When a mass of warm moist air collides with a mass of cold arctic air, the less dense warm air will rise and the moisture may precipitate out in the form of rain. When this rain falls through the colder, more dense air and comes in contact with cold surfaces, the latent heat of fusion is removed by convective and evaporative cooling. Ice forms on these cold surfaces and may continue to form until the ice is quite deep, as much as several inches.
The buildup of radial ice may strain branches of trees, power lines and even transmission towers to the breaking point. One inch of radial ice will bring down tree limbs onto power line. Two inches or radial ice on power lines with bring the lines down because of their own weight, usually taking poles and transformers with them. Either way, ice storms cause widespread power outages.
Debris from downed trees and wires makes roads impassible and makes emergency access, repair and cleanup extremely difficult. Sawyer crews have to clear the way before line crew can begin restoring power.
The state's largest ice storm occurred in December 2008. The National Weather Service had been predicting a major ice storm, but actually arrived was about twice what was expected. More than half of the state's electric utility customers, which included about two-thirds of the population, were out of power. Some for as long as two weeks. All four of the state's electric utilities were overwhelmed and brought in extra sawyer and line crews from as far away was the Midwest and Canada.
An earlier ice storm, in January 1998, also caused power outages around the state as well as significant damage to the state's forests. Read a meteorologist's recollections 15 years after the January 1998 ice storm.