How does an ice storm form?

An ice storm forms through a specific atmospheric process involving temperature layers. Outside the tropics, precipitation typically begins as ice crystals in the upper atmosphere.2

The formation process occurs as follows:

• Melting: These ice crystals fall through a layer of air that is above freezing, causing them to melt into liquid droplets.12
• Supercooling: As these droplets continue to fall, they enter a lower-level layer of air that is at or below freezing. This causes the droplets to become supercooled.12
• Freezing on Contact: When these supercooled droplets strike cold surfaces on the ground—such as trees, roads, or electric power lines—they freeze immediately upon contact, creating a glaze or film of ice.23

This weather event is defined by these accumulations reaching 6.35 mm (0.25 inch) or more, which can lead to significant structural damage and hazardous conditions.2