Those of us that pay close attention to the radar, may have noticed something out of the ordinary on the radar Tuesday in Alabama. An area of what appeared to be rain or thunderstorms developed south of Demoplis and moved north across the state. However, what appeared to be rain on the radar, may not have been.

Radar picks up on all types of things from rain, sleet, snow, and hail, to bugs, birds, and debris. Sometimes the radar returns other types of targets, like buildings, traffic, and mountains. We have a set of radar tools that allow us to interpret the types of things that the radar is seeing.

One of those tools is called Correlation Coefficient (CC) or Debris Tracker. CC is able to determine the size and shape of a target and differentiate between raindrops, and things that are not. Often we use this tool during severe weather to determine if a tornado may be lofting debris. When a tornado lofts debris into the atmosphere it appears as what we call a CC dropout, or appears blue in color. This helps us confirm that a tornado is likely on the ground and causing damage.

In other cases, CC can be used to determine whether or not something is rain or not. Take a look at the radar image over Tuscaloosa from Tuesday that appears to be producing rain. There was no rain reported at that time in Tuscaloosa. When we examine CC, we can see that the target that the radar is seeing is likely something other than rain. So what could it be?

This target that the radar saw traveled from the southern part of the state all the way over to the northern part of the state. That was perpendicular to the movement of actual rain showers that were moving in from the southwest and moving northeast. This is because the target that the radar was picking up on was a little closer to the surface. This means that it was being blown by the winds closer to the surface. These winds at the time would have been out of the southeast, pushing the target against the movement of actual rain showers. This was a big clue as to what the target may have been.

So what was likely the culprit of this strange radar blob? Chaff, or to be more specific military chaff that is used to confuse radar-sensing missiles. Chaff is very thin strands of aluminum metalized glass fiber or plastic that are dropped from planes in order to confuse radars. Unfortunately, weather radar is also susceptible to the tricks of chaff and often appears as rain on the radar.