How often does freezing rain occur in the county? And why can it be so destructive?
A University of Illinois study of freezing rain frequency produced a number of years ago by climatologists Drs. Stanley Changnon and Thomas Karl indicated freezing rain between 1948 and 2000 occurs here an average of 3 to 4 times a year and that freezing rain events most often span a period of time 6 to 9 hours in length here in northern Illinois. The study is an older one–it was published in 2003–but if serves as a guide to just how often we deal with such an event.
The American Meteorological Society (AMS) defines freezing rain as: “Rain that falls in liquid form but freezes upon impact and forms a coating of ice on the ground and or exposed objects.”
The impact of a coating of ice can be significant. We sure saw that Wednesday and Wednesday night. More than 110,000 lost power across Chicago’s northwest and north suburbs. Trees came down as did branches and power lines. It’s been estimated that a layer of ice on a tree can increase the weight on branches by as much as 30 times and that a half inch layer of ice can add 500 lbs to a single pole to pole span of power lines. Add to that the force of wind and the can become more than a tree or power line can support.
Warm, moist air above a cold, sub-freezing atmospheric layer produces freezing rain. At one point Wednesday, with freezing rain pouring down into a 31-degree air mass north and west of Chicago, the temperature a mile aloft was 46-deg. Had you been able to brave the wind and rain and taken a balloon ride, you could have ascended into air which was 15-deg warmer roughly 4,000 to 5,000 ft. above the average terrain here in northern Illinois.