An ice storm is a winter weather event characterized by freezing rain that coats surfaces with a layer of ice. This occurs when rain falls and freezes upon impact. The U.S. National Weather Service defines an ice storm as a storm resulting in at least 0.25 inches (6.4 mm) of ice accumulation on exposed surfaces. These storms can lead to widespread disruptions.
How Ice Storms Develop
Ice storms require a specific atmospheric temperature profile. Precipitation begins high in the atmosphere as snow or ice crystals, which then descend through an above-freezing air layer, melting into raindrops. This warm layer sits above a shallow layer of air at or below freezing temperatures (0°C or 32°F) near the ground.
As raindrops enter this sub-freezing layer, they cool below freezing but do not immediately turn into ice. This process is known as supercooling, where water remains liquid despite being below its freezing point. Upon striking surfaces such as the ground, trees, or power lines, these supercooled droplets instantly freeze, forming a layer of clear ice.
Identifying an Ice Storm
A distinguishing characteristic of an ice storm is glaze ice, a smooth, transparent, and homogeneous coating. This clear ice encapsulates outdoor objects like tree branches, power lines, and vehicles, giving them a distinct glassy appearance. Unlike sleet, which consists of small ice pellets, glaze ice forms as liquid rain freezes directly onto surfaces.
A subtle tapping or clicking sound may be heard as supercooled raindrops collide with surfaces before freezing. Glaze ice allows underlying structures to remain visible, differentiating it from opaque forms of ice like rime. This dense ice adheres firmly to objects, increasing their weight considerably.
Consequences and Safety
Ice storms commonly lead to extensive power outages. The accumulated weight of ice on tree limbs can cause them to break and fall onto power lines. Even without falling trees, the burden of ice directly on power lines can cause them to snap or pull down utility poles, potentially disrupting power for days or weeks. Just 0.25 inches of ice accumulation can add hundreds of pounds of weight per line span.
Roads, sidewalks, and bridges become extremely hazardous due to slick, transparent ice, making both driving and walking highly dangerous. Falling ice shards from trees or structures also pose a direct threat to people and property.
Prolonged power outages can lead to risks such as carbon monoxide poisoning if alternative heating sources are used indoors without proper ventilation. To remain safe, it is advisable to stay indoors and avoid non-essential travel. Prepare an emergency kit with food, water, and battery-powered devices. Never approach downed power lines, as they may still be energized and present a severe electrical hazard.