Freezing rain threat hits Chicago this week: Here's what you need to know

The Chicago area will likely encounter the threat of freezing rain at least once and possibly twice this week. It’s worthwhile to explain what that is and how it forms.  

What is freezing rain?

Dig deeper:

First, almost all precipitation any day of the year begins as snow high up in clouds. When temperatures are warm enough, that snow melts into raindrops.  If it’s below freezing all the way (or almost) to the ground, that precipitation falls as snow. Every once in a while, the temperature profile of the atmosphere is a bit wonky. Such is the case with freezing rain.  

Snow develops up in the clouds and starts to fall. On the way down, it melts as temperatures aloft rise above freezing.  Here’s where the curveball comes in. Close to the ground, a layer of sub-freezing air causes that rain to freeze on contact with objects like traffic signs, power lines, trees and pavement.  Freezing rain turns into a glaze of ice.  Contrast that with sleet.  Sleet forms when raindrops encounter a thicker layer of colder air before reaching the ground.  This freezes the raindrops into tiny pellets. Sleet is crunchy on the ground compared with the smooth glaze of freezing rain. 

Rarity of Chicago ice storms

Local perspective:

True "ice storms" are rare in Chicagoland.

They are much more common farther south with a bias for central Illinois and Indiana over the northern or southern parts of each state. However, it doesn’t take much ice to cause significant travel problems. Even a trace of freezing rain is capable of making untreated surfaces into skating rinks. That includes sidewalks and driveways. 

Generally speaking, to create downed limbs and power outages, at least ¼" of ice accretion is needed.  Those events don’t happen here often as snow is a much more common occurrence. 

The Source: The information in this story came from FOX 32's Mike Caplan.

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