were still generating electricity, it could not get to the substations and the distribution system that would carry it to customers.
The ice storm also damaged the distribution system, which delivers power to customers.
With the distribution system out of commission, customers had no power.