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    Re: Refraction
    From: George Huxtable
    Date: 2005 Aug 7, 01:28 +0100

    At 18:17 06/08/2005, Robert Eno wrote:
    >George wrote:
    >>Refraction works in the opposite direction to geometrical dip, reducing it
    >>by about 8% when seen from a boat. It can be roughly predicted, for
    >>standardised atmospheric conditions. However, the lower few feet of the
    >>air, being strongly influenced by the temperature of the sea surface just
    >>below it, can suffer from unpredictable temperature gradients, which can
    >>upset the predicted dip by several arc-minutes, on a bad day. This can be
    >>the biggest source of error in marine sextant observations.
    >Robert responds:
    >Again, this gets back to a handful of questions I earlier asked but to which
    >no one replied, so I will ask you directly George:
    >In your experience, are there certain weather conditions and/or other
    >observable factors during which these unpredictable errors are likely to
    Response from George-
    Well, Robert's question is phrased rather differently from his previous
    version. Now it's clear he is asking how to avoid anomalous dip. And asks
    me to comment "in your experience".
    Let's be straight. I'm no ocean navigator. I use a plastic sextant on a
    28-foot boat. So my own observations are so rough-and-ready that I would
    probably not notice anomalous dip if I met it.
    Hanry Halboth, however, has as much seagoing experience as the rest of us
    put together. What he says has some authority behind it. And his account of
    the Red Sea as a haunt of anomalous dip is backed up by other accounts I
    have read about that area. Presumably, the reason is the proximity of a
    nearby sandy desert, over which the air gets very hot, and blows across the
    sea, giving rise to a large temperature gradient. I think coral reefs occur
    there too, close to the navigation channels, which could heat up under a
    strong Sun.
    I have read of similar effects off the coasts of Southern California,
    presumably from the same cause, of offshore winds blowing hot air across a
    cooler ocean.
    There have been oceanographic cruises which included some occasional dip
    measurements, such as the good ship "Venus", of France, and the American
    research yacht "Carnegie".
    In general, anomalous dip seems to be mostly reported near coasts, rather
    than in mid-ocean. But you have to consider that all this work was done in
    the days before GPS. Positional errors would show up only if you knew your
    position precisely by other means such as by bearings of landmarks. Though
    you could deduce unexpected dip values from discrepancies between a round
    of star altitudes, at different azimuths. Or with a special-purpose
    dipmeter. I have made myself a dipmeter from a plastic sextant but haven't
    tried it out very often and have never seen a dip differing by more than an
    arc-minute from the expected value, which probably corresponds to the
    scatter inherent in the measurement.
    My best answer to Robert's enquiry is that he is likely to see unexpected
    values of dip whenever there are mirage-type effects near the horizon; when
    distant ships appear to have expanded upperworks or float above the
    horizon. What weather conditions give rise to such effects, I don't know.
    But if there are no nearby vessels to show up such effects, just a clear
    horizon, how can you tell?
    Marcel Tschudin wrote-
    >I can not answer for George. But since you did ask before generally, here my
    >thoughts about this.
    >I think you might have more success to ask this question some meteorologist.
    >As already mentioned temperature inversions are very much disturbing. A
    >meteorologist  may tell you all the signs of this like e.g. type of clouds
    >etc. May be you can start here http://www.net-weather.co.uk/forum/index.php?
    I think Marcel's advice is wrong here. Anomalous dip is generated in the
    very lowest level of the atmosphere, below the eye-level of the observer,
    and within a few feet of the sea surface. It's the temperature gradient in
    that thin layer that creates the problem. I doubt that a meteorologist will
    contribute much information on that matter. Their interest is in effects
    that are much higher up, where the inversions and clouds occur, as Marcel
    Some mariners of old would try to estimate dip effects by measuring the
    temperature difference between air at eye-level and water in a bucket drawn
    from the sea. But the resulting temperature gradient in the air was
    affected by other variables, such as wind strength and turbulence, so there
    was never an agreed consensus on how to determine the dip from the
    temperature difference. Not that I'm aware of, anyway.
    Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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