Ice storms are caused by freezing rain. Despite its name, freezing rain is actually rain that falls at a temperature just below freezing, but it freezes once it lands on a surface that is 32 degrees Fahrenheit or less, creating a glaze of ice over that surface. When a storm results in .25 inch of ice or more on exposed surfaces, it is declared an ice storm.
Ice storms form when a layer of warmer air becomes sandwiched between two layers of cold air. Snow or hail falls from the top cold layer, then it melts when it hits the middle warmer layer of air. As the precipitation hits the bottom colder layer, it refreezes. However, because of its journey through the warmer layer, the precipitation does not become snow again. Instead, it becomes freezing rain.
Ice storms are very dangerous because of the slick roads they create. Because ice is heavy, accumulations of ice can weigh down power lines, often causing severe power outages. An ice accumulation of just half an inch can weigh as much as 500 pounds on a tree branch or power line, according to The Weather Channel. Because the freezing rain associated with ice storms often seems to fall gently, people are not always aware of its potential danger.