They say that contrails are harmless. But then I found out about "Chaff". Chaff is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminum, metallized glass fiber or plastic, which either appears as a cluster of secondary targets on radar screens or swamps the screen with multiple returns. Chaff is intended to act as decoy for radar and/or increase ground clutter at the same time. However, modern pulse-doppler radar can recognize such decoys, especially in the lookdown/shootdown mode. This is particularly true because simple decoys, in contrast to true targets, do not exhibit a corresponding doppler shift in the radar band. I asked a local weather reporter from KTVN-TV and this is what he sent me. "I see chaff on the radar all the time. The bombing ranges out east of Fallon are always pumping it out from the jets there, and it can be a real challenge differentiating it from actual weather occurring at times."...