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    Re: dip, dip short, distance off with buildings, etc.
    From: Frank Reed CT
    Date: 2006 Jan 15, 20:20 EST

    Bill, regarding the refraction from temperature  inversion, you wrote:
    "I should not be surprised given the range of values  Frank suggested, and the
    heat produced by the Chicago metro area and  steel-works, but I was."
    It's not the city or the industrial areas. It's  just normal weather.
    Temperature inversions are quite common. They have been  associated (in local weather
    reports?) with cities and pollution rather by  accident. When a lasting
    temperature inversion exists, convection is capped.  This causes a rapid build-up
    in ozone and smog. But that's the effect rather  than the cause. Inversions can
    occur anywhere including over the open ocean,  especially near major ocean
    currents like the Gulf Stream.
    "Which brings  us back to coasts and thermal inversions and other
    abnormalities.  Not a  rhetorical question--how should the warning label or
    guidelines read?   Can it possibly be quantified?"
    It *can* be quantified, but it's probably  not worth it. I doubt very many
    people use Table 15 or worry about range of  visibility or dip short. As you've
    seen, even on this list of navigation  enthusiasts, it's mostly a yawn.
    "Also, for non coastal situations, can  modifications to dip etc be adjusted
    by some factors?"
    Occasionally  over the past 150 years (or more), people have suggested
    measuring water  temperature and air temperature and modifying dip values under the
    assumption  that the air temperature at the water surface will equal the water
    temperature.  It's not a practical solution since the assumption isn't true,
    the temperature  measurements have to be accurate, and the improvement in
    accuracy is not all  that great.
    42.0N 87.7W, or 41.4N  72.1W.

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