Brutal chill to loosen its grip on Chicago area

Blasts of arctic air as extreme as this week's depart with all the speed of molasses in
January. The sheer density of these brutally cold air masses dictates that the
comparatively low density "mild" air replacing them must initially ride up and over the chilly air
before being mixed to the surface by gusty winds like those which greet Chicagoans
Saturday morning.
Friday morning's low of 18 degrees below zero at O'Hare was the city's coldest in 13
years. Lows of minus 18 or lower have occurred here only 30 times over the past 139
years.
Suburban lows were truly eye-catching! Sugar Grove bottomed out at minus 31. On only two
other days has a Rockford low been colder. Records there date back to 1893. A reading of
36 below in west suburban Ogle County's Rochelle was chilly enough to equal the state's
all-time low -- but the reading was on a thermometer whose calibration isn't monitored
closely enough to meet the criteria for inclusion as an official record.
Except for 20 minutes just after 11:30 a.m. Friday when the official temperature at
O'Hare crept above zero to 1 degree, the span of zero or lower temperatures reached 45
consecutive hours late Friday evening.
--Tom Skilling, Chief Meteorologist, WGN-TV/Chicago Tribune