National Weather Service Weather Forecast Office

Atmospheric conditions were right early Tuesday morning across north central Illinois for the NWS doppler radar in Romeoville to be able to pick up on traffic on Interstate 55 and 57.  The doppler radar typically measures the motions of rain and water droplets toward and away from the radar, just like how a police officer's radar can pick up on a car's speed.  But when a low level inversion (a layer of warmer air up above the surface) develops, it can cause the beam to be re-directed toward the ground and pick up on dense objects like buildings or traffic on highways.  In this instance, the radar was able to pick up on traffic across Livingston County along I-55 as well as Iroquois County along I-57.  The specs of higher returns are where the beam is being deflected back to the radar off of traffic.  It may be hard to make out, but the green and blue specs along I-57 near Danforth show some in-bound velocities of 115 knots (~130mph)...hopefully this was just noise and not someone driving too fast!!!  Click here for more information on how doppler radar works.