If you've misspent any part of your life watching TV you've undoubtedly seen meteorologists pointing at radar--often Doppler radar. Radar is short for radio detection and ranging. A blast of radio energy is sent out to bounce off the hydrometeors we mere mortals usually call raindrops. Measure the time it takes the radar signal to make the roundtrip, divide by the speed of light, add the processing time in the radar's electronics and you've got the distance to your rain. Factor in the direction the radar antenna is pointed and you know exactly where the rain is... well, not exactly. More on that in a second.

Doppler radar (capital "D" because it's named after Christian Doppler) is an added function of the radar that measures the speed those hydrometeors are moving by seeing how the frequency of the returned radio signal has shifted. If it pitches up, the drops are moving toward the radar, pitched down and they're moving away. Stand near a highway listening to cars drive by and you'll experience a similar Doppler shift. The pitch of a car's sound shifts down as it drives past you and away.

Radar is pretty cool when it works. It doesn't always. Sometimes the atmosphere plays tricks with the radar beam, bending it so it bounces in odd and often unpredictable ways. This often happens when there's an atmospheric inversion--warmer air over cold. And inversions often happen in the summer. That was the set-up for what happened in Central Illinois early this morning with the Weather Service NEXRAD Doppler radar in Romeoville, IL.

From NWS Chicago: "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!!!"

Will knowing this radar minutiae get you out of your next speeding ticket? Dream on.