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    Re: Wind drift, was: DR thread from Nov-Dec '04
    From: Bill B
    Date: 2005 Jan 19, 23:07 -0500

    Trevor
    
    No argument, just confusion and a desire for clarification.
    
    1. If I understand it correctly, tides can be amplified by what I will term
    the "natural frequency" of a basin.
    
    2. The Annapolis Book of Seamanship states regarding seiches, "On tideless
    lakes,  strong wind-driven currents called seiches may develop.  The effect
    of wind-driven current is figured like that of a tidal current."
    
    3. If I understand your more complete explanation, that wind/current piles
    up water at the leeward end of the lake, and there are two probable outcomes
    once the fluid sets up (piles up?) on the leeward end of the body.
    a.  A subsurface current in the opposite direction attempts to "level out"
    the basin
     b.  The water remains "set up" (piled up?) at the leeward end of the body.
    
    4.  By your definition of seiche, when the wind decreases/stops, gravity
    once again prevails and the piled up water on the leeward side of the body
    rushes to the former windward end to establish equilibrium (level off),
    setting up a wave.
    
    5.  If the basin is sympathetic, this "sloshing" may be rather dramatic for
    a tideless body of water.
    
    I feel you have adequately dispelled the heartland folk lore concerning why
    a seiche occurs, particularly for a lake that is relatively shallow at its
    southern tip and would have thermoclines.
    
    Given I may have made some logical leaps above, but the definition of seiche
    remains a wee bit unresolved for me. It is either:
    
    A two line reference stating it is a wind driven current.
    
    A knowledgeable explanation stating it is akin to a standing wave, AKA
    sloshing in a basin after the wind dies.
    
    I apologize for the second degree.  A lawyer would say, "asked and
    answered."  I am attempting to "get it right."
    
    I hope my summary is close to accurate. Friends might claim decreasing my
    sphere of ignorance is thankless.  They are incorrect.  Thank you.
    
    Bill
    
    
    
    
    
    > Then Bill wrote:
    >
    >> Lastly, can we count on the wind direction and velocity being identical in
    >> both Japan and California?  From the texts I have read wind-induced current
    >> can be a relatively local event, e.g. Chesapeake Bay or Great Lakes.  On
    >> Lake Michigan we get these wonders called seiches.  High pressure on one end
    >> of the lake, low pressure on the other, plus the wind in the right direction
    >> and bam, a foot or more "tide" over a matter of hours.  The first time we
    >> experienced it was when tied up along the wall in Chicago.  Blew my mind.
    >> Perhaps Frank can offer a better explanation of the seiche from the Chicago
    >> vantage point.
    >
    > That is "set-up", not "seiche". A seiche occurs when the wind which set
    > the water up dies away and the water sloshes back towards its normal,
    > level surface. It will always slosh to far and set up a standing wave,
    > with a period that depends on the length and depth of the lake. Careful
    > observation at either end of the lake should show a few "high tides" and
    > "low tides" over minutes (in a small lake) or many hours (in a big one),
    > though to really detect a seiche you need a recording tide gauge since
    > the amplitude of the wave rapidly drops away as energy is lost to
    > friction between the water and the lake bottom. ("Seiche", by the way,
    > is said to have been a local term for this phenomenon on the shores of
    > Lake Geneva, before the word was taken up by limnologists and then
    > oceanographers.)
    >
    > And in any water body as tiny as Lake Michigan, pressure will not cause
    > a set-up. With a depression centred over Chicago, the air pressure at
    > Mackinaw City will be hardly any higher. Thus, the atmosphere will press
    > just about equally on all parts of the Lake's surface. It is wind alone
    > that sets up the surface of lakes. The North Atlantic is different. It
    > is big enough to have an intense depression over Newfoundland and a
    > well-developed high over the Azores. Then the sea's surface can be
    > forced upwards under the depression by the differential pressure across
    > the surface of the basin.
    
    
    

       
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