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    Re: Table A4 + elevation?
    From: Trevor Kenchington
    Date: 2003 May 2, 17:36 -0300

    George Huxtable wrote, in his (as usual) magisterial comment:
    > Nautical Almanac table A4.
    > Doug says he has used this table to correct for non-standard refractions,
    > but I am not sure how he has done so, at his height which (I think I
    > remember) he quoted as 2100 ft.
    > Vaguely remembering (or maybe misremembering?) the density (relative to
    > water) of air as 1/830 and of Mercury as 13.6, it seems to me that at
    > Doug's height of 2100 ft. the atmospheric pressure will be reduced by 7.5%
    > below what it is at ground level. (Somebody please check!) If that's
    > correct, then the local pressure will reduce to 935 millibars, compared
    > with its "standard" sea-level value of 1010. That takes it well outside the
    > realms of the diagram that goes with table A4, which goes no further down
    > than 970.
    Is Table A4 correcting primarily for refraction in the layers of air
    immediately above the observer or primarily for refraction higher up? If
    the former, then I fully understand George's point: The lower pressure
    at altitude means less refraction and Table A4 doesn't extend far enough
    to cope with observations taken from far up a mountain but the equations
    from which the Table was drawn should be applied to the air pressure
    observed at Doug's location.
    However, if the bulk of the refraction occurs much higher in the
    atmosphere, air pressure at the observer's location can only give an
    approximate indication of the density of the air far above but that
    approximation would need to be based on observed surface pressure
    standardized to sea level. Indeed, if we are dealing with substantial
    refraction high in the atmosphere (even if there is more per metre in
    the few metres nearest the Earth), it might be necessary to use a
    correction drawn from Table A4 using surface pressure standardized to
    sea level and then a second correction based on the pressure difference
    between the observer's altitude and sea level to allow for the lack of
    low-level refraction resulting from the observer's elevation. The
    combination of the two corrections would presumably be intermediate
    between what Table A4 gives for the sea-level pressure and what it would
    give if the pressure at altitude were treated as a sea-level observation.
    I suppose that somebody may have figured out the details of refraction
    at high altitudes for use in aircraft navigation but perhaps aircraft
    move too quickly for anyone to care about the last 0.1 minutes precision.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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