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    Re: Refraction
    From: George Huxtable
    Date: 2008 Jul 4, 11:29 +0100

    I agree with all Dave Walden has written about this, but have questions
    about his plot of errors in the various methods of predicting refraction.
    What were those plots of refraction differences actually differences FROM?
    You can only determine such differences if you have some measure which you
    know to be absolutely true. In the case of refraction, I know of  no such
    Refraction can be calculated from a multi-layer computer simulation, in
    which the different layers have different rates of change of temperature. No
    simulation is better than the data that is fed into it, and depends on the
    validity of the assumptions that it's based on. For refraction, that depends
    on the way in which temperature is presumed to change with altitude, and on
    humidity. Actually, humidity has remarkably little effect; it's mostly
    temperature. And various "standard" models of temperature variation have
    been adopted over the years, to represent some sort of average measured
    value, but there's nothing God-given about any of it.
    And the other big assumption is that the atmosphere is uniformly layered.
    You only need to look at the sky on a day like today, in Oxfordshire, to see
    what a dodgy assumption that must be, in practice. About half of the sky is
    deep blue, the other is made up of great stacks of cumulus cloud, with warm
    air being drawn up from below at the centre, and cold air returning around
    the margins. As the clouds pass, those downdraughts can be clearly felt at
    ground level; they are real! The notion of uniform layering also belies the
    passage of fronts, those wedges of air at differing temperature that drive
    much of our weather.
    Nevertheless, these are the assumptions that have been built into the
    refraction models, on the basis of "what better can you do?". Which is fair
    enough. Having made those assumptions, refraction can be computed as
    accurately as you wish, but those calculations are never better than the
    data that was fed in, and it's as well not to get too dazzled by their
    apparent precision. Differing assumptions make hardly any difference at high
    altitudes, but can be important near the horizon.
    So what Dave's plots show (I suggest) is not absolute error in any
    refraction formula, but just how well they conform with some "standard"
    model, unspecified.
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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