May 30, 2009

Clouds on the coast and chaff

Last night a weak disturbance passed by us to the north. As a result the pressure trough over western Washington shifted eastward and the onshore pressure difference increased to near 2 millibars. The result...a weak influx of marine air that brought clouds to the coast, the Strait, and SW Washington. Thus, today will be a few degrees cooler than yesterday, although the clouds will burn back this morning.

A few weeks ago I talked about birds in the weather radar. Yesterday there was something else. Take a look at the weather radar from late yesterday afternoon...looks like showers near the Olympics and over parts of SW Washington. But it was clear. This is good example of chaff from military aircraft. Once in a while the miltary has some kind of exercise off the Oregon coast and release chaff..which blows inland during the next few hours. The radar really picks this up...making it appear that heavy or extensive showers are approaching the region. Why they release this chaff? I can't answer that.

For those of you that are interested, I will be doing 1.5hr show and call in on KCTS9 this Tuesday at 7 PM. They will offering my book and a DVD of the program as part of their pledge drive.

And check out the op-ed section of the Seattle Times tomorrow (Sunday)...there is a piece on the disastrous math decision of Seattle Public Schools. And Issaquah and some other local school district are considering the Discovering Math series. Hope they will think it though first.


  1. Cliff Said... "Why they(military) release this chaff? I can't answer that."

    Let me guess, the answer is perhaps top secret? lol.

  2. Cliff:

    Any idea of the composition of chaff? Metallic? Would we recognize it blowing down the street?

    BTW, I have just donated $100 to the Friends of Atmospheric Sciences fund to help keep the web page up...I hope we can keep it going. It saves a lot of hunting, and the enhanced satellite with fronts is very helpful to weather foamers like me.

  3. This comment has been removed by the author.

  4. Chaff is used to disrupt radar so the radar operator can't see the approaching aircraft. I think it's metal-coated mylar, but I believe in WWII it was aluminum strips.

  5. Cliff -

    My son was on the Victoria Clipper that was cancelled leaving Victoria around 6PM yesterday (Friday). They said there was a "storm" with 6 foot waves. Didn't they mean a "Sound Breeze", and how unusual is it that it would be strong enough to ground the Clipper?


  6. Atilla has it mostly right though US chaff seems to be aluminum-coated silica glass fiber rather than mylar (but I've heard of mylar and other plastics being used). Until the 1980s it was just aluminum fibers.

    Radar chaff is used to block and confuse anti-aircraft system radar (and also by ships to confuse anti-ship missiles).

    The wikipedia article is OK but a bit WW2 centric. article does have pictures of the US Navy RR-129 and RR-124 chaff countermeasures and containers which may be the ones in use.

    As they say: "Note how the RR-129 chaff, bottom, is different lengths, and the RR-124, top, is all the same length. The RR-124 is designed to prevent interference with civil ATC radar systems."

    As each chaff strip acts like a half-wave (dipole) antenna the length of the chaff is specific to the frequency of the radar. So for example they can use chaff in exercises that doesn't jam air traffic control radar.

    I think the real problem with weather radar is it is designed to exploit very small returns from raindrops whose radar cross-section is much much smaller than an aircraft (or even a bird). So even detuned chaff will probably show up.

    Note also that today's radar would be running in clear air mode to bump up the sensitivity on a dry day. But with all the chaff in the air they've left it in "precip" mode.

    The other value (if you know the altitude of the chaff) is you could use it to track wind patterns. Perhaps there is an undergraduate thesis project in that? :-)

    BTW, NOAA and the GAO have considered this in the past

    CHAFF AND THE WSR-88D PRECIPITATION PROCESSING SYSTEM Protection: DOD Management Issues Related to Chaff (Letter
    Report, 09/22/98, GAO/NSIAD-98-219) Detected by NWS Louisville Radar this last one you can see the curtain chaff lines quite clearly in the radar.

  7. Dear Cliff, I read your interview in Real Change, and I'd like to volunteer as a TA to save your Department some moola. I'm your neighbor, and would like to chat. My email: -- hope to hear from you. And I have donated to a number of organizations lately and will check with my "boss" (wife) about a $50 donation.

  8. Can you show me where the OSPI recommended against the Discovering Math series? I live in Issaquah, and this issue is before us right now. has a stale OSPI link and a link to a recommendation from an outside agency. Thanks.

  9. Releasing chaff gives pilots the chance to actually practice evasive maneuvers and also gives radar operators a chance to see the effects and work around it. Equipment can be tested. I would surmise that a military exercise was on-going and that was one of the training events.

  10. I wonder if the chaff is part of the testing and evaluation flights of the Boeing 737 AEW&C. ( They fly on various flights over land and the Pacific several times a week. I believe there are 3 if not 4 undergoing flight testing right now out of Boeing Field. Here's what a flight track for one of the flights looks like (from May 29):

    Currently, Turkey and the Australian Air Force are evaluating them. 2 weeks ago, several Dornier Alpha Jets were participating in some of the excercises.

  11. Hi Cliff. I was interested in the point you made about the military releasing chaff and wondered what it was. I found this article that gave a nice explanation. Thanks!

  12. Hey Cliff,

    Although it's far from home, the weather event that apparently caused the Air France flight to go down seems very strange--100 MPH updrafts, strongly electrical, etc. I'd be interested in any comments or analysis you might provide.



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