An atmospheric phenomenon this morning allowed the Doppler weather radar in the Chicago area to pick up cars driving down a pair of interstate highways, including one vehicle that appears to be doing 130 mph. Weather radar isn't that dissimilar from what cops use to bust speeders, as both project beams to gauge the speed of something moving away from a point. Typically, the radar is measuring the density of droplets of water in clouds, but a layer of warm air in the atmosphere deflected the weather radar's beams towards the road. So how did they know how fast the car was going?
The weather service office in Chicago shared two images that clearly show clumps of traffic along I-55 and I-57 in North Central Illinois. The first image is the typical green-to-red scale radar image that measures the density of rain in a cloud, which is how the weather service knows how large a storm is. The second measures the direction of those rain drops (either towards the radar site or away from it) and the speed they're traveling. This is how they often detect tornadoes.
Using the scale above, you can see that a car near Danforth, Illinois is dark blue, which corresponds to 115 knots or 130 mph. They appear to be heading away from Chicago which means this is noise in the radar, someone left something important at home or it's the Blues Brothers. It's probably a good thing the weather service can't write tickets. [National Weather Service - Romeoville, IL]