Ice storms form when air layers stacked in a specific way deliver supercooled liquid water that freezes on contact with cold objects. Business Insider notes that ice storms start when clouds release snow. The snow melts while passing through a lower, warmer layer of air. The melted raindrops fall into another layer of air below freezing temperature, but they do not actually freeze until they impact a solid object.
The supercooling of water required for ice storms is actually a relatively common phenomenon in liquids. To freeze, liquids commonly require a starting point that a relatively pure drop of melted snow does not provide, such as a small particle or other solid. The moment that the water impacts a solid object, it rapidly freezes.
The complex atmospheric conditions required for ice storms make them one of the less common forms of precipitation. When ice storms do occur, however, they can be devastating. The solid ice formed during an ice storm is far denser than snow, and it freezes onto surfaces, such as power lines and vertical plant stems, on which snow would accumulate only lightly, if at all. The ice also coats surfaces like roads and walkways, causing slipping hazards.