Freezing rain occurs when rain becomes supercooled and freezes upon impact with any cold surface. Ice storms – defined as long periods of freezing rain – can be very dangerous, destructive, and disruptive events. They cause traffic accidents along with slip and fall accidents to pedestrians. Heavy ice loadings on trees and shrubs can cause limbs to break off or entire trees to be ruined. People have been injured or killed by falling ice, tree limbs, or other debris. Ice loadings on power lines and poles can cause them to fail. Falling tree branches can take out power lines as well. Interruptions in electrical power may take days to fix.
Water Survey publications documenting recent ice storms:
How Freezing Rain Forms
This diagram showing what happens in the atmosphere during freezing rain. In the top-most blue layer where temperatures are below freezing, snow flakes have formed. In the slightly warmer middle layer, temperatures are above freezing, causing the snow flakes to melt into rain drops. In the shallow but very cold bottom layer, temperatures are once again below freezing. This time the rain drops are cooled to slightly below freezing. On impact with any object at or below freezing like a tree, car, or power line, they instantly freeze.
Here is the map of the frequency of freezing rain days in Illinois. Central Illinois is more likely to see freezing rain days than either southern or northern Illinois. However, damaging ice storms do occur throughout the state.