Climate-related hazards, such as heatwaves and floods, have become more frequent and intense in Canada and much of the globe.[1] This article is the first of a new series that will look at several different weather hazards and explore how rising temperatures and shifting climate patterns may affect the frequency, intensity, and duration of these hazards.
Climate represents the long-term average of weather (usually a 30-year average). Climatic zones (regions experiencing similar weather conditions) are often large. Weather hazards tend to be short-lived singular events that can lead to significant damage and disruption. How do changes in climate patterns relate to weather hazards?
This first article looks at freezing rain. Here we provide an overview of the latest data and research to shed light on how climate change could alter the frequency, severity, and geographical distribution of freezing rain events across the country.
Freezing rain forms when frozen precipitation falls through a layer of above-zero air, melting into rain, before passing through a sub-zero layer of air near the surface. This process results in supercooled raindrops that freeze on contact, creating a glaze of ice. The complexity of forecasting freezing rain stems from the complexity of forecasting the precise vertical temperature structure of the atmosphere—too thick a layer of freezing air, and the raindrops re-freeze before hitting the ground; too thin, and they remain liquid.
Supercooled water droplets are droplets that exist in a liquid state at temperatures below the normal freezing point of water, 0°C. Despite being colder than the freezing point, these droplets remain liquid until they come into contact with a surface or particle upon which they can freeze. This phenomenon is a crucial component in the formation of freezing rain and other types of frozen precipitation.
Freezing rain is most often observed in eastern Canada, particularly in regions like southern Québec, southeastern Ontario, and the Atlantic Provinces. During the winter season, these areas are more susceptible to the conditions that lead to freezing rain due to their geographical location and prevailing weather patterns.
The St. Lawrence River Valley is a notable hotspot for freezing rain events in Canada. Here, cold air sinks into the valley, cooling liquid raindrops as they fall from relatively warm air aloft. It is not uncommon for places like Ottawa and Montréal to experience numerous freezing rain warnings per year. In most cases, freezing rain events last a few minutes to a few hours. However, longer and more serious freezing rain events are possible, as witnessed during the infamous 1998 Quebec ice storm. During this event, more than 100 mm of mixed precipitation fell over five consecutive days, downing trees and powerlines, coating roads, and resulting in at least 40 deaths.
The relationship between freezing rain and climate change is an active area of research. A study by McCray, Paquin, Thériault, and Bresson (2022) used the Canadian Regional Climate Model (CRCM5) to project changes in freezing rain patterns across North America. Here are three key takeaways from their study:
A different approach to modelling future freezing rain was undertaken as a part of the Climate-resilient Buildings and Core Public Infrastructure initiative (CRBCPI). The final CRBCPI report contains modelled climate parameters intended for use in infrastructure design (so-called ‘design values’), specifically buildings and bridges. One of these design values is ice accretion. Ice accretion can occur as a result of several different processes; however, freezing rain is the dominant cause of ice accretion in most locations in Canada, making it a suitable proxy – or stand in – for this hazard. CRBCPI’s ice accretion data project a similar poleward shift in future freezing precipitation conditions due to rising surface temperatures as noted by McCray et al. (2022). Confidence in projections of future ice accretion values is low due to the complexity of processes involved in the formation of ice accretion, and the limited spatial and temporal resolution of climate models.[3]
These scientific studies and reports highlight the need for Canadians to adapt their preparedness and response strategies to account for the changing geographical distribution of freezing rain. However, they also highlight the large uncertainties that exist when attempting to model future freezing rain events, indicating the need for more research.
These shifts suggest that as the climate warms, freezing rain in Canada may move northwestward.
For those interested in the raw dataset utilized by McCray, Paquin, Thériault, & Bresson (2022), the annual freezing rain frequencies generated from each simulation and precipitation-type algorithm are available, for all of North America, here.
Ouranos has also made its freezing rain data available via its Climate Portraits data portal. On this site you can explore freezing rain maps and data, for Quebec only, summarized by:
Stay tuned for future articles on other climate-related weather hazards.
[1] Canada’s Changing Climate Report, 2019, https://publications.gc.ca/collections/collection_2019/eccc/En4-368-2019-eng.pdf
[2] McCray, C. D., Paquin, D., Thériault, J. M., & Bresson, É. (2022). A multi-algorithm analysis of projected changes to freezing rain over North America in an ensemble of regional climate model simulations. Journal of Geophysical Research: Atmospheres, 127.
[3] Climate-resilient buildings and core public infrastructure 2020 : an assessment of the impact of climate change on climatic design data in Canada / Authors: Alex J. Cannon, Dae Il Jeong, Xuebin Zhang, and Francis W. Zwiers. https://publications.gc.ca/site/eng/9.893021/publication.html