ice storm

ice storm, weather event characterized by rain freezing on contact with cold surfaces that results in ice accumulations of 6.35 mm (0.25 inch) or more on those surfaces. It can cause ice to build up on roads, vehicles, and residences and other structures as well as on trees and electric power lines. The buildup of ice makes roads slippery and adds weight to trees, which can cause them to break and contribute to property damage and injury when they fall to the ground. Ice buildup on power lines can cause them to snap, creating fire and electrocution hazards if they continue to carry electric current.

Formation and development

Outside the tropics precipitation begins as ice crystals, falling generally as snow if they do not melt before reaching the ground or as rain if they melt into liquid droplets before reaching the ground. Freezing rain occurs when falling ice crystals pass through a layer of air whose temperature is above freezing, causing them to become liquid droplets before entering lower-level air that is below freezing. At this point, droplets can become supercooled, and, when they strike a cold surface, they freeze immediately, which creates a film of ice on roads, trees, and other surfaces.

Notable ice storm events

Ice storms happen in many regions across Earth, and large ice storms can have devastating impacts. Five of the deadliest and most damaging ice storms are listed below:

• In January 1940 a cold snap that froze the River Thames in London contributed to the formation of a rare ice storm that coated parts of southern England with ice up to 30 cm (12 inches) thick.
• In February 1994 an ice storm passed across 11 U.S. states, causing $5.7 billion worth of damage (adjusted for inflation in 2022 dollars). Mississippi was hit particularly hard, with ice accumulations of up to 15 cm (about 6 inches) that resulted in 9 deaths and damaged roughly 1.5 million hectares (3.7 million acres) of forest.
• In January 1998 an ice storm falling on New England and southeastern Canada over four days killed about 40 people and caused an estimated $4 billion worth of damage (adjusted for inflation in 2022 dollars). Some areas recorded as much as 10 cm (4 inches) of ice buildup. The storm caused more than 500,000 residents of New England, including more than three quarters of the population of Maine, to lose power—in some places, for weeks.
• In November 2020 ice up to 12 mm (0.5 inch) thick covered parts of Russia’s far east, leaving some 150,000 people in the city of Vladivostok without water and electricity.
• In April 2023 an ice storm striking the provinces of Quebec and Ontario in Canada resulted in ice accumulations of up to 25 mm (1 inch) that cut power to more than a million people.