Posted May 7, 2007
I have to credit Greg Fishel with the title of this post. I was commenting on how I'd been watching an apparent ran shower run inland and southwest on the west side of the powerful low off the coast this afternoon, and thinking that this rogue shower was ruining my forecasts for sunny skies and strong northwest winds across the eastern coastal plain, but then took a look at a satellite image to get a sense of the storm's structure only to find that it was indeed "severe clear" across that area. That led to some more detailed investigation of radar imagery from different sites and using varying elevation angles, leading to the conclusion that I was looking at a chaff release from military aircraft operating over northeastern NC. Chaff is composed of thin metallic strips designed to either confuse enemy radar and mask the position of aircraft, or in some cases to test or calibrate radar systems. Because chaff is designed to be highly reflective to radar signals, it can produce echoes of comparable intesity to those produced by precipitation, and one has to be careful to double check an echo against other data sources to be sure the echo actually corresponds to a natural phenomena. Since I was finished with my forecasting duties for the day, I had not done so for a while and hence found myself fooled for a bit by the chaff imagery.
There are some other hints that chaff is something other than rain, and those eventually helped out in this case as well. Often, chaff is released along long, thin lines by fairly high speed aircraft, and takes on an elongated appearance that is characteristic. I captured a few images of the chaff in this case that will illustrate what was going on through the early afternoon, and you can step through the images above, with a short caption explaining each. These images were captured using a handy web page for archived radar imagery that is operated by Plymounth State University. If you're interested in checking out that site, see