Ice storm — Ice storms are one of the most dangerous forms of winter storms. When surface temperatures are below freezing, but a thick layer of above-freezing air remains aloft, rain can fall into the freezing layer and freeze upon impact into a glaze of ice. In general, 8 millimetres (0.31 in) of accumulation is all that is required, especially in combination with breezy conditions, to start downing power lines as well as tree limbs.[3] Ice storms also make unheated road surfaces too slick to drive upon. Ice storms can vary in time range from hours to days and can cripple small towns and large urban centers alike.