The answer to the "what is that?!" case from October 6, 2004
So what was it?
The answer is chaff. Chaff is millions of tiny bits of plastic or metal dropped in the air within the range of a RADAR. Chaff was originally used in World War II as a RADAR confusion technique--the beam would either bounce off the chaff and be unable to detect objects (airplanes, missles) farther away, or the chaff itself would be detected as a potential "something out there" and, either way, the RADAR operators would be confused.
Nowadays Chaff is released periodically near air force bases as a training tool.
Now more about anomalous propagation.
Anomalous propagation (AP) is a false RADAR return caused by a temperature inversion. It usually happens early in the morning. Cold, dense air near the ground is blanketed by warm, less dense air higher up. This density change causes the RADAR beam to bend and, if the gradient is large enough, to bounce back toward the ground.
The RADAR beam then bounces off the ground and back up into the sky, where it once again bounces off the temperature inversion and comes back to the RADAR.
The easiest ways to determine the presence of AP are:
- to note whether or not there are clouds present
- to see, using an animation, whether or not the echoes are moving in any organised fashion (if they are not, it is likely AP)
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Last update to this page: January 12, 2005