In the wee hours of the night, an answer to the mystery emerged. WEHT meteorologist Wayne Hart said a pilot told him that the air control tower at Evansville Regional Airport reported a chaff release from a military C-130 northwest of the city.

Read the Latest:Radar mystery may have been solved (again) | Webb

The National Weather Service then added another detail that explained the unusually slow movement of the chaff:

"Upper level winds can be much stronger this time of year. However, winds aloft late Monday averaged 10 to 15 mph from the north northwest. That would explain the rather slow movement and slow dispersion of the chaff debris."

PREVIOUSLY:

EVANSVILLE -- What probably should've been an uneventful night at the National Weather Service office in Paducah, Kentucky has instead turned into a night spent observing a mystery unfold.

Since a little before 3 p.m. CST Monday, the station's crew has been tracking something on its radar. And we say "something" because the meteorologists at the station aren't sure what caused a radar return to begin showing up over Southern Illinois, then drifting over Southern Indiana and into Western Kentucky.

It is a clear night with very little wind and temperatures in the 20s in the Tri-State, so that rules out a shower or a thunderstorm. And here's the thing: The radar return was of the strength of what you'd seen in a strong storm.

As of 9:20 p.m., the radar return had drifted well into Western Kentucky and elongated. The National Weather Service at Paducah, Kentucky issued another message on Twitter seeking reports from people in Madisonville and Owensboro.

The best guess the folks at the weather service have is that it's military chaff that originated in Richland County, Illinois around 2:45 p.m. Monday. For more than six hours, it has floated to the south and east.

Chaff was developed during World War II, and the British and Germans used it to create false returns on radar. The website globalsecurity.org describes it this way:

"Chaff consists of small fibers that reflect radar signals and, when dispensed in large quantities from aircraft, form a cloud that temporarily hides the aircraft from radar detection. The two major types of military chaff in use are aluminum foil and aluminum-coated glass fibers. The aluminum foil-type is no longer manufactured, although it may still be in use."

There are two military installations within a quick flight of the area where the chaff originated: Scott Air Force Base in Shiloh, Illinois (just east of St. Louis) and the Naval Surface Warfare Center Crane Division in Southern Indiana, about 75 miles away.

A third military base in the region is at Fort Campbell, on the Kentucky-Tennessee state line.

Reached by phone late Monday evening, National Weather Service meteorologist Greg Meffert said the office had been tracking the return at altitudes up to 10,000 feet, but returns were also showing up at ground level.

"It might be chaff released from an aircraft, but we've never seen it quite this hot," said Meffert.

(I asked him what he meant by "hot" and Meffert said it was a description of how strong the material was showing up on radar.)

The other thing is that in the past, Meffert said, they got reports from people finding pieces of the chaff in their yards. The early darkness likely hampered that today -- the sunset at Evansville was at 4:31 p.m.

Meffert said he previously worked for the National Weather Service office in Little Rock, Arkansas, which dealt with chaff on occasion from the local U.S. Air Force base. But around the Evansville area? It's out of the ordinary.

"We'll just keep listening like everyone else" to find out what it is, Meffert said.