Freezing Rain (Supercooled Rain)

Primary reference(s)

WMO, 2017. International Cloud Atlas Manual on the Observation of Clouds and Other Meteors. WMO-No. 407. World Meteorological Organization (WMO). Accessed 25 November 2019.

Additional scientific description

Freezing rain or freezing drizzle is precipitation that first falls in liquid form but then descends through a layer of cold air. If this layer is thick enough and the air temperature is below freezing, the precipitation freezes on contact with the ground (or an object that is below freezing temperature), forming a coating of ice on its surface. Driving, and even walking can be dangerous in such conditions. Ice-coated utility lines or poles can be brought down due to the excess weight of the ice (Environment and Climate Change Canada, 2019).

Freezing rain can sometimes land on surfaces exposed to the air (such as tree limbs) in air temperatures slightly above freezing in strong winds. Local evaporational cooling may result in freezing. Freezing rain frequently occurs, therefore, as a transient condition between the occurrence of rain and ice pellets (sleet). When encountered by an aircraft in flight, freezing rain can cause a dangerous accretion of clear icing (AMS, 2012).

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Freezing rain is a rare type of liquid precipitation that strikes a cold surface and freezes almost instantly. The weight of the ice can sometimes be enough to bring down trees and power lines, and the glaze of ice on the ground (‘black ice’) effectively turns roads and pathways into an ice rink. The freezing rain can also be extremely hazardous for aircraft (UK Met Office, no date).

Freezing rain tends to occur in those parts of the world, for example the USA, where weather systems can produce freezing rain. These are associated with ice storms, and if enough glaze collects on trees or power lines, the weight of the ice can cause them to break and result in large-scale disruption (UK Met Office, no date).

The impact of freezing rain leading to ice storms can be significant. This is illustrated by the impact of the 1998 ice storm in Canada and the United States.

Late on 4 January 1998 freezing rain began to fall on eastern Ontario, southwestern Quebec, and southern New Brunswick and Nova Scotia, Canada. This continued for six days, ending on 10 January. These areas were pelted with 80 mm or more of freezing rain and the event doubled the amount of precipitation experienced in any prior ice storm. The result was a catastrophe that produced the largest estimated insured loss (CAD 1.44 billion) in the history of Canada (Lecomte et al., 1998).

The storm slashed across northern New York and parts of Vermont, New Hampshire and Maine in the United States, leaving a vast trail of damage and destruction (approximately USD 200 million in insured losses). Nevertheless, the damage in the United States paled in contrast to that sustained in Canada (Lecomte et al., 1998).

The combined Canadian and United States insured loss stands in excess of USD 1.2 billion or CAD 1.75 billion, as of 1 October 1998 (Lecomte et al., 1998).

In Canada, 28 deaths were attributed to the storm; in the United States, 17 people lost their lives (Lecomte et al., 1998).

According to Emergency Preparedness Canada, electrical outages in the affected areas of Canada deprived 4.7 million people or 16% of the Canadian population of power. In the United States, 546,000 people were without electricity. Thus, in both countries over 5 million people were without power (heat, light and in many instances, water) in the cold of mid-winter, which intensified the human suffering (Lecomte et al., 1998).

In terms of national alerting thresholds, Canada issues a freezing rain warning when freezing rain is expected to pose a hazard to transportation or property, or when freezing rain is expected for at least two hours (in some provinces four hours) (Environment Canada, no date). A winter weather watch is issued in the USA when any accretion of freezing rain or freezing drizzle on road surfaces is observed. A winter storm warning is issued when a half inch (1.3 cm) or greater accretion of freezing rain is expected (NOAA, no date).