The Northeast is a prime area for ice storms, and Pennsylvania is no exception. Ice storms are a type of winter storm that is caused by freezing rain. As snow falls, it encounters a warm layer of air in the middle of the atmosphere that turns it to rain. Before the rain hits the ground or the surface of an object, it encounters a very shallow layer of cold air that causes the rain to freeze on the cold exposed surfaces. The ice accumulates on trees, power lines, bridges, and roadways causing significant damage and dangerous driving conditions.
Source: Ice Storm Power Lines| Wikipedia