When warm and cold air meet, the resulting weather phenomena can be an ice storm. Ice storms can be particularly dangerous because the freezing rain makes an icy glaze on roads and other outdoor surfaces. Very heavy amounts of ice brutally damage trees and weigh down power lines. In fact, the weight of the ice can be increased by 30 times! The weight of the ice is made worse if the wind speed is high. The areas with the highest frequency of freezing rain occasions are the Midwest and Northeast.
The accumulations of ice are classified according to the impact they have. They are classified as nuisance, disruptive, and crippling. During a nuisance ice storm event, less than ¼ of an inch of ice accumulates. Travel, especially on roads with bridges, can be difficult. Disruptive ice storms are typically an ice accumulation of ¼ of an inch to ½ of an inch. During these storms, there can be power outages and sagging or breaking tree limbs. The most dangerous ice storm is the crippling variety. This category of storm causes widespread ice accumulations of over ½ inch, and may reach accumulations of one inch. Due to severe damage to trees and power lines, power may be interrupted for many days. This is especially dangerous for those who do not have back-up power solutions and go for prolonged periods with no heat.
Sources:
http://mrcc.isws.illinois.edu/living_wx/icestorms/
http://www.wisegeek.com/what-is-an-ice-storm.htm#didyouknowout
http://www.weather.com/news/news/ice-storm-damage-impacts-20121123