Ice storms happen when a warm cloud rains above a layer of colder air. This lowers the temperature of the droplets to below zero, however it remains in liquid form. The supercooled droplets freeze into ice on impact when they fall onto a surface the temperature of which is close to, or below, freezing.
This freezing rain covers everything with heavy, smooth glaze ice. Ice-covered roads become slippery and dangerous. Driving becomes extremely hazardous as the ice causes all types of vehicles to skid out of control, which can cause devastating car crashes as well as pile ups. Even pedestrians are severely affected as sidewalks become slippery, which can cause people to slip and fall and outside stairs can become an extreme injury hazard.
In addition to hazardous driving or walking conditions, branches or even whole trees may break from the weight of ice. Falling branches can block roads, tear down power and telephone lines and cause other damage. Even without overhead tree branches, the weight of the ice itself can snap power lines, and power poles as well (even the big steel frame electrical pylons have crumbled under the weight of ice before). This can leave people without power for as long as several days to even weeks. According to most meteorologists, just one quarter of an inch of ice accumulation can add about 500 pounds of weight per line span. Damage from ice storms is highly capable of shutting down entire metropolitan areas.
An example of an ice storm is the North American ice storm, which occurred from January 5-9, 1998. One of the worst ice storms in Canadian history, it caused massive power failures in several large cities in the eastern provinces. Whole trees snapped and electrical pylons were completely flattened under the weight of the accumulated ice.