United States

Doppler Radar Catches Car Going 130 MPH?

August 13, 2008; 2:30 PM

The NWS in Chicago posted a story yesterday that got picked up by Gizmodo, saying:

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!!!

Here are some radar images from the time and area in question (courtesy Plymouth State); you can download larger versions of the entire radar scope here.

CLICK TO LOOP 10:30 PM - 1:30 AM C STORM-RELATIVE VELOCITY | SEE ALSO: VELOCITY | REFLECTIVITY

First, I have heard of birds, bats, buildings, wind turbines, and mountains showing up on Doppler (NEXRAD) radar before, but this was the first time I had heard of cars. That doesn't mean, however, that it didn't happen. Clearly the radar is picking up *something* along the interstates though I am not convinced it is cars. What else could it be? Signage, light poles, the road itself... I'm not sure. Given that it's picking up something around Dawson Lake, I'd bet on trees or poles before cars.

If it is cars, why did it misjudge that one was going 130 mph? First of all, I can't be sure that's the number, although the NWS scale goes farther than Plymouth State; the color (on the Plymouth State Storm-Relative Velocity radar) represents 50 (purple) to 122 (max readout) knots (58-140 mph), although that max of 122 could have been anywhere on the radar. The radar shows all kinds of random, outlandish velocities for non-meteorological objects - this is not big news (in fact to detect a mesocyclone or tornado, multiple pixels must be used). Weather radars aren't built to detect objects the size of a car; if they really could, they certainly couldn't tell one from another (another question, why is all the traffic going towards the radar (pink/purple) and none going away (blue/green)?

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com