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    Re: Measuring (and Calculating) Dip - sighting technique
    From: Brad Morris
    Date: 2013 Mar 20, 23:49 -0400

    Hi Bill

    Swell waves are wind waves, just generated elsewhere.  They appear as monotonic frequencies because the various frequencies get sorted out with distance.  The velocity of a wave (celerity) is related to the frequency.  In general, the longer the period, the faster the wave.  So a jumble of frequencies far away travel towards us a different speeds, so the longer period swell gets here first.  The longer the distance, the finer the sorting. 

    If you saw the ocean swing that 30 arc seconds in less than 16 seconds of time, then its most likely a wave.  If its moving up and down all the time, its likely local wind waves.  If its just now and then, its likely swell.  Note: you can get swell that moves up and down all the time, its just not as common.  Surfers in the States call that corduroy and that's the good stuff!

    Its just something to look for next time.


    On Mar 20, 2013 11:31 PM, "Bill Morris" <engineer@clear.net.nz> wrote:

    Hi Bruce

    As far as I remember, most of the time the horizon, though difficult to define exactly because of its irregularity, remained in the same place and then suddenly rose or fell (and not because I leant on the tripod!). When I have time, I will see if there is indeed a periodic rise or fall, but I am inclined to think that if there had been I would have adopted some sort of averaging process. At the time, we were in the middle of a large area of high pressure and there was no wind to make my anemometer rotate, so practically no waves to superimpose on the swell.

    Bill Morris
    New Zealand
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