Ice storms occur when a layer of warm air is between two layers of cold air. Frozen precipitation melts while falling into the warm air layer, and then proceeds to refreeze in the cold layer above the ground. If the precipitate is partially melted, it will land on the ground as sleet. However, if the warm layer completely melts the precipitate, becoming rain, the liquid droplets will continue to fall, and pass through a thin layer of cold air just above the surface. This thin layer of air then cools the rain to a temperature below freezing (0 °C or 32 °F). However, the drops themselves do not freeze, a phenomenon called supercooling (or forming "supercooled drops"). When the supercooled drops strike ground or anything else below 0 °C (32 °F) (e.g power lines, tree branches, aircraft), they instantly freeze, forming a thin film of ice, hence freezing rain.
- Ice storms are more common in valleys and foothills.
- Ice storms occur most often during the months of December and January and near sunrise, usually the coldest time of the day.
- According to most meteorologists, just one quarter of an inch of ice accumulation can add about 500 pounds of weight per power line span.
- One of the damaging and costly ice storms in recent history struck North America in January of 1998. Phone and power lines collapsed, electricity pylons buckled, and 4 million people were left without power. Falling ice or fires set off by collapsing electrical cables killed 25 people, and the damage cost around $1 billion.
- The major ice storm that struck the Northeastern U.S. in December 2008 left 1.25 million homes and businesses without power. In what was described as the worst storm in the last ten years, a state of emergency has been declared in Massachusetts, New Hampshire, and parts of Maine.
- An ice storm which struck northern Idaho in January 1961 set a record for thickest recorded ice accumulation from a single storm in the United States, at 8 inches.
Here are some photos of ice storms from the Internet.