You may have noticed this on the Chicago-Romeoville WSR-88D Doppler radar Friday afternoon, February 10, 2012, between 230-400 PM CST. An unusual "halo" in radar imagery developed in the vicinity of O'Hare International Airport...roughly in the middle of the image loop below.
The winds at KORD up through about 200 PM CST were light and from the northwest (320 degrees) at less than 10 knots. The cold front came through and winds turned to the north-northeast at 20 mph with gusts to around 30 mph. Air traffic controllers probably had to reconfigure the runways and put some aircraft in a holding pattern. A circular pattern became evident for a time in the radar display to the north-northwest of KORD. We've seen this before where the exhaust particles from the aircraft feed/seed the clouds and coalescence accelerates, resulting in the development and fallout of ice crystals. Doppler radar can detect these ice crystals along with precipitation.
Below is a 4-panel display submitted by Milwaukee Area Skywarn Association. Note how the pattern dissapears with height - or with higher elevation slices in the radar imagery.
Kapela/SD/Wimberley, WFO Milwaukee/Sullivan