Chaff on Radar?

Introduction
On the afternoon on December 10, 2018, an area of what initially appeared to be precipitation (Figure 1) was detected northwest of Evansville, Indiana by the National Weather Service (NWS) Doppler Weather Radar station at Paducah, Kentucky (KPAH). When first detected, the blob was 100 nm northeast of KPAH at an elevation of approximately 12,000 feet. A check of surface weather stations near the blob and visible satellite imagery (Figure 2) indicated that no precipitation was occurring and that skies were clear in the area.

Figure 1: Radar image from NWS station KPAH at 2:49 pm CDT (2049Z) on December 10, 2018.

Figure 2: Visible satellite image from December 10, 2018 at 2:37 pm.

The blob was visible by the KPAH radar for nearly ten hours (from 2:49 pm on December 10 to 12:49 am on December 11, 2018 CDT). During this time, it traveled approximately 120 nautical miles (average of 12 knots) in a southeasterly direction from southeastern Illinois to western Kentucky (see animation below).

If it wasn’t precipitation, what was the radar station detecting? It’s not uncommon for migrating birds to appear on radar, but at a height of 7,000 to 10,000 feet, the blob was too high to be birds. Dust and insect swarms sometimes appear on radar, but the elevation was too high for these also. Not to mention it is the wrong time of year for insect swarms. While impossible to confirm, the evidence suggests the blob was chaff from a military aircraft.

Military Chaff
Traditional military aircraft lack radar-avoiding technology and must rely upon defensive countermeasures. Primary among these is chaff, a fibrous substance composed of glass fibers and aluminum packed into small bundles. The pilot launches the bundle into the aircraft’s slipstream, where it opens and releases nearly five million small fibers (1 to 2cm in length) into the atmosphere forming a highly-reflective masking “cloud.”

The dispersal of the chaff is intended to confuse a missile’s radar guidance system by creating a more attractive target than the plane that released it. Since the rapid dispersal of the chaff cloud would be counter-productive, the material is designed to linger in the atmosphere for an extended period. Falling at a rate of approximately ten meters per minute, chaff dispersed at an altitude of ten thousand feet doesn’t reach the ground for nearly ten hours. The fibers are also highly resistant to breaking apart during descent helping to prolong their protective mission.

The Evidence For Chaff
A zoomed-in reflectivity image (Figure 3) at 2:49 pm CDT on December 01, 2018 shows the blob shortly after it initially appeared on radar. The high values of dBZ (55 dBZ) associated with the blob are consistent with chaff when the radar station is in clear air mode. (Clear air mode is slower than the standard precipitation mode and increases the station’s ability to detect small particles such as dust and mist.)

Correlation coefficient (CC) is a radar product that measures the uniformity of the target’s shape. Targets will high CC values (close to 100%) are round, such as perfectly formed raindrops or snowflakes. The correlation coefficient image at 2:49 pm CDT on December 10, 2018 (Figure 4) shows the blob had values less than 60% which typically suggests chaff or other non-weather related targets.

Figure 3: (Left) Base reflectivity at 2:49 pm CDT on December 10, 2018. Figure 4: (Right) Base correlation coefficient at 2:49 pm CDT on December 10, 2018.

Based on data associated with the KPAH radar beam, the presumed cloud of chaff extended from an altitude of approximately 7,000 to 10,000 feet. This height roughly correlates to 700 mb, one of the standard atmospheric levels analyzed by meteorologists. The impact of friction is virtually absent at 700 mb, therefore the wind flows nearly parallel to the contours of constant height (the dark lines on Figure 5).

Figure 5: Analysis of 700 mb heights and wind speed at 6:00 pm on December 10, 2018.

A review of the 700 mb analysis valid at 7:00 pm on December 10, 2018 (Figure 5) indicates the wind above southeastern Illinois and western Kentucky was likely northwesterly to northerly at approximately 15 to 20 knots.

The observed sounding at Nashville, TN at 7:00 pm on December 10, 2018 (Figure 6) confirms that wind speeds at the height of the presumed chaff cloud were rather light at approximately 15 knots.

Figure 6: Observed sounding at Nashville, TN at 7:00 pm on December 10, 2018.

Conclusion
The absence of precipitation, the longevity of the cloud’s appearance on radar, and its movement consistent with the speed and direction of the wind at 700 mb, supports the conclusion that the cloud was military chaff.