Warm over cold
ILLUSTRATION PROVIDED BY ENVIRONMENT CANADA
Freezing rain forms in winter when snowflakes pass through a layer of the atmosphere (orange) that is warmer than the layer on the surface of the ground. Water that falls to the ground is said to be frozen.
How does freezing rain form? In a nutshell: warm over cold. “Normally, it is warmer on the surface [au sol], and the higher you go in the atmosphere, the colder it gets, says Julie Mireille Thériault, professor in the department of earth and atmospheric sciences at the University of Quebec in Montreal (UQAM). In the case of freezing rain, it is the opposite. It is cold on the surface and warmer as it rises in the atmosphere. So the snowfall, which starts from very high, passes through a warmer layer (a “hot nose”, in the jargon), melts, but does not reform into flakes before reaching the ground. These droplets are said to be frozen. They remain liquid due to their microphysical properties.
From freezing rain to sleet
PHOTO ROBERT SKINNER, THE PRESS
Marie-Ève Giguère, outreach meteorologist at Environment Canada
“As soon as a droplet of frozen water hits a surface, be it a tree, a clothesline, an electric wire or the ground, it loses its microphysical properties and freezes immediately”, explains Marie-Ève Giguère, meteorologist at outreach to Environment Canada. This is when the ice forms. The rest follows. New frozen drops fall on the first layer of ice, which thickens over time. “Sleet is also a form of freezing precipitation. It’s a shell of ice with a heart of frozen water,” explains Marie-Ève Giguère.
Danger in the Valley
ENVIRONMENT CANADA CREDIT
The warm air arriving from the south aloft settles over the cold air blowing in the St. Lawrence Valley. Conditions are then conducive to the formation of freezing rain.
We like it, our St. Lawrence Valley, with its lowlands surrounded by mountains. This is the ideal place to create freezing precipitation. “When a cold air dome is found in the St. Lawrence estuary, the river channels the winds that remain trapped in the lowlands,” notes Marie-Ève Giguère, outreach meteorologist at Environment Canada. And when a warm air system arrives from the southern United States, it rises above the cold air layer. The conditions conducive to creating freezing rain are then met.
A revealing card
IMAGE PROVIDED BY ENVIRONMENT CANADA
Each of these lines represents the course of a low pressure system. North of each of these lines, which can be compared to the eye of a hurricane, the temperature is colder.
This map gives a good idea of the effect of the St. Lawrence Valley on the passage of weather systems. With the exception of St. John’s, Newfoundland and Labrador, the Montreal region and the St. Lawrence Valley (orange dots) are those that receive the most freezing rain on average in North America.
IMAGE PROVIDED BY THE AMERICAN METEOROLOGICAL SOCIETY
Brownish dots in St. John’s and orange dots near Montreal indicate that these locations receive the highest annual average of freezing rain.
Five episodes, three storms, one blockage
Freezing rain fell on southern Quebec for five consecutive days, from January 5 to 9, 1998. Meteorologists group these five days into three storms (episode): January 5-6, 7-8 and 9. What is remarkable is that the air masses were not moving. This is called an atmospheric blockage in which a corridor is formed through which all the precipitation passes. In addition, this system has positioned itself further south than normal. With the effect that southern Quebec found itself in the cold and abundant precipitation part of the depression. With the results that we know.
PHOTO DENIS COURVILLE, LA PRESSE ARCHIVES
In Montérégie, the freezing rain storm left heavy layers of ice on the ground, on trees and on all Hydro-Québec equipment. Here, January 9, 1998, in the region of Saint-Césaire, Rougemont.
More than 100 mm in Montérégie
According to an Environment Canada report, the regions of Montreal, Montérégie, Drummondville and Bois-Francs received between 60 and 100 mm of freezing rain. With a peak at nearly 120 mm in the Acton Vale sector. The Quebec region, where it was colder at ground level, received nearly 80 centimeters of snow and sleet. The – warmer – border sectors of Estrie and Montérégie received nearly 100 mm… of rain.